Thread: Is it possible to change the value of a variable in main using a function?

A simple approach is to use a reference parameter:

Code:

void getValue(int& value)
{
    value = 4;
}

but for something so simple, this function may or may not be overkill.

More likely, you would create a class to represent a player, and then an object of this class would store the particular player's hand value. In this way, you can create a container of such player objects.

You either return a value:

Code:

include <iostream>
 
using namespace std;
 
int getValue()
{
    return 4;
}
 
 
int main()
{
    int value;
 
 
    value = getValue();
    cout <<"the value is: " <<value;
}

Or you pass the argument into the function by reference:

Code:

include <iostream>
 
using namespace std;
 
void getValue(int& value)
{
    value = 4;
}
 
 
int main()
{
    int value;
 
 
    getValue(value);
    cout <<"the value is: " <<value;
}

Or as a pointer:

Code:

include <iostream>
 
using namespace std;
 
void getValue(int * value)
{
    *value = 4;
}
 
 
int main()
{
    int value;
 
 
    getValue(&value);
    cout <<"the value is: " <<value;
}

The reference route seems like the least amount of modification to your code.

Parameters are passed by value in C/C++. What you give to your getValue function is a copy of the value of what was passed in. Within the getValue function itself, changes made to this copy do not affect the content of the original variable you passed in by in main (having them named the same thing doesn't change this aspect BTW). When the getValue function exits and control returns to main, the variable has the same value as it did before the call to getValue.

When you pass a pointer, even though what you are passing in is still a copy, it is now a copy of an address, a location that does exist and represents the place in memory of the variable back in the main function. This copy of the address can therefore be used to affect a change to the variable back within the main function.

Originally Posted by SandDune

Yeah ofcourse! You can use pointers.
Pass the address of the variable as a parameter and then just assign the value of the address to whatever.

No, use references. This is not C...
For this, references are more than enough. Pointers are more complicated and left for those instance where simple isn't enough.
Pointers open up a can of worms, so let's not go there.

Originally Posted by Grae

i'm reading a book on C++ and have not yet learnt about pointers or references yet so I think i'll just use a few global variables for now and fix it up later

No, don't do that. If you have not learnt about pointers or references, then follow hk_mp5kpdw's first example. It is critical that you do not develop bad habits at this early stage because bad habits don't matter now, but by the time they do, you would have to work so much harder to get rid of them.

If you not understand references yet, then you are not ready for your first program, I would say.