在 C++ 中遇到对象和类的问题

Question

So, I defined the method displayStudInfo in the 'Student' Class and called it in the main function. But I'm getting the error "Function not declared in this scope". Can anyone please tell me why this is happening and what I can do to solve this problem?

#include <iostream>
#include <string>
using namespace std;


class Student{

  public:
    int age;
    string name;


    void enterInfo(){

    cout << "Enter your age = " ; cin >> age;
    cout << "Enter your name = "; cin >> name;

    }


    void displayStudInfo(Student s)
    {
        cout << "Age = " << s.age << ", name=" << s.name << endl;
    }

};




int main(){

    int size;
    Student stud[100];
    Student abir;

    abir.enterInfo();
    displayStudInfo(abir);

}

Answer 1

void displayStudInfo(Student s) hidden in side class. So, its not accessible in main().

Try:

 void displayStudInfo()
    {
        cout << "Age = " << age << ", name=" << name << endl;
    }

call in main():

abir.displayStudInfo();

Answer 2

In your case void displayStudInfo(Student s) is a member function of Student so you have to call it on an instance of Student , the same way you did with enterInfo .

You can solve that in different ways. One way is to make that member function a free function by moving it out of the body of the Student

class Student{

  public:
    // … 
};

void displayStudInfo(Student s)
{
  cout << "Age = " << s.age << ", name=" << s.name << endl;
}

int main(){
    // … 

    displayStudInfo(abir);

}

displayStudInfo is, in fact, a good candidate for a free function. Or you make it static which is similar to a free function, and access the static member function using Student::displayStudInfo(abir) .

The other way would be to call displayStudInfo on abir in that case you don't need the Student argument, as abir is implicitly passed to displayStudInfo .

class Student{

  public:
    // … 

    void displayStudInfo()
    {
      cout << "Age = " << age << ", name=" << name << endl;
    }

};

int main(){
    // … 

    abir.displayStudInfo();
}

Answer 3

In C++, all member functions implicitly receive a parameter which points to the current object. This parameter is the this pointer .

Therefore, it doesn't make sense for you to specify an additional (explicit) parameter for the object in your definition of the function displayStudInfo .

It would make sense to rewrite the function definition to

void displayStudInfo()
{
    cout << "Age = " << age << ", name=" << name << endl;
}

and to call it with

abir.displayStudInfo();

instead of

displayStudInfo(abir);

Alternatively, you could make the function displayStudInfo a non-member function, by putting it outside the declaration of class Student. In that case, you would have to keep the explicit parameter, because that parameter is only passed implicitly to member functions.