multiple return values in function

Question

I've faced the next one sample:

#include <stdio.h>

// test multiple return                                                                                                                                                             
int foo()
{
    return 1,2,3,4,5,6;
}

// main entry point                                                                                                                                                                 
int main(int argc, char* argv[])
{
    printf("foo returns: %d\n", foo());
    return 0;
}

compile it, then run:

gcc main.cpp -o main
./main

The results are confusing me:

foo returns: 6

The question is: why there is no compile time error?

Answer 1

In this context:

return 1,2,3,4,5,6;

is actually the comma operator. It evaluates everything between the commas in order (left-to-right), and returns the last one.

That's why it returns and prints 6. So yes, it's valid code. That's why there's no compiler error. (Although the 1,2,3,4,5 part doesn't do anything in this case.)

In C and C++, you can't return multiple values. You'd have to use a struct or a class to do that.

Answer 2

Because you are using the comma operator: the expression a,b where a and b are arbitrary (usually side-effecting) sub-expressions mean: evaluate the left-hand side a and discard its result (so a is only evaluated for side-effects), then evaluate b and give it as result.

You cannot return several things from a C function. You should return e.g. an aggregate (usually a struct) or a dynamically heap-allocated pointer.

As to the question, why the compiler don't say anything? Because you didn't ask it. You really should compile with gcc -Wall (for C code) or g++ -Wall (for C++ code), and then you get warnings:

 egor7.c: In function ‘foo’:
 egor7.c:6:13: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:15: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:17: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:19: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:21: warning: left-hand operand of comma expression has no effect [-Wunused-value]