Re: Define friend operator << for class template.

From:

terminator <farid.mehrabi@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 6 Dec 2007 09:26:22 -0800 (PST)

Message-ID:

<7edd5862-69d3-42ee-85ec-2c725b919077@e23g2000prf.googlegroups.com>

On Dec 6, 8:05 pm, "Joe Hesse" <joe_he...@actcx.com> wrote:

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

********************************************
#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>&)
*/

********************************************

Instein says "take it simple, as simple as posible but not simpler".
Why did you not use the simplest imaginable syntax?
I would write:

friend std::ostream & operator << (std::ostream &, const Foo &);

but this is not what the compiler complaigns about;Please learn to
read:

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>&)

Since you have defined a constructor ,C++ will no more automatically
generate a default constructor. this is the errorneous line:

Foo<int> fi;

In order to resolve this add the following inside the braces for
declaration of your template class:

Foo(){};

regards,
FM.