Fdtxjr

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:

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.