Declaring a local variable within class scope with same name as a class attribute

Question

While observing another person's code, I realized that within class A's method he declared a local int with the same name as a variable of class A. For example:

classA.h:

class A{
    int Data;

    void MethodA();
};

classA.cpp:

#include "classA.h"

using namespace std;

void A::MethodA(){
    int Data; //local variable has same name as class attribute
    
    Data = 4;

    //Rest of Code
}

I found it weird that the compiler would accept this without returning an error. In the above case, would the 4 be assigned to the local Data or A::Data, and what problems could this cause in more complex situations?

Answer 1

The local variable will shadow the member one (it has the more narrow scope). If you just write

Data = 4;

you will assign to the local variable Data. You can still access the member variable with

this->Data = 4;

This works basically just as

{
int data = 4;
    {
    int data = 2;
    data++; // affects only the inner one
    }
}

As for problems in the future: As long as you and everyone who will ever work with your code understands the rules and is aware that you did this on purpose there is no problem. If you do not intend to do such things on purpose, make your compiler warn about it.

However, it would certainly be saver if you followed a naming scheme for member variables, e.g. append an underscore like

class A{
    int Data_;

    void MethodA();
};