在类中创建静态变量（C ++）

Question

So I'm a noob with programming, and I am unsure why I am unable to make a static variable in my class? I got a question from class and I'm not sure if I'm going about it the right way. The question is: Create a class with a static member item so that whenever a new object is created, the total number of objects of the class can be reported.

This is my code so far:

#include <iostream>

class ObjectCount
{
public:
    ObjectCount();
    void reportObjectNo();

private:
    static int objectNo = 0;

};


ObjectCount::ObjectCount()
{
    objectNo++;
}

void ObjectCount::reportObjectNo()
{
    std::cout << "Number of object created for class ObjectCount: " << objectNo << std::endl;
}

int main()
{
    ObjectCount firstObject;
    firstObject.reportObjectNo();

    ObjectCount secondObject;
    secondObject.reportObjectNo();

    ObjectCount thirdObject;
    thirdObject.reportObjectNo();
    return 0;
}

And the error I get back is:

ISO C++ forbids in-class initialization of non-const static member 'objectNo'
line 9

I sincerely apologize if this has already been asked, but I couldn't find anything that helped me, if there is a link would be appreciated :)

Answer 1

The error message is telling you that you cannot initialize a non- const static member from inside a class. This would mean that you would need to change the code to look something more like:

class ObjectCount
{
public:
    ObjectCount();
    void reportObjectNo();

private:
    static int objectNo;

};

int ObjectCount::objectNo = 0;

Answer 2

C++ lets you declare and define in your class body only static const integral types.

class Foo
{
    static const int xyz = 1;

};

non-const static member variables must be declared in the class and then defined outside of it.you define it in implementation file ie .cpp

int ObjectCount::objectNo = 0;

Also, the proper way to use it would be

ObjectCount::objectNo++;

since, objectNo is associated with class and not with any object.