Re: Define friend operator << for class template.

Hi,

I have a template class and I want to define an operator << as a friend
function. For each instantiation of the class I want a corresponding
instantiation of operator <<.
The following example fails to compile with g++ version 4.1.2.
I would appreciate it if you could help me fix it or point me to a
suitable reference.

Thank you,
Joe Hesse

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#include <iostream>

// forward declaration
template <typename T>
class Foo;

// forward declaration
template <typename T>
std::ostream & operator << (std::ostream &, const Foo<T> &);

template <typename T>
class Foo {
private:
 T value;
public:
 Foo(const T & v) : value(v) {}
 friend std::ostream & operator << <> (std::ostream &, const Foo<T> &);
};

// implement operator <<
template <typename T>
std::ostream & operator << (std::ostream &o, const Foo<T> &f) {
 return o << f.value ;
}

int main() {
 Foo<int> fi;
 std::cout << fi;

 return 0;
}

/* Here are the compiler error messages
Test.cpp: In function int main():
Test.cpp:26: error: no matching function for call to Foo<int>::Foo()
Test.cpp:16: note: candidates are: Foo<T>::Foo(const T&) [with T = int]
Test.cpp:12: note: Foo<int>::Foo(const Foo<int>&)
*/

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