Vector of pair with const member

7

As stated in this answer a std::vector<T> cannot contain const T, or classes with const-members. However, this is not the case when T = std::pair<const int, int>, as shown below. Why is this the case? How is std::pair special?

#include <utility>

#include <vector>



struct foo

{

    const int first;

    int second;

};



int main() {

    std::vector<std::pair<const int, int>> V1;

    V1.resize(3); // This compiles



    std::vector<foo> V2;

    V2.resize(3); // This gives the error listed below

}

error: use of deleted function 'foo::foo()'

note: 'foo::foo()' is implicitly deleted because the default definition would be ill-formed:

c++ vector language-lawyer

share|improve this question

edited 1 hour ago

Usman

1,090717

asked 1 hour ago

JonasJonas

5,87682444

• 
  
  
  
  
  

  
  
  1
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.
  
  
  
  – StoryTeller
  
  1 hour ago
  
  

  
  
  
  
  
  

add a comment | 

7

As stated in this answer a std::vector<T> cannot contain const T, or classes with const-members. However, this is not the case when T = std::pair<const int, int>, as shown below. Why is this the case? How is std::pair special?

#include <utility>

#include <vector>



struct foo

{

    const int first;

    int second;

};



int main() {

    std::vector<std::pair<const int, int>> V1;

    V1.resize(3); // This compiles



    std::vector<foo> V2;

    V2.resize(3); // This gives the error listed below

}

error: use of deleted function 'foo::foo()'

note: 'foo::foo()' is implicitly deleted because the default definition would be ill-formed:

c++ vector language-lawyer

share|improve this question

edited 1 hour ago

Usman

1,090717

asked 1 hour ago

JonasJonas

5,87682444

• 
  
  
  
  
  

  
  
  1
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.
  
  
  
  – StoryTeller
  
  1 hour ago
  
  

  
  
  
  
  
  

add a comment | 

7

7

7

1

As stated in this answer a std::vector<T> cannot contain const T, or classes with const-members. However, this is not the case when T = std::pair<const int, int>, as shown below. Why is this the case? How is std::pair special?

#include <utility>

#include <vector>



struct foo

{

    const int first;

    int second;

};



int main() {

    std::vector<std::pair<const int, int>> V1;

    V1.resize(3); // This compiles



    std::vector<foo> V2;

    V2.resize(3); // This gives the error listed below

}

error: use of deleted function 'foo::foo()'

note: 'foo::foo()' is implicitly deleted because the default definition would be ill-formed:

c++ vector language-lawyer

share|improve this question

edited 1 hour ago

Usman

1,090717

asked 1 hour ago

JonasJonas

5,87682444

As stated in this answer a std::vector<T> cannot contain const T, or classes with const-members. However, this is not the case when T = std::pair<const int, int>, as shown below. Why is this the case? How is std::pair special?

#include <utility>

#include <vector>



struct foo

{

    const int first;

    int second;

};



int main() {

    std::vector<std::pair<const int, int>> V1;

    V1.resize(3); // This compiles



    std::vector<foo> V2;

    V2.resize(3); // This gives the error listed below

}

error: use of deleted function 'foo::foo()'

note: 'foo::foo()' is implicitly deleted because the default definition would be ill-formed:

c++ vector language-lawyer

c++ vector language-lawyer

share|improve this question

edited 1 hour ago

Usman

1,090717

asked 1 hour ago

JonasJonas

5,87682444

share|improve this question

edited 1 hour ago

Usman

1,090717

asked 1 hour ago

JonasJonas

5,87682444

share|improve this question

share|improve this question

edited 1 hour ago

Usman

1,090717

edited 1 hour ago

Usman

1,090717

edited 1 hour ago

Usman

1,090717

1,090717

asked 1 hour ago

JonasJonas

5,87682444

asked 1 hour ago

JonasJonas

5,87682444

asked 1 hour ago

JonasJonas

5,87682444

5,87682444

• 
  
  
  
  
  

  
  
  1
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.
  
  
  
  – StoryTeller
  
  1 hour ago
  
  

  
  
  
  
  
  

add a comment | 

• 
  
  
  
  
  

  
  
  1
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.
  
  
  
  – StoryTeller
  
  1 hour ago
  
  

  
  
  
  
  
  

1

1

You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.



– StoryTeller

1 hour ago

You linked to a Q&A about C++03. Not saying it's necessarily irrelevant, but a lot has been tweaked in the C++ standard since.



– StoryTeller

1 hour ago

add a comment | 

1 Answer

1

active



oldest



votes

13

You are mixing two things here. The error that you get is due to the implicitly deleted foo() default constructor that std::vector::resize(size_type count) invokes:

If the current size is less than count,

1) additional default-inserted elements are appended

The std::pair template has a default constructor, this is why the call to V1.resize succeeds. If you provide one for foo as well, or allow its implicit generation by in class initialization, e.g.

struct foo

{

    const int first = 42;

    int second = 43;

};

then

std::vector<foo> V2;

V2.resize(3);

will happily compile. The operation that won't work out for both std::pair<const int, int> and foo is assignment. This won't compile:

V1[0] = std::pair<const int, int>(42, 43); // No way

V2[0] = { 42, 43 }; // Also not ok, can't assign to const data member

which doesn't have anything to do with std::vector, but with the const-qualified data members in both cases.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?
  
