Is it possible to get a pointer to one subobject via a pointer to a different, unreleated subobject?Where in the C++ Standard is the memory layout of objects documented?What are the differences between a pointer variable and a reference variable in C++?Is the Empty Base Class Optimization now a mandatory optimization (at least for standard-layout classes)?Pointer arithmetic across subobject boundariesWhy is f(i = -1, i = -1) undefined behavior?What makes it possible for glibc malloc to compare pointers from different “objects”?Can an equality comparison of unrelated pointers evaluate to true?Compatibility between structs with differently const-qualified pointersGuaranteed memory layout for standard layout struct with a single array member of primitive typeUsing std::launder to get a pointer to an active union member from a pointer to an inactive union member?Accessing the members of a struct referenced by a void pointer

Floating Point XOR

Audire, with accusative or dative?

Debussy as term for bathroom?

What is the maximum viable speed for a projectile within earth's atmosphere?

Make Interviewee Comfortable in Potentially Intimate Environment

Why is the stock market so unpredictable?

Prove that the area of the trangles are equal.

Minimize taxes now that I earn more

Is a global DNS record a security risk for phpMyAdmin?

Who are the people reviewing far more papers than they're submitting for review?

What are the end bytes of *.docx file format

Can I separate garlic into cloves for storage?

Persuading players to be less attached to a pre-session 0 character concept

How often is duct tape used during crewed space missions?

Flying pigs arrive

How should errors be reported in scientific libraries?

Borrowing observations for prior probability in Bayesian Inference

Should the pagination be reset when changing the order?

How to influence manager to not schedule team meetings during lunch?

SMTP banner mismatch with multiple MX records

Is there any reason nowadays to use a neon indicator lamp instead of a LED?

What do solvers like Gurobi and CPLEX do when they run into hard instances of MIP

Why do things cool down?

Twelve Minesweeper mines that make twelve 4s



Is it possible to get a pointer to one subobject via a pointer to a different, unreleated subobject?


Where in the C++ Standard is the memory layout of objects documented?What are the differences between a pointer variable and a reference variable in C++?Is the Empty Base Class Optimization now a mandatory optimization (at least for standard-layout classes)?Pointer arithmetic across subobject boundariesWhy is f(i = -1, i = -1) undefined behavior?What makes it possible for glibc malloc to compare pointers from different “objects”?Can an equality comparison of unrelated pointers evaluate to true?Compatibility between structs with differently const-qualified pointersGuaranteed memory layout for standard layout struct with a single array member of primitive typeUsing std::launder to get a pointer to an active union member from a pointer to an inactive union member?Accessing the members of a struct referenced by a void pointer






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;








12















Look at this simple code:



struct Point 
 int x;
 int y;
;

void something(int *);

int main() 
 Point p1, 2;

 something(&p.x);

 return p.y;



I expect, that main's return value can be optimized to return 2;, as something doesn't have access to p.y, it only gets a pointer to p.x.



But, none of the major compilers optimize the return value of main to 2. Godbolt.



Is there something in the standard, which allows something to modify p.y, if we only give access to p.x? If yes, does this depend on whether Point has standard layout?



What if I use something(&p.y);, and return p.x; instead?






c++ language-lawyer c++17







share|improve this question





















  • 1





    I don't understand the downvote and the flag. This is perfectly reasonable question to me.

    – Zereges
    8 hours ago











  • stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

    – alter igel
    8 hours ago











  • @alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

    – geza
    8 hours ago












  • If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

    – t.niese
    8 hours ago






  • 1





    @zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

    – Jeffrey
    8 hours ago


















12















Look at this simple code:



struct Point 
 int x;
 int y;
;

void something(int *);

int main() 
 Point p1, 2;

 something(&p.x);

 return p.y;



I expect, that main's return value can be optimized to return 2;, as something doesn't have access to p.y, it only gets a pointer to p.x.



But, none of the major compilers optimize the return value of main to 2. Godbolt.



Is there something in the standard, which allows something to modify p.y, if we only give access to p.x? If yes, does this depend on whether Point has standard layout?



