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Why is nullptr a part of the core language, but nullptr_t is a part of STL?

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6

As far as I'm aware nullptr is a part of the core language.

Quoting C++11: (18.2/9)

nullptr_t is defined as follows:

namespace std typedef decltype(nullptr) nullptr_t;

and is defined in the header <cstddef>.

c++ c++11 nullptr

share|improve this question

edited 10 hours ago

Jarod42

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asked 10 hours ago

embedcembedc

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• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  related/maybe dupe: stackoverflow.com/questions/39080709/…
  
  – NathanOliver
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  You don't need nullptr_t if you are willing to live with decltype(nullptr). nullptr_t isn't a core concept, it's a helper.
  
  – François Andrieux
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You need nullptr_t sometimes: "If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument." (cppreference for std::nullptr_t).
  
  – Adrian
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Adrian I'm assuming you meant to reply to my comment. If so, I said you don't need nullptr_t if you are willing to live with decltype(nullptr). You can use decltype(nullptr) instead of nullptr_t. nullptr_t is just a convenient alias for decltype(nullptr);. You should use nullptr_t, my comment is meant to indicate why nullptr_t might not be part of the core language.
  
  – François Andrieux
  10 hours ago
  
  

  
  
  
  

add a comment | 

6

As far as I'm aware nullptr is a part of the core language.

Quoting C++11: (18.2/9)

nullptr_t is defined as follows:

namespace std typedef decltype(nullptr) nullptr_t;

and is defined in the header <cstddef>.

c++ c++11 nullptr

share|improve this question

edited 10 hours ago

Jarod42

129k1212 gold badges115115 silver badges202202 bronze badges

asked 10 hours ago

embedcembedc

6871212 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  related/maybe dupe: stackoverflow.com/questions/39080709/…
  
  – NathanOliver
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  You don't need nullptr_t if you are willing to live with decltype(nullptr). nullptr_t isn't a core concept, it's a helper.
  
  – François Andrieux
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You need nullptr_t sometimes: "If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument." (cppreference for std::nullptr_t).
  
  – Adrian
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Adrian I'm assuming you meant to reply to my comment. If so, I said you don't need nullptr_t if you are willing to live with decltype(nullptr). You can use decltype(nullptr) instead of nullptr_t. nullptr_t is just a convenient alias for decltype(nullptr);. You should use nullptr_t, my comment is meant to indicate why nullptr_t might not be part of the core language.
  
  – François Andrieux
  10 hours ago
  
  

  
  
  
  

add a comment | 

As far as I'm aware nullptr is a part of the core language.

Quoting C++11: (18.2/9)

nullptr_t is defined as follows:

namespace std typedef decltype(nullptr) nullptr_t;

and is defined in the header <cstddef>.

c++ c++11 nullptr

share|improve this question

edited 10 hours ago

Jarod42

129k1212 gold badges115115 silver badges202202 bronze badges

asked 10 hours ago

embedcembedc

6871212 bronze badges

share|improve this question

edited 10 hours ago

Jarod42

129k1212 gold badges115115 silver badges202202 bronze badges

asked 10 hours ago

embedcembedc

6871212 bronze badges

share|improve this question

edited 10 hours ago

Jarod42

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asked 10 hours ago

embedcembedc

6871212 bronze badges

edited 10 hours ago

Jarod42

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edited 10 hours ago

Jarod42

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asked 10 hours ago

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asked 10 hours ago

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3 Answers
3

active

oldest

votes

7

Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.

nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*)? So a brand new literal was added to the core language: nullptr. Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:

namespace std 
 using nullptr_t = decltype(nullptr);

share|improve this answer

edited 9 hours ago

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, but the nullptr itself can be a part of the STL... Is nullptr included in the core language to not require developers to include any header files?
  
  – embedc
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @embedc Any std typedefs like e.g. std::stize_tare provided through headers, so what?
  
  – πάντα ῥεῖ
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @embedc nullptr is a literal like 5, 'a' or "hello". While I can't imagine what it is, I would bet there is a good technical reason why a null pointer literal is better than having a nullptr-like object in std. Edit : For one thing, being a literal it's a prvalue so you can't take it's address. It would seem strange to me to be able to take nullptr's address. It would be like getting true's address.
  
  – François Andrieux
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No headers are required. It is a built-in keyword / pointer literal.
  
  – Ron
  10 hours ago
  
  

  
  
  
  

add a comment | 

3

The proposal that introduced nullptr, N2431, indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr.

It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.

share|improve this answer

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this actually answers the question.
  
  – bolov
  8 hours ago
  

  
  
  
  

add a comment | 

1

From cppreference.com:

std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type.

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.

You can then solve overloaded function call ambiguity with std::nullptr_t.

