STL container assignment and const pointers

Question

This compiles:

int* p1;
const int* p2;
p2 = p1;

This does not:

vector v1;
vector v2;
v2 = v1;         


        

Answer 1

An important point not mentioned in any of the previous answers is that template specializations make this impossible to implement on a language-wide bases. Consider:

template
class Test
{
    T t;
};

template<>
class Test
{
    char array[1000];
};

Thus Test contains an array of chars, whereas Test contains a single int.

#include 
using namespace std;

int main()
{
    Test t1;
    Test t2;
    cout << sizeof(t1) << endl; // gives 4
    cout << sizeof(t2) << endl; // gives 1000
    return 0;
}

In reality vector and vector may hardly differ at all --- in particular, they may have the same size. However, the possibility of explicit template specialization means that they could differ spectacularly, hence the reluctance of the compiler to allow conversion.

(This answer is mostly copied from http://bytes.com/topic/c/answers/449611-cast-vector-foo-vector-const-foo#post1717570)