模板类中的好友运算符<<

Question

From what I know about friend functions this should work. I'm not sure what's going on.

In my code i define a class

template < class IType = unsigned int >
class BitArray {
    ...
    friend ostream& operator<<(ostream&, const BitArray&);
    friend istream& operator>>(istream&, BitArray&);
    ...
}

then later in the same header file

template < class IType >
ostream& operator<<(ostream& os, const BitArray<IType>& that)
{
    ...
}

template < class IType >
istream& operator>>(istream& is, BitArray<IType>& that)
{
    ...
}

and it's giving me

error LNK2019: unresolved external symbol

when I try to compile.

I've re-read and re-written this half a dozen times, and reviewed the use of the "friend" keyword, and can't find what's wrong.

does this implementation follow different rules because of the template

I also overwrote << and >> as shift operators, but again that shouldn't matter since they have different arguments;

Answer 1

With warning up, you have:

warning: friend declaration 'std::ostream& operator<<(std::ostream&, const BitArray<IType>&)' declares a non-template function [-Wnon-template-friend]
note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)

So declare the template function before:

template <class IType> class BitArray;
template <class IType> std::ostream& operator<<(std::ostream&, const BitArray<IType>&);
template <class IType> std::istream& operator>>(std::istream&, BitArray<IType>&);

template < class IType = unsigned int >
class BitArray {
    friend std::ostream& operator<< <>(std::ostream&, const BitArray&);
    friend std::istream& operator>> <>(std::istream&, BitArray&);
};

template <class IType>
std::ostream& operator<<(std::ostream& os, const BitArray<IType>& b)
{
    /* Your implementation */
}

Live example

Answer 2

I think you want this:

template < class IType = unsigned int >
class BitArray {
    template<class> friend ostream& operator<<(ostream&, const BitArray&);
    template<class> friend istream& operator>>(istream&, BitArray&);
};

Your BitArray declaration is telling the compiler to look for a non-templated operator<< and operator>>