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 <iostream>
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 <iostream>
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 <stdio.h>
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。
