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Temporary Variables: Chasing Temporaries Away


There are situations in C++ when it is good to return the result of a function by value rather than by reference. There is usually a price to be paid, however...unless the compiler can be made to help. Jun Nakamura explains.

Author Info:
By: J. Nakamura
Rating: 5 stars5 stars5 stars5 stars5 stars / 5
October 10, 2005
TABLE OF CONTENTS:
  1. · Temporary Variables: Chasing Temporaries Away
  2. · Return Value Optimization
  3. · Vector3 test
  4. · Details, details… what’s that… small print?
  5. · Help the compiler help you

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Temporary Variables: Chasing Temporaries Away
(Page 1 of 5 )

>There are only two qualities in the world: efficiency and inefficiency.

>And only two sorts of people: the efficient and the inefficient.

 (G. Bernard Shaw 1856-1950)

Finding the Closest Enemy

In the previous article we discovered that there are situations where it is beneficial to return the result of a function by value. The example used was the subtraction operator implemented for a Vector3 class:

class Vector3 {
 float mX, mY, mZ;
public:
      ...
      Vector3 const operator– (Vector3 const &rhs) const {
  return Vector3(mX-rhs.mX, mY-rhs.mY, mZ-rhs.mZ);
      }
      ...
};

This operator is needed to determine a vector between two points, where the length of the vector determines the distance between the two points. It is not possible for this function to return a reference one way or the other, so it has to return the result by value.

What we are going to examine now is how we can help the compiler to optimize the temporary variable away, so we won’t have pay a price for returning by value at all!


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