  
  
  – StoryTeller
  
  49 mins ago
  
  

  
  
  
  
  
  



• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  @StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)
  
  
  
  – lubgr
  
  37 mins ago
  
  

  
  
  
  
  
  

add a comment | 

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13

You are mixing two things here. The error that you get is due to the implicitly deleted foo() default constructor that std::vector::resize(size_type count) invokes:

If the current size is less than count,

1) additional default-inserted elements are appended

The std::pair template has a default constructor, this is why the call to V1.resize succeeds. If you provide one for foo as well, or allow its implicit generation by in class initialization, e.g.

struct foo

{

    const int first = 42;

    int second = 43;

};

then

std::vector<foo> V2;

V2.resize(3);

will happily compile. The operation that won't work out for both std::pair<const int, int> and foo is assignment. This won't compile:

V1[0] = std::pair<const int, int>(42, 43); // No way

V2[0] = { 42, 43 }; // Also not ok, can't assign to const data member

which doesn't have anything to do with std::vector, but with the const-qualified data members in both cases.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?
  
  
  
  – StoryTeller
  
  49 mins ago
  
  

  
  
  
  
  
  



• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  @StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)
  
  
  
  – lubgr
  
  37 mins ago
  
  

  
  
  
  
  
  

add a comment | 

13

You are mixing two things here. The error that you get is due to the implicitly deleted foo() default constructor that std::vector::resize(size_type count) invokes:

If the current size is less than count,

1) additional default-inserted elements are appended

The std::pair template has a default constructor, this is why the call to V1.resize succeeds. If you provide one for foo as well, or allow its implicit generation by in class initialization, e.g.

struct foo

{

    const int first = 42;

    int second = 43;

};

then

std::vector<foo> V2;

V2.resize(3);

will happily compile. The operation that won't work out for both std::pair<const int, int> and foo is assignment. This won't compile:

V1[0] = std::pair<const int, int>(42, 43); // No way

V2[0] = { 42, 43 }; // Also not ok, can't assign to const data member

which doesn't have anything to do with std::vector, but with the const-qualified data members in both cases.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?
  
  
  
  – StoryTeller
  
  49 mins ago
  
  

  
  
  
  
  
  



• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  @StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)
  
  
  
  – lubgr
  
  37 mins ago
  
  

  
  
  
  
  
  

add a comment | 

13

13

13

You are mixing two things here. The error that you get is due to the implicitly deleted foo() default constructor that std::vector::resize(size_type count) invokes:

If the current size is less than count,

1) additional default-inserted elements are appended

The std::pair template has a default constructor, this is why the call to V1.resize succeeds. If you provide one for foo as well, or allow its implicit generation by in class initialization, e.g.

struct foo

{

    const int first = 42;

    int second = 43;

};

then

std::vector<foo> V2;

V2.resize(3);

will happily compile. The operation that won't work out for both std::pair<const int, int> and foo is assignment. This won't compile:

V1[0] = std::pair<const int, int>(42, 43); // No way

V2[0] = { 42, 43 }; // Also not ok, can't assign to const data member

which doesn't have anything to do with std::vector, but with the const-qualified data members in both cases.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

You are mixing two things here. The error that you get is due to the implicitly deleted foo() default constructor that std::vector::resize(size_type count) invokes:

If the current size is less than count,

1) additional default-inserted elements are appended

The std::pair template has a default constructor, this is why the call to V1.resize succeeds. If you provide one for foo as well, or allow its implicit generation by in class initialization, e.g.

struct foo

{

    const int first = 42;

    int second = 43;

};

then

std::vector<foo> V2;

V2.resize(3);

will happily compile. The operation that won't work out for both std::pair<const int, int> and foo is assignment. This won't compile:

V1[0] = std::pair<const int, int>(42, 43); // No way

V2[0] = { 42, 43 }; // Also not ok, can't assign to const data member

which doesn't have anything to do with std::vector, but with the const-qualified data members in both cases.

share|improve this answer

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

share|improve this answer

share|improve this answer

edited 1 hour ago

edited 1 hour ago

edited 1 hour ago

answered 1 hour ago

lubgrlubgr

13.7k31952

answered 1 hour ago

lubgrlubgr

13.7k31952

answered 1 hour ago

lubgrlubgr

13.7k31952

13.7k31952

• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?
  
  
  
  – StoryTeller
  
  49 mins ago
  
  

  
  
  
  
  
  



• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  @StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)
  
  
  
  – lubgr
  
  37 mins ago
  
  

  
  
  
  
  
  

add a comment | 

• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?
  
  
  
  – StoryTeller
  
  49 mins ago
  
  

  
  
  
  
  
  



• 
  
  
  
  
  

  
  
  

  
  
  
  
  
  
  
  
  

  
  
  
  
  @StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)
  
  
  
  – lubgr
  
  37 mins ago
  
  

  
  
  
  
  
  

Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?



– StoryTeller

49 mins ago

Isn't it also related to that fact that in C++03 (the Q&A the OP quoted) the requirements were container-wide, while now they are more member function specific?



– StoryTeller

49 mins ago

@StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)



– lubgr

37 mins ago

@StoryTeller Good question. Comparing the container requirements between two versions of the standard seems a bit above my pay grade, but I'll try to think about it :)



– lubgr

37 mins ago

add a comment | 

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Vector of pair with const member 2019-03-18T04:07:00-07:00 Rating: 4.5 Diposkan Oleh: Admin