Class private member wont change in function

Question


using namespace std;

class map
{
    private:
        float result= 0;
    public:
        void func_1();
        void setres(float counter);
        float getres();
};
void map::setres(float counter)
{
    result= counter;
}
float map::getres()
{
    return result;
}
void map::func_1()
{
    float num_0=0, num_1=0, sum=0, i;
    
    map run;
    
    cout << run.getres() << " Result." << endl;
    if(result != 0)
        cout << "We already have a result saved and it's: " << run.getres() << endl;
    else
    {
        cout << "Give me first number: ", cin >> num_0;
        cout << "Give me second number: ", cin >> num_1;
        sum= num_0+num_1;
        cout << "Result is: " << sum << endl;
    }
    cout << "The program will save the result." << endl;
    run.setres(sum);
    cout << "The saved result is: " << run.getres() << "\nPress 1 to repeat the function and check\nif the result is saved." << endl;
    cin >> i;
    if(i==1)
        run.func_1();    
}

int main()
{
    map go;
    go.func_1();
    return 0;
}

I don't know why the private variable result is not saved. And how can i make it work.

Then i start compiling it works fine, private result is changing, but then i reopen the function, the result is back to 0 and i wanted it ro be the last result.

Example: I put 4 I put 7 Sum is 11 And saved result is 11 Then i press 1 to go to the start the result is 0 again, but i wanted it to be 11 not 0.

Vlad from Moscow · Accepted Answer · 2021-05-07T23:45:10.733

Within the function you are creating a local variable of the type map

map run;

the data member result of which is changed. That is the function does not change the data member result of the object for which the function is called.

Moreover for example in this code snippet

cout << run.getres() << " Result." << endl;
if(result != 0)

you are accessing the data member result of two different object. In the first statement

cout << run.getres() << " Result." << endl;

you are accessing the data member of the local object run while in the next statement

if(result != 0)

you are accessing the data member result of the object (the object go declared in main) for which the member function is called.

So remove the declaration in the function

map run;

and instead of expressions like for example run.getres() use either just getres() or this->getres().

score 0 · Answer 2 · answered May 07 '21 at 23:41

The issue is that your function is not using members of the object the method is called on. Instead you create a new instance inside the function:

void map::func_1()
{
    float num_0=0, num_1=0, sum=0, i;
    
    map run;  // <---------- here
    //...

Thats why every time you call the function you get a new fresh object. You do not need to create that instance. You already create one in main and inside the member functions you can access its members. As a fix you can remove all run. from the code. Eg

cout << run.getres() << " Result." << endl;

->

cout << getres() << " Result." << endl;

or if you prefer

cout << this->getres() << " Result." << endl;

score 0 · Answer 3 · answered May 07 '21 at 23:45

The value is saved to run, and run.func_1() is called for checking, but then run.getres() is called there. This run is new map object and differ from the run in which the data is saved.

You checked result != 0, but you used run.getres() to print the result. Inconsisitency here.

Instead of

    cout << run.getres() << " Result." << endl;
    if(result != 0)
        cout << "We already have a result saved and it's: " << run.getres() << endl;

You should do

    cout << result << " Result." << endl;
    if(result != 0)
        cout << "We already have a result saved and it's: " << result << endl;

or

    cout << getres() << " Result." << endl;
    if(getres() != 0)
        cout << "We already have a result saved and it's: " << getres() << endl;

Class private member wont change in function

3 Answers3