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The Return Value Optimization[1]

Resource from the book:
Dov Bulka & David Mayhew <Efficient C++--- Performance programming Techiques>

Anytime you can skip the creation and destruction of an object, you are looking at a performance gain. We will discuss an optimization often performed by compilers to speed? up your source code by transforming it and eliminating object creation. This optimization is referred to as the Return Value Optimization(RVO). Prior to delving into the RVO we need to understand how return-by-value works. We will walk through it with a simple example.

The Mechanics of Return-by-Value
The Complex class implements a representation for complex numbers:

?1 class ?Complex
?2 {
?3 ??? // ?Complex?addition?operator
?4 ??? friend?Complex? operator ? + ( const ?Complex & ,? const ?Complex & );
?5 public :
?6 ??? // ?default?constructor
?7 ??? // ?Value?defaults?to?0?unless?otherwise?specified
?8 ??? Complex( double ?r? = ? 0.0 ,? double ?i? = ? 0.0 ):real(r),?imag(i)? {?}
?9
10 ??? // ?copy?constructor
11 ??? Complex( const ?Complex & ?c):real(c.real),?imag(c.imag)? {?}
12
13 ??? // ?Assignment?operator
14 ??? Complex & ? operator ? = ( const ?Complex & ?c);
15
16 ??? ~ Complex()? {?}
17 private :
18 ??? double ?real;
19 } ;

1 Complex? operator ? + ( const ?Complex & ?a,? const ?Complex & ?b)
2 {
3 ????Complex?retVal;
4 ????retVal.real? = ?a.real? + ?b.real;
5 ????retVal.imag? = ?a.imag? + ?b.imag;
6 ???? return ?retVal;
7 }

1 Complex & ?Complex:: operator ? + ( const ?Complex & ?c1,? const ?Complex & ?c2)
2 {
3 ??
4 }

1 void ?Complex_Add( const ?Complex & ?__result,? const ?Complex & ?c1,? const ?Complex & ?c2)
2 {
3 ???
4 }

1 struct ?Complex?__tempResult;? // ?Storage.?No?constructor?here.
2 Complex_Add(__tempResult,?c1,?c2);? // ?All?argument?passed?by?reference.
3 c3? = ?__tempResult;? // ?Feed?result?back?into?left-hand-side.

posted on 2006-11-13 19:01 Zero Lee 閱讀(263) 評(píng)論(0)  編輯 收藏 引用 所屬分類(lèi): C++ Performance