Why return values from a function by ref, address

**jim_0** · 02-05-2014

You can return values from functions by ref, address or value you can also do this with parameters, so what is the difference, if you have full return of a passed parameter by ref or address why would you need to ever return the function as a whole?

For ex

Code:

int nValue(int& y){
y++;
}

or int& nVlaue(int y){
return y;
}

**grumpy** · 02-05-2014

Your two examples are both flawed. The fact a compiler allows you to do something doesn't mean you should do it (although most compilers, if configured to picky warning levels, will issue warnings about the problems).

The first version falls off the end of the function, so the caller will exhibit undefined behaviour if it tries to use the return value.

The second function returns a reference to an argument passed by value. Since an argument passed by value ceases to exist when the function return, the caller will also exhibit undefined behaviour when it attempts to use the returned reference.

An example that would demonstrate the usage of references would be

Code:

int &biggest(int &x, int &y)
{
     return (x >= y ? x  : y);    //   this selects the biggest of x and y, and returns a reference to it
}

int main()
{
     int a = 5, b = 7;
     biggest(a,b) = 32;     //   b is larger, so this sets b to be 32.

     int p = 16, q = 2;
     biggest(p, q) = 32;    //   p is larger, so this statement sets p to 32
     return 0;
}

Pointers can be used in similar ways, albeit the syntax is different. There are also the same caveats - a function should not return the address of a local variable, nor should it return a pointer to a argument passed by value.

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