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push_backing to a vector of non-const elements works as expected:
std::vector<int> foo;
int bar = 0;
foo.push_back(bar);
But why is the following not possible?
std::vector<const int> foo;
const int bar = 0;
foo.push_back(bar);
More precisely, why is creating the foo object possible but not calling push_back on it?
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You can't put items into a const vector, the vectors state is the items it holds, and adding items to the vector modifies that state.
If the vector itself is declared const (as in const std::vector<T*>), then you can't modify the vector, but you can modify the objects. If the pointers are declared const (as in std::vector<const T*>), then you can modify the vector, but not the objects.
vector::push_back() push_back() function is used to push elements into a vector from the back. The new value is inserted into the vector at the end, after the current last element and the container size is increased by 1.
push_back effectively increases the vector size by one, which causes a reallocation of the internal allocated storage if the vector size was equal to the vector capacity before the call.
According to this answer (with commentary from one of the C++11 designers), std::vector<const T> is not permitted by the Standard.
The answer suggests that it might be possible to supply a custom allocator which permits a vector with that allocator to hold const objects.
You're probably better off not attempting to do this.
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