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Vector of pair with const member

Vector of structs with const members?What is the difference between const int*, const int * const, and int const *?Appending a vector to a vectorWhy can I not call reserve on a vector of const elements?Unable to emplace_back instance of a class with const membersWhy is it not possible to instantiate pair with “non const” copy constructor while it is possible to instantiate one without?How can I use std::rotate to rotate an array of pairs whose first member is const?Well-formed program containing an ill-formed template member function?Error while trying to use const int std::arrayHow to return a static const int std::array from a method?C++ initializing const

9

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 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  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
  2 days ago
  

  
  
  
  

add a comment | 

9

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 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  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
  2 days ago
  

  
  
  
  

add a comment | 

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 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

#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

share|improve this question

edited 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

share|improve this question

edited 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

edited 2 days ago

Usman

1,131717

edited 2 days ago

Usman

1,131717

asked 2 days ago

JonasJonas

5,88982445

asked 2 days ago

JonasJonas

5,88982445

1 Answer
1

active

oldest

votes

18

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 2 days ago

answered 2 days ago

lubgrlubgr

13.8k32052

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  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
  2 days 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
  2 days ago
  

  
  
  
  

add a comment | 

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18

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 2 days ago

answered 2 days ago

lubgrlubgr

13.8k32052

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  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
  2 days 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
  2 days ago
  

  
  
  
  

add a comment | 

18

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 2 days ago

answered 2 days ago

lubgrlubgr

13.8k32052

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  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
  2 days 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
  2 days ago
  

  
  
  
  

add a comment | 

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 2 days ago

answered 2 days ago

lubgrlubgr

13.8k32052

struct foo

 const int first = 42;
 int second = 43;
;

std::vector<foo> V2;
V2.resize(3);

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

share|improve this answer

edited 2 days ago

answered 2 days ago

lubgrlubgr

13.8k32052

answered 2 days ago

lubgrlubgr

13.8k32052

answered 2 days ago

lubgrlubgr

13.8k32052