return value optimization

最新推荐文章于 2025-03-02 13:11:13 发布

转载最新推荐文章于 2025-03-02 13:11:13 发布 · 107 阅读

本文通过一个简单的示例详细解释了返回值优化（NRVO）的概念及其在C++中的实现方式。对比开启与关闭NRVO的情况，展示了NRVO如何减少对象复制，提高程序效率。

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http://en.wikipedia.org/wiki/Return_value_optimization

http://msdn.microsoft.com/en-us/library/ms364057(VS.80).aspx

simple example

A MyMethod (B &var)
{
   A retVal;
   retVal.member = var.value + bar(var);
   return retVal;
}

valA = MyMethod(valB);

Hidden argument code without NRVO (pseudo code)

A MyMethod (A &_hiddenArg, B &var)
{
   A retVal;
   retVal.A::A(); // constructor for retVal
   retVal.member = var.value + bar(var);
   _hiddenArg.A::A(retVal);  // the copy constructor for A
   return;
retVal.A::~A();  // destructor for retVal

}

Hidden argument code with NRVO (pseudo code)

A MyMethod(A &_hiddenArg, B &var)
{
   _hiddenArg.A::A();
   _hiddenArg.member = var.value + bar(var);
   Return
}

Code Samples

Sample 1: Simple example

#include

<stdio.h>

class

RVO

{

public

RVO()

{

printf(

"I am in constructor %d\n" , this );

}

RVO (

const RVO& c_RVO)

{

printf (

"I am in copy constructor %d\n" , this );

}

~RVO()

{

printf (

"I am in destructor %d\n" , this );

}

RVO&

operator =( const RVO& rhs)

{

printf (

"I am in operator %d = %d\n" , this , &rhs);

return * this ;

}

int mem_var;

};

RVO MyMethod (

int i)

{

printf(

"3\n" );

RVO rvo;

//I am in constructor 1244896

printf(

"4\n" );

rvo.mem_var = i;

printf(

"5\n" );

return (rvo); //1244948CC, ~1244896

printf(

"6\n" );

}

int

main()

{

printf(

"1\n" );

RVO rvo;

//I am in constructor 1245016

printf(

"2\n" );

rvo=MyMethod(5);

//I am in copy constructor

printf(

"7\n" );

return 0; //~1245016

}

Figure 4. Sample1.cpp

Compiling sample1.cpp with and without NRVO turned on will yield different behavior.

Without NRVO (cl /Od sample1.cpp), the expected output would be:

1
I am in constructor 1245016
2
3
I am in constructor 1244888
4
5
I am in copy constructor 1244944
I am in destructor 1244888
I am in operator 1245016 = 1244944
I am in destructor 1244944
7
I am in destructor 1245016

With NRVO (cl /O2 sample1.cpp), the expected output would be:

1
I am in constructor 1245048
2
3
I am in constructor 1245052
4
5
I am in operator 1245048 = 1245052
I am in destructor 1245052
7
I am in destructor 1245048