Eager vs. Lazy Evaulation

Programming languages can be categorized into eager or lazy evaluation. Most languages are in the eager evaluation.

But we can do somethings in C++ and Java to create lazy evaluation by constructing objects that store parameters for later evaluation.

class BigInteger;
class IntermediateSum {
  const BigInteger &a, &b;
};

class BigInteger {
  IntermediateSum operator+(const BigInteger &b) { 
    return new Intermediate(*this, b); 
  }
  BigInteger& operator=(const IntermediateSum &i) {
    /* now add */
  }
};

The sum isn't evaluated until it hits the assignment. This lazy evaluation shows precisely when you can start evaluation and when you really need it.