What if I use something(&p.y);, and return p.x; instead?






c++ language-lawyer c++17







share|improve this question





















  • 1





    I don't understand the downvote and the flag. This is perfectly reasonable question to me.

    – Zereges
    8 hours ago











  • stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

    – alter igel
    8 hours ago











  • @alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

    – geza
    8 hours ago












  • If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

    – t.niese
    8 hours ago






  • 1





    @zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

    – Jeffrey
    8 hours ago














12












12








12


1






Look at this simple code:



struct Point 
 int x;
 int y;
;

void something(int *);

int main() 
 Point p1, 2;

 something(&p.x);

 return p.y;



I expect, that main's return value can be optimized to return 2;, as something doesn't have access to p.y, it only gets a pointer to p.x.



But, none of the major compilers optimize the return value of main to 2. Godbolt.



Is there something in the standard, which allows something to modify p.y, if we only give access to p.x? If yes, does this depend on whether Point has standard layout?



What if I use something(&p.y);, and return p.x; instead?






c++ language-lawyer c++17







share|improve this question
















Look at this simple code:



struct Point 
 int x;
 int y;
;

void something(int *);

int main() 
 Point p1, 2;

 something(&p.x);

 return p.y;



I expect, that main's return value can be optimized to return 2;, as something doesn't have access to p.y, it only gets a pointer to p.x.



But, none of the major compilers optimize the return value of main to 2. Godbolt.



Is there something in the standard, which allows something to modify p.y, if we only give access to p.x? If yes, does this depend on whether Point has standard layout?



What if I use something(&p.y);, and return p.x; instead?






c++ language-lawyer c++17




c++ language-lawyer c++17






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 8 hours ago







geza

















asked 8 hours ago









gezageza

16.6k3 gold badges40 silver badges97 bronze badges




16.6k3 gold badges40 silver badges97 bronze badges










  • 1





    I don't understand the downvote and the flag. This is perfectly reasonable question to me.

    – Zereges
    8 hours ago











  • stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

    – alter igel
    8 hours ago











  • @alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

    – geza
    8 hours ago












  • If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

    – t.niese
    8 hours ago






  • 1





    @zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

    – Jeffrey
    8 hours ago













  • 1





    I don't understand the downvote and the flag. This is perfectly reasonable question to me.

    – Zereges
    8 hours ago











  • stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

    – alter igel
    8 hours ago











  • @alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

    – geza
    8 hours ago












  • If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

    – t.niese
    8 hours ago






  • 1





    @zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

    – Jeffrey
    8 hours ago








1




1





I don't understand the downvote and the flag. This is perfectly reasonable question to me.

– Zereges
8 hours ago





I don't understand the downvote and the flag. This is perfectly reasonable question to me.

– Zereges
8 hours ago













stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

– alter igel
8 hours ago





stackoverflow.com/questions/50803202/… "Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other"

– alter igel
8 hours ago













@alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

– geza
8 hours ago






@alterigel: this shouldn't matter, as we can use offsetof. But I'm not sure that we can actually get a pointer to p.y from p.x, with the new pointer semantic rules of C++17. std::launder cannot be used, as a pointer to p.x can only reach p.x. So even if I could move the pointer to the correct location, maybe that pointer won't point to the object of p.y.

– geza
8 hours ago














If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

– t.niese
8 hours ago





If you change it to void something(int * x) *(x+1) = 6; it optimizes the return value to a fixed 6, so the compiler at least sees that as a valid option to do.

– t.niese
8 hours ago




1




1





@zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

– Jeffrey
8 hours ago






@zereges, didn't downvote, but almost did. The question is very hard to parse. I believe the OP is asking: "why can't the compiler realize the something() function will not modify p.y ?" If the OP asked that directly, there'd be less downvotes.

– Jeffrey
8 hours ago













2 Answers
2






active

oldest

votes


















14
















This is perfectly well-defined:



void something(int *x) 
 reinterpret_cast<Point*>(x)->y = 42;



The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:




Two objects a and b are pointer-interconvertible if:



  • [...]

  • one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:

  • [...]

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.




That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.



