Zero Lee的专栏

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 阅读(257) 评论(0)  编辑 收藏 引用 所属分类: C++ Performance