Question

Suppose we have the following code:

#include <iostream>

class Person{
public:
    Person(int age);
    int get_a();
private:
    int a;
};

Person::Person(int age)
{
    a = age;
}

int Person::get_a()
{
    return a;
}

void Show_Age(Person P)
{
    std::cout<<P.get_a()<<std::endl;
}

int main() {

    Person P(10);
    Show_Age(P);
    return 0;
}

Now suppose we have a heavy object, we should pass Person by reference, so we proceed:

void Show_Age(Person &P)
{
    std::cout<<P.get_a()<<std::endl;
}

There isn’t a problem, but a good observation is P should be const, we try with it:

void Show_Age(const Person &P)
{
    std::cout<<P.get_a()<<std::endl;
}

A compiler failure:

error: passing ‘const Person’ as ‘this’ argument of ‘int Person::get_a()’ discards qualifiers [-fpermissive]

How to solve it?

Answer 1

You should mark get_a const in order for this to compile:

class Person{
public:
    Person(int age);
    int get_a() const;
private:
    int a;
};

int Person::get_a() const
{
    return a;
}

Doing so tells the compiler that the member function does not modify the state of the object, making it compatible with const pointers and references.