Analysis and explanation of common usage of const constants in C++

Let’s first explain const in C language and const in C. The variable modified by const in the former is not a real constant. It just tells the compiler that the variable cannot appear on the left side of the assignment symbol. The latter C evolves const based on C.

1. const in C language:

• const modified variables are read-only and are still variables in nature.

• constModified local variables allocate space on the stack

• constModified global variables Variables allocate space in the read-only storage area

• const is only useful during compilation and is invalid during runtime

• constCannot define a real constant

constThe modified variable is not a real constant, it just tells the compiler that the variable cannot appear in The left side of the assignment symbol. const Local variables allocate space on the stack, and the value in this space can be changed through a pointer. After the compilation period, the constant characteristics and read-only characteristics of const variables are gone. The read-only characteristics are only valid during the compilation period and have no effect at runtime. Global variables modified by const allocate space in the read-only storage area. Therefore, if a pointer is used to modify the global variable modified by const, the program will crash because the read-only storage of the program has been modified. content in the area, most programs will crash.

2. const in C:

C has evolved const on the basis of C, as shown in:

• const When declaring, put the constant in the symbol table

• During the compilation process, it is found that the constant is directly used in the symbol table Value replacement (constant folding)

• During the compilation process, storage space may also be allocated for the corresponding constant:

• const Used globally or using the static keyword description, stored in the read-only data area

  extern const int i = 10;
  static const int i = 10;
  
  // 或者修饰全局变量
  const int a =10;
  int main()
  {}

• to const in local variables The constant uses the & operator to allocate space in the stack area

Note: Although the C compiler may allocate const constants space, but the value in its storage space will not be used

The symbol table is a data structure generated during the compilation process

#include <iostream>
#include <string>
using namespace std;

const int i = 10;          // 如果通过指针去改变i，就会出现段错误：尝试修改只读数据区数据
int main()
{
    const int a = 5;
    int *p = (int *)&a;    // &a， 给a标识符分配空间了，并用p指向了该空间，
                           // 可以通过*p访问这个地址，但是不能通过a来访问
    *p = 10;               // 不能通过指针去改变a的值
    cout << a << endl;
    cout << *p << endl;
    
    return 0;
}

3. Comparison with macro definition

C The const constant in is similar to the macro definition

const int c = 5;
// 类似于
#define c 5

but the difference between cosnt and the macro definition is:

• constConstants are processed by the compiler

• The compiler performs type checking and scope checking on cosnt constants

• Macro definitions are processed by the preprocessor, and only simple text replacement is performed

#include <stdio.h>

void f()
{
    #define a 3
    const int b = 4;
}

void g()
{
    printf("a = %d\n", a);
    // 在g函数中访问f函数中的宏定义，完全没有问题
    // 在预处理的时候就进行了宏替换，对编译器来说，就是printf("a = %d\n", 3);
    // 宏是没有作用域的概念
    
    // const 定义的常量，被编译器处理，是有作用域的，不能访问b
    printf("b = %d\n", b);
}

int main()
{
    const int A = 1;
    const int B = 2;
    int array[A + B] = {0};    
    /* C编译
    const修饰得到的只是具有只读特性的变量，数组的大小是由两个变量的大小决定的，
    两个变量相加的结果需要在运行的时候才能直到，因此编译器编译的时候不知道这个数组长度，直接报错
    */
    
    /* C++编译
    const是定义的真正意义上的常量，直接从符号表中取值，编译的时候就知道A和B的值，
    可以得到数组的长度，不会报错
    */
    int i = 0;
    
    for(i=0; i<(A + B); i++)
    {
        printf("array[%d] = %d\n", i, array[i]);
    }
    
    f();
    g();
    
    return 0;
}

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