Question

Why does the following code compile for the line using a non-const iterator but fails for the const iterator (using Visual Studio 2008) ?

#include <vector>

using std::vector;

int main(int argc, char* argv[])
{
    vector<int> test;

    test.push_back(1);
    test.push_back(2);

    vector<int>::const_iterator cit = test.end();
    std::distance(test.begin(), cit); // error: template parameter is ambiguous

    vector<int>::iterator it = test.end();
    std::distance(test.begin(), it);

    return 0;
}

---

Note: In Visual Studio 2008 there is no vector member function cbegin() to avoid the ambiguousness, but an overloaded begin() method:

iterator begin()
    {   // return iterator for beginning of mutable sequence
    return (iterator(_Myfirst, this));
    }

const_iterator begin() const
    {   // return iterator for beginning of nonmutable sequence
    return (const_iterator(_Myfirst, this));
    }

Answer 1

Note: In Visual Studio 2008 there is no vector member function cbegin() to avoid the ambiguousness, but an overloaded begin() method:

I think the compiler always picks the non-const overload for a non-const objects, and the const method only for const objects.

Calling

std::distance(test.begin(), cit);

doesn’t look at the overload set for begin and the type of cit, and figure out whether it can make a match. It resolves the overload first (to the non-const version), and hence fails.

The cleanest way to express your intent that will also work for the compiler is probably:

vector<int> const &cref = test;
vector<int>::const_iterator cit = cref.end();
std::distance(cref.begin(), cit);