[C ++ in-depth analysis learning summary] 20 Use of initialization list

[C ++ in-depth analysis learning summary] 20 Use of initialization list

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Can const members be defined in a class?
can

Are the following class definitions legal?

illegal

Const members in the class

#include <stdio.h>
class Test
{
private:
    const int ci;
public:
    Test() : ci(10)
    {
        //ci = 10;    //这里编译会出错的
    }
    int getCI()
    {
        return ci;
    }
};
int main()
{
    Test t;
   
    printf("t.ci = %d\n", t.getCI());
   
    return 0;
}

1. Initialization of class members
C ++ provides an initialization list to initialize member variables
Syntax rules

Precautions

• The initialization order of members is the same as the declaration order of members (prone to bugs)
• The initialization order of members is independent of the position in the initialization list
• The initialization list is executed before the function body of the constructor

Use of initialization list

class Test
{
private:
    value m2;
  value m3;
  value m1;
public:
Test() : m1(1),m2(2),m(3)
  {

  }
};

输出： 2 3 1  和声明顺序相同

2. Const members in the class

• Const members in the class will be allocated space
• The essence of const members in a class is read-only variables
• Const members in the class can only specify initial values ​​in the initialization list
• The compiler cannot directly get the initial value of the const member, so it cannot enter the symbol table to become a constant in the true sense.

Initialization and assignment are different

• Initialization: Initial value setting for the object being created
• Assignment: set the value of an existing object

Summary
You can use the initialization list to initialize members in the class. The initialization list
is executed before the constructor body
. The const member variable can be defined in the class. The
const member variable must specify the initial value in the initialization list. The
const member variable is read-only.