For example:

void Foo(int* ptr) 
void Foo(double* ptr) 
void Foo(std::nullptr_t ptr) // This overload is called if Foo(nullptr) is invoked

Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t

share|improve this answer

edited 3 hours ago

Remy Lebeau

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answered 9 hours ago

Adrien GivryAdrien Givry

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add a comment | 

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7

Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.

nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*)? So a brand new literal was added to the core language: nullptr. Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:

namespace std 
 using nullptr_t = decltype(nullptr);

share|improve this answer

edited 9 hours ago

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, but the nullptr itself can be a part of the STL... Is nullptr included in the core language to not require developers to include any header files?
  
  – embedc
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @embedc Any std typedefs like e.g. std::stize_tare provided through headers, so what?
  
  – πάντα ῥεῖ
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @embedc nullptr is a literal like 5, 'a' or "hello". While I can't imagine what it is, I would bet there is a good technical reason why a null pointer literal is better than having a nullptr-like object in std. Edit : For one thing, being a literal it's a prvalue so you can't take it's address. It would seem strange to me to be able to take nullptr's address. It would be like getting true's address.
  
  – François Andrieux
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No headers are required. It is a built-in keyword / pointer literal.
  
  – Ron
  10 hours ago
  
  

  
  
  
  

add a comment | 

7

Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.

nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*)? So a brand new literal was added to the core language: nullptr. Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:

namespace std 
 using nullptr_t = decltype(nullptr);

share|improve this answer

edited 9 hours ago

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, but the nullptr itself can be a part of the STL... Is nullptr included in the core language to not require developers to include any header files?
  
  – embedc
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @embedc Any std typedefs like e.g. std::stize_tare provided through headers, so what?
  
  – πάντα ῥεῖ
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @embedc nullptr is a literal like 5, 'a' or "hello". While I can't imagine what it is, I would bet there is a good technical reason why a null pointer literal is better than having a nullptr-like object in std. Edit : For one thing, being a literal it's a prvalue so you can't take it's address. It would seem strange to me to be able to take nullptr's address. It would be like getting true's address.
  
  – François Andrieux
  10 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No headers are required. It is a built-in keyword / pointer literal.
  
  – Ron
  10 hours ago
  
  

  
  
  
  

add a comment | 

Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.

nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*)? So a brand new literal was added to the core language: nullptr. Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:

namespace std 
 using nullptr_t = decltype(nullptr);

share|improve this answer

edited 9 hours ago

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

namespace std 
 using nullptr_t = decltype(nullptr);

share|improve this answer

edited 9 hours ago

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

answered 10 hours ago

Paul EvansPaul Evans

22.2k33 gold badges2424 silver badges4343 bronze badges

3

The proposal that introduced nullptr, N2431, indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr.

It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.

share|improve this answer

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this actually answers the question.
  
  – bolov
  8 hours ago
  

  
  
  
  

add a comment | 

3

The proposal that introduced nullptr, N2431, indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr.

It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.

share|improve this answer

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this actually answers the question.
  
  – bolov
  8 hours ago
  

  
  
  
  

add a comment | 

The proposal that introduced nullptr, N2431, indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr.

It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.

share|improve this answer

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

share|improve this answer

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

answered 9 hours ago

BrianBrian

71.5k77 gold badges104104 silver badges202202 bronze badges

answered 9 hours ago

BrianBrian

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1

From cppreference.com:

std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type.

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.

You can then solve overloaded function call ambiguity with std::nullptr_t.

For example:

void Foo(int* ptr) 
void Foo(double* ptr) 
void Foo(std::nullptr_t ptr) // This overload is called if Foo(nullptr) is invoked

Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t

share|improve this answer

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges

add a comment | 

1

From cppreference.com:

std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type.

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.

You can then solve overloaded function call ambiguity with std::nullptr_t.

For example:

void Foo(int* ptr) 
void Foo(double* ptr) 
void Foo(std::nullptr_t ptr) // This overload is called if Foo(nullptr) is invoked

Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t

share|improve this answer

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges

add a comment | 

From cppreference.com:

std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type.

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.

You can then solve overloaded function call ambiguity with std::nullptr_t.

For example:

void Foo(int* ptr) 
void Foo(double* ptr) 
void Foo(std::nullptr_t ptr) // This overload is called if Foo(nullptr) is invoked

Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t

share|improve this answer

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges

void Foo(int* ptr) 
void Foo(double* ptr) 
void Foo(std::nullptr_t ptr) // This overload is called if Foo(nullptr) is invoked

share|improve this answer

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

edited 3 hours ago

Remy Lebeau

360k2121 gold badges287287 silver badges485485 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges

answered 9 hours ago

Adrien GivryAdrien Givry

67733 silver badges1010 bronze badges