重载运算符<<用于类模板

Question

let's take this code to implement the operator<< for two classes:

#include <iostream>

using std::cout;
using std::endl;

class A
{
    int a1_;
public:
    A(int a1) : a1_(a1){}
    std::ostream& print(std::ostream& os) const
    {
        return os << "a1_ : " << a1_ << endl;
    }
};

class B
{
    int b1_;
    double b2_;
public:
    B(int b1,double b2) : b1_(b1),b2_(b2){}
    std::ostream& print(std::ostream& os) const
    {
        os << "b1_ : " << b1_ << endl;
        os << "b2_ : " << b2_ << endl;
        return os;
    }
};

std::ostream& operator<<(std::ostream& os, const A& in)
{
    return in.print(os);
}

std::ostream& operator<<(std::ostream& os, const B& in)
{
    return in.print(os);
}

int main(int argc,char* argv[])
{
    A myA(10);
    B myB(20,30.14);

    cout << myA << myB << endl;
    return 0;
}

Because I am lazy I'd like to provide a template version of operator<< instead of the two versions as above. I can do it easily replacing with:

template< class T>
std::ostream& operator<<(std::ostream& os, const T& in)
{
    return in.print(os);
}

So far so good. If I have several classes I can implement the operator<< in one go. The trouble start when one of my classes is a class template. Let's take the previous example but with B class template:

#include <iostream>

using std::cout;
using std::endl;

class A
{
    int a1_;
public:
    A(int a1) : a1_(a1){}
    std::ostream& print(std::ostream& os) const
    {
        return os << "a1_ : " << a1_ << endl;
    }
};

template <class T>
class B
{
    int b1_;
    T b2_;
public:
    B(int b1,T b2) : b1_(b1),b2_(b2){}
    std::ostream& print(std::ostream& os) const
    {
        os << "b1_ : " << b1_ << endl;
        os << "b2_ : " << b2_ << endl;
        return os;
    }
};


std::ostream& operator<<(std::ostream& os, const A& in)
{
    return in.print(os);
}

template <class T>
std::ostream& operator<<(std::ostream& os, const B<T>& in)
{
    return in.print(os);
}

int main(int argc,char* argv[])
{
    A myA(10);
    B<A> myB(20,myA);

    cout << myA << myB << endl;
    return 0;
}

This version works and I have the expected result, however I have provided two operator<< functions (one for each class), let's imagine that I have 200 classes that already implement a public ostream& print(ostream& os) const. Some of them are template class (with also multiple parameters).

How can I write a template version of the operator<< in this scenario?

Thanks for you help.

Answer 1

Same as above:

template< class T>
std::ostream& operator<<(std::ostream& os, const T& in)
{
    return in.print(os);
}

However, a "catch all" overload like that is a bit like dynamite fishing. You can constrain the range of the operator to all T's which define a suitable "print" member using SFINAE (http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error):

template<int X, typename T>
struct enabler
{
    typedef T type;
};
template<class T>
typename enabler< sizeof(&T::print), std::ostream&>::type
operator << (std::ostream &o, const T &t)
{
    t.print(o);
    return o;
}

This effectively disables the operator<< when searching for a suitable overload, if T has no member print(std::ostream&)

Answer 2

This is, actually, what the Concepts were intended for. You can emulate them with Boost.Concepts at the moment.

However, there is one issue with your solution: Argument Dependent Lookup.

When you use an operator, it need be:

• either in the current scope (the search will radiate outward to consider more and more global scope)
• or in the namespace of one of the argument.

However if you define your template overload, it cannot be present in the namespace of all those other classes.

I suggest cheating .

If you wrap std::ostream& in a class of your own, you can then, in its namespace, provide all the operator overloads that you wish for:

namespace X {

struct MyStream
{
  MyStream(std::ostream& o): _o(o) {}
  std::ostream& _o;
};

template <typename T>
MyStream& operator<<(MyStream& s, T const& t)
{
  t.print(s._o);
  return s;
}

} // namespace X

You can then add opportunistic overloads for common types:

inline MyStream& operator<<(MyStream& s, bool b)
{
  s._o << (b ? 'Y' : 'N');
  return s;
}

Without risking a clash with functions defined in std .

Note that it trades reworking the class hierarchy (having a common PrintableInterface would be great too) vs reworking the calls. The latter can be done with a search and replace though.