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CppOverloadStreamOut

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(C++) Overloading the stream out operator, <<

Overloading can be done by making it a free function or a method. Overload it with a free function [1], see method for guideline.

A two-dimensional std::vector

template <class T>
std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<T> >& stlImage)
{
  const int maxx = stlImage.size();
  if (maxx == 0)
  {
    os << "{empty image}";
  }
  else
  {
    const int maxy = stlImage[0].size();
    for (int y=0; y!=maxy; ++y)
    {
      assert(maxy == static_cast<int>(stlImage[y].size()); //Assume it is a rectangular 2D vector
      for (int x=0; x!=maxx; ++x)
      {
        os  << static_cast<int>(stlImage[x][y]);
      }
      os << '\n';
    }
  }
  return os;
}

int main()
{
  const std::vector<std::vector<unsigned char> > image(20,std::vector<unsigned char>(10,127));
  std::cout << std::hex << image << std::dec << '\n';
}

A custom class

include <iostream>


struct MyClass
{
  MyClass() : mValue(0) {}
  int mValue;
};
std::ostream& operator<<(std::ostream& os, const MyClass& myClass)
{
  os << "MyClass.value: " << myClass.mValue;
  return os;
}

int main()
{
  MyClass myClass;
  myClass.mValue = 1;
  std::cout << myClass << std::endl;
}

However, this code works only on statically allocated instances of MyClass. To be also able to cout pointers and auto_ptr's, you could easily expand the code to:

include <iostream>

include <memory>


struct MyClass
{
  MyClass() : mValue(0) {}
  int mValue;
};
std::ostream& operator<<(std::ostream& os, const MyClass& myClass)
{
  os << "MyClass.value: " << myClass.mValue;
  return os;
}
std::ostream& operator<<(std::ostream& os, const MyClass* const myClass)
{
  os << *myClass;
  return os;
}
std::ostream& operator<<(std::ostream& os, const std::auto_ptr<MyClass>& myClass)
{
  os << myClass.get();
  return os;
}
int main()
{
  {
    MyClass myClass;
    myClass.mValue = 1;
    std::cout << myClass << std::endl;
  }
  {
    MyClass * myClass = new MyClass;
    myClass->mValue = 2;
    std::cout << myClass << std::endl;
    delete myClass;
  }
  {
    std::auto_ptr<MyClass> myClass(new MyClass);
    myClass->mValue = 3;
    std::cout << myClass << std::endl;
  }
}

Reference

• 1) Herb Sutter. Exceptional C++. ISBN: 0-201-61562-2. Item 20: Class mechanics.

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