box1.cpp

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//: C05:Box1.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Defines template operators.
#include <iostream>
using namespace std;

// Forward declarations
template<class T> class Box;

template<class T>
Box<T> operator+(const Box<T>&, const Box<T>&);

template<class T>
ostream& operator<<(ostream&, const Box<T>&);

template<class T> class Box {
  T t;
public:
  Box(const T& theT) : t(theT) {}
  friend Box operator+<>(const Box<T>&, const Box<T>&);
  friend ostream& operator<< <>(ostream&, const Box<T>&);
};

template<class T>
Box<T> operator+(const Box<T>& b1, const Box<T>& b2) {
  return Box<T>(b1.t + b2.t);
}

template<class T>
ostream& operator<<(ostream& os, const Box<T>& b) {
  return os << '[' << b.t << ']';
}

int main() {
  Box<int> b1(1), b2(2);
  cout << b1 + b2 << endl;  // [3]
//  cout << b1 + 2 << endl; // No implicit conversions!
} ///:~