[C ++ deep analysis learning summary] 25 types of static member variables

[C ++ deep analysis learning summary] 25 types of static member variables

Author CodeAllen , please indicate the source

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1. Member Variables

• Can access public member variables by object name
• The member variables of each object are unique
• Member variables cannot be shared between objects

2. New demand

• Count the number of objects of a certain class during the running of the program
• Ensure the safety of the program (global variables cannot be used)
• The number of current objects can be obtained at any time

Solution attempt (failed solution)

#include <stdio.h>

class Test
{
private:
    int mCount;
public:
    Test() : mCount(0)
    {
        mCount++;
    }
    ~Test()
    {
        --mCount;
    }
    int getCount()
    {
        return mCount;
    }
};


Test gTest;


int main()
{
    Test t1;
    Test t2;
    
    printf("count = %d\n", gTest.getCount());
    printf("count = %d\n", t1.getCount());
    printf("count = %d\n", t2.getCount());
    
    return 0;
}

3. Static member variables-start to introduce new concepts
in C + + can define static member variables

• Static member variables belong to the entire class
• The lifetime of static member variables does not depend on any objects
• You can directly access public static member variables by class name
• Static member variables of all objects sharing classes
• Public static member variables can be accessed by object name

Characteristics of static member variables

• Modified directly by the static keyword at the time of definition
• Static member variables need to be allocated separately outside the class
• Static member variables are located in the global data area inside the program

Grammar rules:

• Type ClassName::VarName = value;

Use of static member variables

#include <stdio.h>

class Test
{
private:
    static int cCount;
public:
    Test()
    {
        cCount++;
    }
    ~Test()
    {
        --cCount;
    }
    int getCount()
    {
        return cCount;
    }
};


int Test::cCount = 0; //需要在类的外部单独定义，隶属于test


Test gTest;


int main()
{
    Test t1;
    Test t2;
    
    printf("count = %d\n", gTest.getCount());
    printf("count = %d\n", t1.getCount());
    printf("count = %d\n", t2.getCount());
    
    Test* pt = new Test();  //指向堆上的一个指针对象，动态生成，触发构造函数就 ++，输出为4
    
    printf("count = %d\n", pt->getCount());
    
    delete pt;
    
    printf("count = %d\n", gTest.getCount());
    
    return 0;
}

Summary
Static member variables can be defined through the static key in a class.
Static member variables belong to all classes.
Every object can access static member variables.
Static member variables are allocated in the global data area
. The life time of static member variables is the program runtime.