Let's start by observing the following:
std::pair<const int, int> a, b; a=b;
This will not compile either. Why? Because you are in effect declaring the following class:
class my_pair { public: // ... Bunch of irrelevant stuff const int first; int second; };
This class has its default assignment operator deleted. The default assignment operator is equivalent to assigning class members individually:
pair<first,second> &operator=(const pair<first, second> &other) { first=other.first; second=other.second; return *this; }
This is a rough example of what a default assignment operator looks like.
But you can't modify the first class member, here, because it is a const class member. Hence, the default assignment operator is deleted. End result: cannot assign a std::pair with a const value to another such pair.
You should now be able to figure out by yourself why insert() on your vector of pairs doesn't work.
insert() is, roughly, equivalent to shifting over all existing values by one, in the vector, after the insertion point, to make room for the new value. In other words:
value[n+1]=values[n];
For all values past the insertion point (I'm ignoring for the moment the work done to grow the vector by one value, which is irrelevant to this discussion).
But the default assignment operator, as I explained, is deleted because the value has a const class member.
push_back works because nothing needs to be inserted, only a new value needs to be added to the end of the vector. If the vector needs to be reallocated, to accomodate its growth, that uses copy or move constructors, which are still valid for a std::pair with a const value.
