const关键字的实验

今天做了一个实现，想看看const变量的值能不能被修改：
程序代码：

#include 

int main()
{
        int arr[10];
        int b = 10;

        arr[10] = 20;
        std::cout << "b = " << b << std::endl;

        return 0;
}

然而编译正确，运行时程序崩溃，这是编译和运行结果：

➜  AssemblyLearnC++ make hello
g++   -Werror -g  hello.cc   -o hello
➜  AssemblyLearnC++ ./hello 
arr = 0x7fffc2481dd0
&b = 0x7fffc2481dcc
b = 10
*** stack smashing detected ***: terminated
[1]    6354 abort (core dumped)  ./hello

查阅资料发现，数组arr和常量b在内存中并不是顺序存放的，报错也很明显，数组越界。并且通过运行结果也可以看出常量b的地址在arr数组之前。

然后我将数组arr和常量b放到了结构体中：
程序代码：

#include 

struct test_st
{
        int arr[10];
        int b = 10;
};

int main()
{
        test_st test;
        std::cout << "arr = " << test.arr << std::endl;
        std::cout << "&b = " << &test.b << std::endl;
        test.arr[10] = 20;
        std::cout << "b = " << test.b << std::endl;

        return 0;
}

编译和运行结果：

➜  AssemblyLearnC++ make hello    
g++   -Werror -g  hello.cc   -o hello
➜  AssemblyLearnC++ ./hello 
arr = 0x7ffffb9597a0
&b = 0x7ffffb9597c8
b = 20

可以看到运行结果是正确的，成功地修改了常量b的值。

一种假设：const和汇编没有关系，const只是在编码的层面限制了程序员的行为，到了汇编层面，const的限制已经荡然无存了。

然后我在网上读到了这篇文章：
从汇编角度看c++的const实现

我通过作者的方式运行了如下代码：

#include 

int main(void)
{
        const int a = 1;
        int *p = (int *)&a;
        printf("&a = %p\n", &a);
        printf("p = %p\n", p);

        *p = 3;
        printf("p = %p\n", p);
        int c = *p;
        int d = a;
        int e = a;

        printf("the value of c is: %d\n", c);
        printf("the value of d is: %d\n", d);
        printf("the value of e is: %d\n", e);
        printf("the value of a is: %d\n", *(&a));

        return 0;
}

发现运行结果和编译器有关：

➜  AssemblyLearnC++ cc  main.c -o main   #cc是我配置的clang编译器
➜  AssemblyLearnC++ ./main 
&a = 0x7fffd179aa68
p = 0x7fffd179aa68
p = 0x7fffd179aa68
the value of c is: 3
the value of d is: 1
the value of e is: 1
the value of a is: 3
➜  AssemblyLearnC++ gcc main.c -o main
➜  AssemblyLearnC++ ./main 
&a = 0x7fffd73b44e0
p = 0x7fffd73b44e0
p = 0x7fffd73b44e0
the value of c is: 3
the value of d is: 3
the value of e is: 3
the value of a is: 3

然后我将常量a加上了volatile关键字，强制让编译器从内存中取出a的值，两个编译器的输出结果就相同了:

➜  AssemblyLearnC++ cc main.c -o main
➜  AssemblyLearnC++ ./main 
&a = 0x7ffff5ad2a08
p = 0x7ffff5ad2a08
p = 0x7ffff5ad2a08
the value of c is: 3
the value of d is: 3
the value of e is: 3
the value of a is: 3
➜  AssemblyLearnC++ gcc main.c -o main
➜  AssemblyLearnC++ ./main 
&a = 0x7ffff81d99e0
p = 0x7ffff81d99e0
p = 0x7ffff81d99e0
the value of c is: 3
the value of d is: 3
the value of e is: 3
the value of a is: 3

综上所述：运行结果与编译器有关。擅自修改const变量的值，这属于未定义行为。