Although it's not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.






share|improve this answer






















  • 1





    Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

    – geza
    8 hours ago











  • @geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

    – Barry
    8 hours ago






  • 1





    @geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

    – Brian
    8 hours ago











  • Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

    – sturcotte06
    8 hours ago







  • 1





    @sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

    – NathanOliver
    8 hours ago


















-4
















Given a POD structure (trivial with a standard layout, such as Point), you could change y as follows, even if x was not the first field in the object:



void something(int *data)

 data[(offsetof(Point, y)- offsetof(Point, x))/sizeof(int)] = 3;






share|improve this answer



























  • You might want to reread the question. The OP is not asking about this.

    – NathanOliver
    8 hours ago











  • The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

    – Gonen I
    8 hours ago











  • Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

    – NathanOliver
    8 hours ago











  • The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

    – Gonen I
    8 hours ago











  • Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

    – NathanOliver
    8 hours ago













Your Answer






StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/4.0/"u003ecc by-sa 4.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);














draft saved

draft discarded
















StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f57979009%2fis-it-possible-to-get-a-pointer-to-one-subobject-via-a-pointer-to-a-different-u%23new-answer', 'question_page');

);

Post as a guest















Required, but never shown

























2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









14
















This is perfectly well-defined:



void something(int *x) 
 reinterpret_cast<Point*>(x)->y = 42;



The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:




Two objects a and b are pointer-interconvertible if:



  • [...]

  • one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:

  • [...]

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.




That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.



Although it's not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.






share|improve this answer






















  • 1





    Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

    – geza
    8 hours ago











  • @geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

    – Barry
    8 hours ago






  • 1





    @geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

    – Brian
    8 hours ago











  • Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

    – sturcotte06
    8 hours ago







  • 1





    @sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

    – NathanOliver
    8 hours ago















14
















This is perfectly well-defined:



void something(int *x) 
 reinterpret_cast<Point*>(x)->y = 42;



The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:




Two objects a and b are pointer-interconvertible if:



  • [...]

  • one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:

  • [...]

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.




That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.



Although it's not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.






share|improve this answer






















  • 1





    Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

    – geza
    8 hours ago











  • @geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

    – Barry
    8 hours ago






  • 1





    @geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

    – Brian
    8 hours ago











  • Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

    – sturcotte06
    8 hours ago







  • 1





    @sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

    – NathanOliver
    8 hours ago













14














14










14









This is perfectly well-defined:



void something(int *x) 
 reinterpret_cast<Point*>(x)->y = 42;



The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:




Two objects a and b are pointer-interconvertible if:



  • [...]

  • one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:

  • [...]

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.




That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.



Although it's not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.






share|improve this answer















This is perfectly well-defined:



void something(int *x) 
 reinterpret_cast<Point*>(x)->y = 42;



The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:




Two objects a and b are pointer-interconvertible if:



  • [...]

  • one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:

  • [...]

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.




That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.



Although it's not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.







share|improve this answer














share|improve this answer



share|improve this answer








edited 8 hours ago









T.C.

111k14 gold badges229 silver badges338 bronze badges




111k14 gold badges229 silver badges338 bronze badges










answered 8 hours ago









BarryBarry

200k22 gold badges373 silver badges665 bronze badges




200k22 gold badges373 silver badges665 bronze badges










  • 1





    Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

    – geza
    8 hours ago











  • @geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

    – Barry
    8 hours ago






  • 1





    @geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

    – Brian
    8 hours ago











  • Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

    – sturcotte06
    8 hours ago







  • 1





    @sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

    – NathanOliver
    8 hours ago












  • 1





    Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

    – geza
    8 hours ago











  • @geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

    – Barry
    8 hours ago






  • 1





    @geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

    – Brian
    8 hours ago











  • Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

    – sturcotte06
    8 hours ago







  • 1





    @sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

    – NathanOliver
    8 hours ago







1




1





Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

– geza
8 hours ago





Thanks, I knew that I left a hole in my question. So, if I used something(&p.y); return p.x;, then it is clearly a missed optimization?

