C++ -- return x,y; What is the point?

Question

I have been programming in C and C++ for a few years and now I'm just now taking a college course in it and our book had a function like this for an example:

int foo(){   int x=0;   int y=20;   return x,y; //y is always returned } 

I have never seen such syntax. In fact, I have never seen the , operator used outside of parameter lists. If y is always returned though, then what is the point? Is there a case where a return statement would need to be created like this?

(Also, I tagged C as well because it applies to both, though my book specifically is C++)

Answer 1

According to the C FAQ:

Precisely stated, the meaning of the comma operator in the general expression

e1 , e2

is "evaluate the subexpression e1, then evaluate e2; the value of the expression is the value of e2." Therefore, e1 had better involve an assignment or an increment ++ or decrement -- or function call or some other kind of side effect, because otherwise it would calculate a value which would be discarded.

So I agree with you, there is no point other than to illustrate that this is valid syntax, if that.

If you wanted to return both values in C or C++ you could create a struct containing x and y members, and return the struct instead:

struct point {int x; int y;}; 

You can then define a type and helper function to allow you to easily return both values within the struct:

typedef struct point Point;  Point point(int xx, int yy){   Point p;   p.x = xx;   p.y = yy;   return p; } 

And then change your original code to use the helper function:

Point foo(){   int x=0;   int y=20;   return point(x,y); // x and y are both returned } 

And finally, you can try it out:

Point p = foo(); printf("%d, %d\n", p.x, p.y); 

This example compiles in both C and C++. Although, as Mark suggests below, in C++ you can define a constructor for the point structure which affords a more elegant solution.

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On a side note, the ability to return multiple values directly is wonderful in languages such as Python that support it:

def foo():   x = 0   y = 20   return x,y # Returns a tuple containing both x and y  >>> foo() (0, 20)