[C++知识库]c ++a/a++汇编原理

代码

用汇编解释前++和后++

	int a = 5;
	int b = ++a + 1;
	std::cout << "b = " << b << std::endl;

	int c = 5;
	int d = c++ + 1;
	std::cout << "c = " << c << std::endl;

前++

	int a = 5;
01051CF8  mov         dword ptr [a],5  //将5赋值给a
	int b = ++a + 1;
01051CFF  mov         eax,dword ptr [a]  //将a存储到寄存器 eax = 5
01051D02  add         eax,1  //加1  eax = 5 + 1 = 6
01051D05  mov         dword ptr [a],eax //寄存器赋值给a  a = eax = 6
01051D08  mov         ecx,dword ptr [a]  //将a存储到寄存器 ecx = a = 6
01051D0B  add         ecx,1  //加1 ecx = ecx + 1 = 6 + 1 = 7
	int b = ++a + 1;
01051D0E  mov         dword ptr [b],ecx  //将运算结果赋值给b
	std::cout << "b = " << b << std::endl; //输出结果

由汇编可以看到，++a首先进行加1处理，然后在赋值给a。

后++

	int c = 5;
01051D50  mov         dword ptr [c],5  //赋值给c c = 5
	int d = c++ + 1;
01051D57  mov         eax,dword ptr [c]  //将c存储到eax eax = c = 5
01051D5A  add         eax,1  //eax = eax + 1 = 5 + 1 = 6
01051D5D  mov         dword ptr [d],eax  //d = eax = 6
01051D60  mov         ecx,dword ptr [c]  //ecx = c = 5
01051D63  add         ecx,1  //ecx = c + 1
01051D66  mov         dword ptr [c],ecx  //c = ecx + 1 = 5 + 1 = 6
	std::cout << "c = " << c << std::endl;

由汇编可以看到，a++首先进行运算处理，将a加1后直接赋值给d，如此就将d的结果运算出来了。而后再对c进行加1的运算，最后赋值给c，完成后++运算。