– geza
8 hours ago













@geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

– Barry
8 hours ago





@geza For some definition of "clearly". There almost certainly exists a lot of code that does things like this that such an optimization would break.

– Barry
8 hours ago




1




1





@geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

– Brian
8 hours ago





@geza Technically, yes, it would be a missed optimization then, but I don't think major compilers, at this time, are prepared to break all the existing code that uses offsetof.

– Brian
8 hours ago













Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

– sturcotte06
8 hours ago






Doesn't that violate the strict-aliasing rule? Or is that a C-only rule?

– sturcotte06
8 hours ago





1




1





@sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

– NathanOliver
8 hours ago





@sturcotte06 it does not because of If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast.

– NathanOliver
8 hours ago













-4
















Given a POD structure (trivial with a standard layout, such as Point), you could change y as follows, even if x was not the first field in the object:



void something(int *data)

 data[(offsetof(Point, y)- offsetof(Point, x))/sizeof(int)] = 3;






share|improve this answer



























  • You might want to reread the question. The OP is not asking about this.

    – NathanOliver
    8 hours ago











  • The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

    – Gonen I
    8 hours ago











  • Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

    – NathanOliver
    8 hours ago











  • The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

    – Gonen I
    8 hours ago











  • Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

    – NathanOliver
    8 hours ago















-4
















Given a POD structure (trivial with a standard layout, such as Point), you could change y as follows, even if x was not the first field in the object:



void something(int *data)

 data[(offsetof(Point, y)- offsetof(Point, x))/sizeof(int)] = 3;






share|improve this answer



























  • You might want to reread the question. The OP is not asking about this.

    – NathanOliver
    8 hours ago











  • The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

    – Gonen I
    8 hours ago











  • Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

    – NathanOliver
    8 hours ago











  • The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

    – Gonen I
    8 hours ago











  • Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

    – NathanOliver
    8 hours ago













-4














-4










-4









Given a POD structure (trivial with a standard layout, such as Point), you could change y as follows, even if x was not the first field in the object:



void something(int *data)

 data[(offsetof(Point, y)- offsetof(Point, x))/sizeof(int)] = 3;






share|improve this answer















Given a POD structure (trivial with a standard layout, such as Point), you could change y as follows, even if x was not the first field in the object:



void something(int *data)

 data[(offsetof(Point, y)- offsetof(Point, x))/sizeof(int)] = 3;







share|improve this answer














share|improve this answer



share|improve this answer








edited 7 hours ago

























answered 8 hours ago









Gonen IGonen I

2,07714 silver badges31 bronze badges




2,07714 silver badges31 bronze badges















  • You might want to reread the question. The OP is not asking about this.

    – NathanOliver
    8 hours ago











  • The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

    – Gonen I
    8 hours ago











  • Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

    – NathanOliver
    8 hours ago











  • The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

    – Gonen I
    8 hours ago











  • Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

    – NathanOliver
    8 hours ago

















  • You might want to reread the question. The OP is not asking about this.

    – NathanOliver
    8 hours ago











  • The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

    – Gonen I
    8 hours ago











  • Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

    – NathanOliver
    8 hours ago











  • The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

    – Gonen I
    8 hours ago











  • Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

    – NathanOliver
    8 hours ago
















You might want to reread the question. The OP is not asking about this.

– NathanOliver
8 hours ago





You might want to reread the question. The OP is not asking about this.

– NathanOliver
8 hours ago













The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

– Gonen I
8 hours ago





The OP says Is there something in the standard, which allows something to modify p.y, if we only give access to p.x?

– Gonen I
8 hours ago













Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

– NathanOliver
8 hours ago





Correct. You are talking about an array, not whether or not something could modify y given a pointer to x.

– NathanOliver
8 hours ago













The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

– Gonen I
8 hours ago





The result will be the same. It is not possible for the compiler to optimize, as the data is freely changeable by something given its prototype.

– Gonen I
8 hours ago













Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

– NathanOliver
8 hours ago





Sure, but arrays are different than structs and you cannot treat a pointer to a member as a pointer to an array. You also don't back up you claim with any text from the standard.

