Global variable inside the class and local function variable

Question

I tried out a c++ program, where i declared a variable of type int named "a" inside a class. And created a function named "b" inside which again i declared a variable named "a" and assigned the value to it. The variable "a" inside the function is considered as local variable. And if i want to assign the value to the variable a which is present in the class definition(not inside the function), how can i do it?

#include <iostream>

using namespace std;

class a{
    public:int a;//need to assign value for this "a" inside the function how can i do it
    int b(){
        int a=5;
        a=7;
        cout<<a;
    }
};
int main() {
    a A;
    A.b();
}

put int & ra = a; before you decare a in the function. Then use ra to refre to the class scope a. — Richard Critten
– Richard Critten, Commented Feb 10, 2018 at 19:10
This is purely academic, right? You don't actually want a member variable and a member function local variable with the same name, do you? — Beta
– Beta, Commented Feb 10, 2018 at 19:12
Hang on. You can't have a data member a in a class named a, can you? — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Feb 10, 2018 at 19:12
@StoryTeller yes i tried this program. It works when the name of the class and data members are same — Muthu
– Muthu, Commented Feb 10, 2018 at 19:17
You sent me checking, and it does indeed seem valid. I learned something today. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Feb 10, 2018 at 19:21

oarcas · Accepted Answer · 2018-02-10 19:28:38Z

8

To access the class variable you can use this keyword . For more explanation and getting knowledge of 'this` keyword you can go here

#include <iostream>

using namespace std;

class a{
    public:int a;//need to assign value for this "a" inside the function how can i do it
    int b(){
        int a=5;
        a=7;
        this->a = 8; // set outer a =8
        cout<< "local variable a: " << a << endl;
        cout<< "class a object variable a: " << this->a << endl;
        return 0;
    }
};
int main() {
    a A;
    A.b();
    cout << "A's variable a: " << A.a << endl; //should print 8
    return 0;
}

edited Feb 10, 2018 at 19:28

oarcas

1591 silver badge8 bronze badges

answered Feb 10, 2018 at 19:11

Shiv Kumar

1,1141 gold badge9 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

oarcas · Accepted Answer · 2018-02-10 19:23:46Z

5

Use this->a.

this allows you to access the members and methods of an instance from within.

EDIT: this is an academical exercise, but a bad programming practice. Just use different names for classes, members and variables.

edited Feb 10, 2018 at 19:23

answered Feb 10, 2018 at 19:10

oarcas

1591 silver badge8 bronze badges

1 Comment

StoryTeller - Unslander Monica Over a year ago

... even if they are hidden by something in the member functions scope.

jfMR · Accepted Answer · 2018-02-10 19:11:28Z

2

Specifying the class-qualified name of a, i.e.: a::a, will do:

a::a=7;

answered Feb 10, 2018 at 19:11

jfMR

25.2k5 gold badges69 silver badges88 bronze badges

4 Comments

StoryTeller - Unslander Monica Over a year ago

That can't work. This will refer to the constructor.

StoryTeller - Unslander Monica Over a year ago

Well, this says it shouldn't. I'd check your compiler flags if I were you.

StoryTeller - Unslander Monica Over a year ago

Well, that's distressing

StoryTeller - Unslander Monica Over a year ago

Aha, so it is indeed correct. The injected class name is hidden. +1.

knoxgon · Accepted Answer · 2018-02-10 19:35:32Z

The problem is lying under the lack of this pointer to the class variable.

If you have a local variable inside your class function that is identical to the class variable, then you must use this keyword for the compiler to distinguish.

An easy mistake people tend to do is when having exactly the same names for the mutator's parameter variable as the class' variable.

#include <iostream>

using namespace std;

class Foo {
    int numberA;
public:
    void setNumberA(int numberA) {
        numberA = numberA; /*Incorrect way, the compiler thinks 
                             you are trying to modify the parameter*/

        this->numberA = numberA; //Correct way
    }
};

In your case, you must use this->a inside the function b().

Collectives™ on Stack Overflow

Global variable inside the class and local function variable

4 Answers 4

Comments

1 Comment

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related