How to change this->value from constructor with a function inside the same class in C++?

Question

I have a class student and constructor for name and age but latter on student must be able to edit that info. I tried following but edit info dont work:

class Student{
public:
    string name;
    int age;
    int grades[3];
    int assignments[4];

Student(string name, int age){
    this->name = name;
    this->age = age;
}

void edit_info(){
    string newName;
    int newAge;

    cout<<"Set new name:";
    cin>>newName;
    cout<<"Set new age:";
    cin>>newAge;

    this->name=newName;
    this->age=newAge;
    
}
void show_info(){
    cout<<name<<"\n";
    cout<<age<<"\n";
}

Code when I chose which student to "play":

if(input == "studentOne"){
        cout<<"You are now student 1\n";
        cout<<"What is your name:";
        cin >> s1_name;
        cout<<"How old are you:";
        cin >> s1_age;
        Student student1(s1_name, s1_age);
        while(true){
            CMD(student1);
        }

and CMD function:

void CMD(Student student){
string command;
cout<<"Type your command(edit_info, submit_assignments, sitFortest, show_info):";
cin>>command;

if(command=="edit_info"){
    student.edit_info();
}
else if(command=="show_info"){
    student.show_info();
}

}

Answer 1

To initialize member variables, give members a distinct naming scheme. (There are a few conventions around, I am used to prefixing with m_). And have a look at this : https://en.cppreference.com/w/cpp/language/constructor member initialization lists.

#include <vector>
#include <string>
#include <iostream>

class Student
{
public:
    Student(std::string name, unsigned int age) :
        m_name{ name }, 
        m_age{ age }
    {
        // don't initialize members in body use member initialization list.
        //this->name = name;
        //this->age = age;
    }

    void edit_info() 
    {
        // you can directly input into members.
        std::cout << "Set new name:";
        std::cin >> m_name;
        std::cout << "Set new age:";
        std::cin >> m_age;
    }

private: // class members should be private!
    std::string m_name;
    unsigned int m_age; // you don't execpt age to be < 0 so unsigned
    std::vector<int> grades;
    std::vector<int> assignments;
};

Answer 2

This code

Student student1(s1_name, s1_age);
while(true){
    CMD(student1);
}

void CMD(Student student){
    ...
}

creates a new Student object each time that CMD is called. The object student in the CMD function is not the same as the object student1 in the calling function. So changes to this object have no effect on the object in the calling function.

This is because by default C++ uses call by value, meaning that objects are copied when passed as parameters. If you want to avoid this happening then you must use references.

The change is very simple

void CMD(Student& student){ // & means use call by reference

The extra & turns student into a reference to the calling variable, instead of a separate object. Now changes to student will actually change the object in the calling function, instead of changing a copy of the object in the calling function.

This is a common misunderstanding among beginners. Some languages do use call by reference, but in C++ the default is call by value.