How to make the variable m_i have its own value for each object, and when a certain function is called, the value of m_i for all objects should be set to zero, no matter how many objects of the class CMyClass were created?

#include <iostream>

using namespace std;
class CMyClass {
public:
   static int m_i;
};

int CMyClass::m_i = 0;
CMyClass myObject1;
CMyClass myObject2;

int main() {
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject1.m_i = 1; // set m_i to 1 for first object
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject2.m_i = 2; // set m_i to 2 for second object
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   CMyClass::m_i = 0; // set m_i to zero for all objects
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;
}

Output

0
0
1
1
2
2
0
0

Expected output should be:

Output

0
0
1
0
1
2
0
0

CodePudding user response：

Save each instance in a static list like so:

#include <iostream>
#include <vector>

using namespace std;
class CMyClass {
public:

    CMyClass(int i): m_i{i} {
        instances.push_back(this);
    }
    int m_i;

    static std::vector<CMyClass*> instances;

    static void reset() {
        for (auto &instance: instances) {
            instance->m_i = 0;
        }
    }
};

std::vector<CMyClass*> CMyClass::instances;

int main() {
    CMyClass myObject1{0};
    CMyClass myObject2{0};

   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject1.m_i = 1;
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

   myObject2.m_i = 2;
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;

    
   CMyClass::reset();
   cout << myObject1.m_i << endl;
   cout << myObject2.m_i << endl;
}

Possible microcontroller (heapless) implementation.

#include <iostream>
#include <array>

using namespace std;
class CMyClass {
public:

    // The member variable should not be static because
    // it is unique for each class instance.
    int m_i = 0;
};

// If you use microcontrollers that does not implement the standar library
// replace this with
// CMyClass instances[2];
std::array<CMyClass, 2> instances;

void resetInstances() {
    for (auto &instance: instances) {
        instance.m_i = 0;
    }
}

int main() {

   cout << instances.at(0).m_i << endl;
   cout << instances.at(1).m_i << endl;

   instances.at(0).m_i = 1;
   cout << instances.at(0).m_i << endl;
   cout << instances.at(1).m_i << endl;

   instances.at(1).m_i = 2;
   cout << instances.at(0).m_i << endl;
   cout << instances.at(1).m_i << endl;

   resetInstances();
   cout << instances.at(0).m_i << endl;
   cout << instances.at(1).m_i << endl;
}

Note: I do not recommend using the name convention with m_ for public class members. In this case i would call the member variable m_i for simply i and then use m_... for private variables that is only used in private functions of the class.

CodePudding user response：

How to make the variable m_i have its own value for each object, and when a certain function is called, the value of m_i for all objects should be set to zero, no matter how many objects of the class CMyClass were created

These are 2 requirements that cannot be both fulfilled trivially:

1. If int m_i is a static member, it will be "shared" between all instances (requirement 1 not fulfilled).
2. If int m_i is a non-static member, each object can have its own value. But a method cannot trivially set it for all instances (requirement 2 not fulfilled).

The solution:

Keep a static array of instances. Your ctor should add the object to this array, and the dtor should remove it. Thus each object can have its own value of m_i, but a specific method can iterate over the said array and set it for all instances.