– NathanOliver
8 hours ago


















draft saved

draft discarded















































Thanks for contributing an answer to Stack Overflow!


  • Please be sure to answer the question. Provide details and share your research!

But avoid …


  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f57979009%2fis-it-possible-to-get-a-pointer-to-one-subobject-via-a-pointer-to-a-different-u%23new-answer', 'question_page');

);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







-

Popular posts from this blog

ParseJSON using SSJSUsing AMPscript with SSJS ActivitiesHow to resubscribe a user in Marketing cloud using SSJS?Pulling Subscriber Status from Lists using SSJSRetrieving Emails using SSJSProblem in updating DE using SSJSUsing SSJS to send single email in Marketing CloudError adding EmailSendDefinition using SSJS

Image
Is there any way to stop a user from creating executables and running them? What is this "Table of astronomy" about? How far did Gandalf and the Balrog drop from the bridge in Moria? Can "être sur" mean "to be about" ? Is there a Morita cocycle for the mapping class group Mod(g,n) when n > 1? Why isn’t SHA-3 in wider use? AsyncDictionary - Can you break thread safety? What is this 1990s horror game of otherworldly PCs dealing with monsters on modern Earth? On the Rømer experiments and the speed of light visible indication that a cell is not evaluatable Is 悪いところを見つかった proper Japanese? Is this n-speak? Can sampling rate be a floating point number? Creating multi-attribute primary key in table in ArcGIS Desktop TEMPO: play a sound in animated GIF/PDF/SVG Can a PC use the Levitate spell to avoid movement speed reduction from exhaustion? How much maintenance time did it take to make an F4U Corsair ready for another flight? I
Read more

Кампала Садржај Географија Географија Историја Становништво Привреда Партнерски градови Референце Спољашње везе Мени за навигацију0°11′ СГШ; 32°20′ ИГД / 0.18° СГШ; 32.34° ИГД / 0.18; 32.340°11′ СГШ; 32°20′ ИГД / 0.18° СГШ; 32.34° ИГД / 0.18; 32.34МедијиПодациЗванични веб-сајту

Image
Главни градови у АфрициГрадови у Уганди енгл.Уганде1890ЕнтебеТанзанијом19781979Иди АминВикторијиног језера (function()var node=document.getElementById("mw-dismissablenotice-anonplace");if(node)node.outerHTML="u003Cdiv class="mw-dismissable-notice"u003Eu003Cdiv class="mw-dismissable-notice-close"u003E[u003Ca tabindex="0" role="button"u003Eсакријu003C/au003E]u003C/divu003Eu003Cdiv class="mw-dismissable-notice-body"u003Eu003Cdiv id="localNotice" lang="sr" dir="ltr"u003Eu003Cdiv style="position:relative; overflow:hidden; background-color:#5E9DC8; text-align:center; color:white; font-size:1.25em; font-weight:bold; line-height:1.5em; margin-top: 5px;"u003Eu003Cuu003Eu003Ca href="/wiki/%D0%92%D0%B8%D0%BA%D0%B8%D0%BF%D0%B5%D0%B4%D0%B8%D1%98%D0%B0:%D0%A2%D0%B0%D0%BA%D0%BC%D0%B8%D1%87%D0%B5%D1%9A%D0%B5_%D1%83_%D0%BF%D0%B8%D1%81%D0%B0%D1%9A%D1%83_%D1%87%D0%BB%D0%B0%D0%BD%D0%B0%D0%BA
Read more

19. јануар Садржај Догађаји Рођења Смрти Празници и дани сећања Види још Референце Мени за навигацијуу

Image
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30. ЈануарДани деветнаестиГрегоријанском календарупреступним годинама (function()var node=document.getElementById("mw-dismissablenotice-anonplace");if(node)node.outerHTML="u003Cdiv class="mw-dismissable-notice"u003Eu003Cdiv class="mw-dismissable-notice-close"u003E[u003Ca tabindex="0" role="button"u003Eсакријu003C/au003E]u003C/divu003Eu003Cdiv class="mw-dismissable-notice-body"u003Eu003Cdiv id="localNotice" lang="sr&
Read more