Extract signed-ness of template type?

Question

The code below generates a warning with -Wsign-conversion. It generates the warning at the line T digit = a % base.

I want to extract the signed-ness of T and then cast base to that signed-ness to squash the warning.

I'm trying to avoid specializations because that just duplicates code (the only thing that's going to change is the signed-ness of base). I'm also trying to avoid casting base to a T in case its a non-POD type like a Integer (which is optimized for the reduction with longs).

How do I extract the signed-ness of T?

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Related, the code base is effectively C++98 and C++03, so it does not have some features (like discussed in Partial template specialization based on “signed-ness” of integer type?).

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template <class T>
std::string IntToString(T a, unsigned int base = 10)
{
    if (a == 0)
        return "0";
    bool negate = false;
    if (a < 0)
    {
        negate = true;
        a = 0-a;    // VC .NET does not like -a
    }
    std::string result;
    while (a > 0)
    {
        T digit = a % base;
        result = char((digit < 10 ? '0' : ('a' - 10)) + digit) + result;
        a /= base;
    }
    if (negate)
        result = "-" + result;
    return result;
}

Answer 1

Pre-C++11 you can use this implementation of std::conditional:

template<bool B, class T, class F>
struct conditional { typedef T type; };
template<class T, class F>
struct conditional<false, T, F> { typedef F type; };

Then we can write a struct to extract the signedness of a type:

template <typename T>
struct signedness {
    typedef typename conditional<T(-1)<T(0),int,unsigned>::type type;   
};

Then just declare base as being that type:

std::string IntToString(T a, 
    typename signedness<T>::type base = 10){