常量折叠

　 c++里的常量折叠（或者常量替换)是将const常量放在符号表中，而并不给其分配内存。编译器直接进行替换优化。


#include <iostream>
using namespace std;

int main(void)
{
  const int a=10;
  int * p = (int*)&a;//为了能尝试通过*p修改常量a的值,用了强制转换

  cout<<"a = "<< a<<endl; //输出 a = 10
  cout<<"*p= "<<*p<<endl; //输出 *p = 10
  *p=100; //试图修改a的值
  cout<<"a = "<< a<<endl; //仍然输出: a = 10
  cout<<"*p= "<<*p<<endl; //这里却输出: *p = 100

  system("pause");
}


以下是 cout<<"a = "<< a<<endl;
这一句在DEBUG时的汇编代码:
004114DB mov esi,esp 
004114DD mov eax,dword ptr [__imp_std::endl (41A344h)] 
004114E2 push eax  
004114E3 mov edi,esp 
004114E5 push 0Ah ;;;;;;;注意这里是个10,是个立即数
004114E7 push offset string "*p=" (417704h) 
004114EC mov ecx,dword ptr [__imp_std::cout (41A348h)] 
004114F2 push ecx  
004114F3 call std::operator<<<std::char_traits<char> > (411159h) 
004114F8 add esp,8 
004114FB mov ecx,eax 
004114FD call dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (41A34Ch)] 
00411503 cmp edi,esp 
00411505 call @ILT+410(__RTC_CheckEsp) (41119Fh) 
0041150A mov ecx,eax 
0041150C call dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (41A32Ch)] 
00411512 cmp esi,esp 
00411514 call @ILT+410(__RTC_CheckEsp) (41119Fh)