Question

I’m getting the following error when I compile the following code on Visual Studio 2008 / Windows SDK 7

const UINT a_uint;
UINT result;

throw std::runtime_error( std::string("we did ") + a_uint +
                          " and got " + result );

Ironically, I ended up with this result:

error C2782: 'std::basic_string<_Elem,_Traits,_Alloc> std::operator +(
                 const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem
             )' : template parameter '_Elem' is ambiguous

Can someone explain why the error message doesn’t explain the real problem (that there is no operator for +ing ints to strings)?

Answer 1

You can reduce that to this

template<typename T>
void f(T, T);

int main() {
 f('0', 0); // is T int or char?
}

You try to add an unsigned int to a string. That does not make sense, and the std::string class does not need to take any precautions to add implicit conversions to char here because that would hide such potential programming bugs.

Try to convert the unsigned int to std::string into a decimal/hexadecimal/octal/etc form and then concatenate (you can do that using std::ostringstream or boost::lexical_cast) or fix the bug in other ways you see fit.