运算符<<中的执行顺序 [英] Order of execution in operator <<
问题描述
我很难理解下面代码中的调用顺序. 我期待看到下面的输出
I have difficulties in understanding the sequence of calls in the code below. I was expecting to see the output below
A1B2
虽然我看到的输出是
BA12
我认为通话std::cout<< b->fooA() << b->fooB() << std::endl
等同于通话
std::cout.operator<<( b->fooA() ).operator<< ( b->fooB() )
但是我可以看到情况并非如此.您能帮助我更好地了解它是如何工作的以及与全局operator<<
的关系吗?这是此顺序中最后一次被调用吗?
but I can see that this is not the case. Can you help me understanding better how this does it work and the relationship with the global operator<<
? Is this last ever called in this sequence?
#include <iostream>
struct cbase{
int fooA(){
std::cout<<"A";
return 1;
}
int fooB(){
std::cout <<"B";
return 2;
}
};
void printcbase(cbase* b ){
std::cout << b->fooA() << b->fooB() << std::endl;
}
int main(){
cbase b;
printcbase( &b );
}
推荐答案
编译器可以按以下方式评估函数printcbase()
:
The compiler can evaluate the function printcbase()
as this:
void printcbase(cbase* b ){
int a = b->FooA(); // line 1
int b = b->FooB(); // line 2
std::cout << a; // line 3
std::cout << b; // line 4
stc::cout << std::endl;
}
或标记为1-4的行的许多置换中的一些.仅保证行1在行3之前完成,行2在行4之前完成(当然行3在行4之前完成). Standard并没有多说什么,实际上您可以期望使用不同的C ++编译器得到不同的结果.
or some of many permutatins of lines marked as 1 - 4. You are only guaranteed that that the line 1 is done before the line 3, and line 2 before the line 4 (and of course line 3 before line 4). Standard does not say more and indeed you can expect different results with different C++ compilers.
这篇关于运算符<<中的执行顺序的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!