Side-effect not working

Question

I have created a buffer class to use with networking, and I use a side-effect to get the buffer pointer, as well as the size. I have created a simple test that displays the same behaviour as the class's getptr() function.

char SomeBuffer[100];

void* getbuf(int& size) {
  size = 100;    
  return SomeBuffer;
}

int testrecv(void* ptr, int size) {
 int i = 0;//BREAKPOINT HERE
 return 0;
}

int main(int argc, char**argv) {
 int size;
 testrecv(getbuf(size), size);
}

when I view the variables from within the testrecv() function, size is some random value that was left on the stack. Shouldn't the size in testrecv() be 100, due to the side-effect in getbuf()?

http://stackoverflow.com/questions/621542/compilers-and-argument-order-of-evaluation-in-c — Tilman Vogel, Dec 29 '12 at 00:05

score 4 · Answer 1 · answered Dec 29 '12 at 00:02

The order of evaluation of function arguments is implementation-defined. That means that you cannot rely on getbuf being called before the size argument is passed to testrecv.

What its happening here for your particular compiler, is that arguments to testrecv are evaluated from last to first. size is evaluated first, and at that time it has an unspecified value (random value). Only then getbuf is evaluated, modifying your size variable to the value you expect, but its too late for the function argument.

score 2 · Answer 2 · answered Dec 29 '12 at 00:04

The order of evaluation of function arguments is unspecified. It seem, the system you are using first evaluates size followed by getbuf(size). As a result, the argument doesn't have the expected value. The easiest fix is probably to return both pointer and the size:

std::pair<void*, int> getbuf() { return std::make_pair(someBuffer, 100); }
int testrcv(std::pair<void*, int> buffer) { ... }

(or you could use a std::vector<T> of a suitable type...)

score 2 · Accepted Answer · edited May 23 '17 at 12:27

The problem is that you are assuming an oder of evaluation:

testrecv(getbuf(size), size);

// What seems to be happening is
1) size is evaluated and set up for the function call.
2) getbuf() is called. This sets the local copy of size
   but the value that is being passed to the function call has already been
   evaluated. So this value will not be passed (but the random value that
   was in the variable at the time the `size` parameter was evaluated).
3) Function testrecv() called.

Don't rely on side affects.

 int size;
 void* buf = getbuf(size);  // Now size will be set
 testrecv(buf, size);

see: https://stackoverflow.com/a/367663/14065

I'm accepting Loki's answer because the explanation is a little more clear. Thank you all who answered — erai, Dec 29 '12 at 00:27

Side-effect not working

3 Answers3