变更常数 [英] Change constant value

问题描述

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#include<iostream>
using namespace std;

int main()
{
    const int a = 5;
    const int *ptr1 = &a;
    int *ptr = (int *)ptr1;
    *ptr = 10;
    cout<<ptr<<" = "<<*ptr<<endl;
    cout<<ptr1<<" = "<<*ptr1<<endl;
    cout<<&a<<" = "<<a;
    return 0;
}

输出:

0x7ffe13455fb4 = 10
0x7ffe13455fb4 = 10
0x7ffe13455fb4 = 5

这怎么可能?

推荐答案

这肯定是未定义的行为，但是我坚决支持理解未定义行为的症状以利于发现一个行为.观察到的结果可以用以下方式解释:

It is certainly undefined behavior, but I am strong proponent of understanding symptoms of undefined behavior for the benefit of spotting one. The results observed can be explained in following manner:

const int a = 5

定义的整数常量.现在，编译器假定在整个函数期间不会修改该值，因此在看到

defined integer constant. Compiler now assumes that value will never be modified for the duration of the whole function, so when it sees

cout<<&a<<" = "<<a;

它不会生成代码来重新加载a的当前值，而是仅使用初始化时使用的数字-比从内存中加载要快得多.

it doesn't generate the code to reload the current value of a, instead it just uses the number it was initialized with - it is much faster, than loading from memory.

这是一种非常常见的优化技术-当某个条件只有在程序表现出未定义的行为时才会发生，优化器会认为该条件永远不会发生.

This is a very common optimization technique - when a certain condition can only happen when the program exhibits undefined behavior, optimizers assume that condition never happens.

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