Template operator friend of template class with enable_if

Question

How would I go about making a templated operator with enable_if a friend of a templated class?

Here is an example of my problem:

#include <type_traits>

template<typename CHAR_TYPE>
class BasicString;

template<typename CHAR_TYPE, typename OTHER_CHAR_TYPE,
    typename std::enable_if<((sizeof(CHAR_TYPE)==sizeof(OTHER_CHAR_TYPE))
        && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type = nullptr>
BasicString<CHAR_TYPE> operator+(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);

template<typename CHAR_TYPE, typename OTHER_CHAR_TYPE,
    typename std::enable_if<((sizeof(CHAR_TYPE)!=sizeof(OTHER_CHAR_TYPE))
        && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type = nullptr>
BasicString<CHAR_TYPE> operator+(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);

template<typename CHAR_TYPE>
class BasicString
{
private:
    CHAR_TYPE* characters;
    size_t length;

    template<typename OTHER_CHAR_TYPE,
        typename std::enable_if<((sizeof(CHAR_TYPE)==sizeof(OTHER_CHAR_TYPE))
            && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
    friend BasicString<CHAR_TYPE> operator+<CHAR_TYPE, OTHER_CHAR_TYPE>(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);
    template<typename OTHER_CHAR_TYPE,
        typename std::enable_if<((sizeof(CHAR_TYPE)!=sizeof(OTHER_CHAR_TYPE))
            && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
    friend BasicString<CHAR_TYPE> operator+<CHAR_TYPE, OTHER_CHAR_TYPE>(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);

public:
    BasicString()
        : length(0),
        characters((CHAR_TYPE*)std::malloc(sizeof(CHAR_TYPE)))
    {
        characters[0] = 0;
    }

    BasicString(const BasicString& str)
        : length(str.length),
        characters((CHAR_TYPE*)std::malloc(sizeof(CHAR_TYPE)*(str.length+1)))
    {
        for(size_t i=0; i<length; i++)
        {
            characters[i] = str.characters[i];
        }
    }

    ~BasicString()
    {
        delete characters;
    }
};

template<typename CHAR_TYPE, typename OTHER_CHAR_TYPE,
    typename std::enable_if<((sizeof(CHAR_TYPE)==sizeof(OTHER_CHAR_TYPE))
        && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
BasicString<CHAR_TYPE> operator+(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right)
{
    BasicString<CHAR_TYPE> newStr;
    CHAR_TYPE* characters = newStr.characters;
    //do some stuff. irrelevant to question
}

template<typename CHAR_TYPE, typename OTHER_CHAR_TYPE,
    typename std::enable_if<((sizeof(CHAR_TYPE)!=sizeof(OTHER_CHAR_TYPE))
        && std::is_integral<CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
BasicString<CHAR_TYPE> operator+(const BasicString<CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right)
{
    BasicString<CHAR_TYPE> newStr;
    CHAR_TYPE* characters = newStr.characters;
    //do some stuff. irrelevant to question
}

int main()
{
    BasicString<char> str1;
    BasicString<wchar_t> str2;
    BasicString<char> newStr = str1 + str2;
    return 0;
}

I know that I can use helper functions or a helper class, but I'd rather not if I can avoid it.

When I compile this, I get the error that the operator functions can't access the private members of the class, which means my code for befriending isn't working. How would I go about befriending these functions?

lufinkey · Answer 1 · 2016-04-22T18:56:13.770

So shortly after posting this question, I figured out the answer.

When declaring the friend, I need to use another template parameter to represent CHAR_TYPE

template<typename _CHAR_TYPE=CHAR_TYPE, typename OTHER_CHAR_TYPE,
        typename std::enable_if<((sizeof(_CHAR_TYPE)==sizeof(OTHER_CHAR_TYPE))
            && std::is_integral<_CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
friend BasicString<_CHAR_TYPE> operator+<_CHAR_TYPE, OTHER_CHAR_TYPE>(const BasicString<_CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);

    template<typename _CHAR_TYPE=CHAR_TYPE, typename OTHER_CHAR_TYPE,
        typename std::enable_if<((sizeof(_CHAR_TYPE)!=sizeof(OTHER_CHAR_TYPE))
            && std::is_integral<_CHAR_TYPE>::value && std::is_integral<OTHER_CHAR_TYPE>::value), std::nullptr_t>::type>
friend BasicString<_CHAR_TYPE> operator+<_CHAR_TYPE, OTHER_CHAR_TYPE>(const BasicString<_CHAR_TYPE>& left, const BasicString<OTHER_CHAR_TYPE>& right);

EDIT: after testing this in standards compliant compilers instead of visual C++, I've found that the standard actually doesn't allow this, and there doesn't seem to be any way of actually achieving it. I've conceded and made a helper class and just befriended that.

Template operator friend of template class with enable_if

1 Answers1