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Overloading « Operator For ostream

A common and useful practise is to overload the << operator for an ostream on your own classes. This allows you to then write things such as:

MyClass myClass;
std::cout << "My class = " << myClass << std::endl;

One way to do this is to overload the operator from within your class header file, declaring std::ostream as a friend:

#include <sstream>

class MyClass {
    friend std::ostream& operator<<(std::ostream &out, const MyClass& rhs);
    int myVar_;

And then the definition in the .cpp would look like:

std::ostream &operator<<(std::ostream &out, const MyClass& rhs) {
    out << "{ ";
    out << std::to_string(myVar_);
    out << " }";
    return out;

A Generic Overload For All Classes

Since C++11, you can make a generic overload that will work for all classes that contain a public method called Print(std::ostream&)const.

template<class T>
auto operator<<(std::ostream& os, const T& t) -> decltype(t.Print(os), os) { 
    return os; 

This saves you to work of having to overload the << operator for each class you design!


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