C++常量折叠

• const关键字修饰的变量被称为常量
• 在编译过程中，计算出常量表达式的值，并保存在常量表中，引用处替换成常量的值。以期优化编译速度。

#include
#include

using namespace std;

int main()
{
    const int a = 1;
    int * p = (int *)&a;

    *p = 2;

    cout << "P is:" << *p << "tA is :" << a << endl;
    cout << "P's address is:" << p << "tA's address is :" << &a << endl;


    int b = 1;
    int * q = &b;

    *q = 2;

    cout << "Q is:" << *q << "tB is :" << b << endl;
    cout << "Q's address is:" << q << "tB's address is :" << &b << endl;

    return 0;
}

• 输出：

P is:2  A is :1
P's address is:0x61feb4 A's address is :0x61feb4
Q is:2  B is :2
Q's address is:0x61feb0 B's address is :0x61feb0

• 从上面代码可以看出：
• p 和 a 指向同一块内存，但是值却不一样，原因就是 a 作为常量在编译过程中就被替换成了 1，p指向的地址虽然改变了，编译过程中已经变成了 1 这个常数。所以打印的 a 值就是1。