Is this short program legal C++?

Question

while solving a test on http://cppquiz.org I found this interesting piece of code :

#include <iostream>

int f(int& a, int& b) {
    a = 3;
    b = 4;
    return a + b;
}

int main () {
    int a = 1;
    int b = 2;
    int c = f(a, a);// note a,a
    std::cout << a << b << c;
}

My question is this program legal C++ or it isnt? Im concerned about strict aliasing.

Have you tried running it? (I'm really not sure what you think is illegal- the fact that you pass the same argument twice to a function?) — David Robinson, Apr 18 '13 at 15:38
My guess is that it's legal (most things are), but I couldn't tell you what it will output, the compiler could rearrange those statements. — Salgar, Apr 18 '13 at 15:39
The output is pretty easy to predict and shouldn't be subject to the whims of the compiler. — tadman, Apr 18 '13 at 15:40
@user2280716: In the function? No, that's undefined behavior, when `a` and `b` reference the same variable. — Benjamin Lindley, Apr 18 '13 at 15:41
Please see [What is the strict aliasing rule?](http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule) — gongzhitaao, Apr 18 '13 at 15:42
@BenjaminLindley yep that's what I thought. But this is interesting. — stardust, Apr 18 '13 at 15:42
@gongzhitaao, that's regarding C and there you have a keyword for it (`restrict`). How does that apply to references in C++? — Shahbaz, Apr 18 '13 at 15:44

score 7 · Accepted Answer · edited May 23 '17 at 12:12

7

You mention strict aliasing – but strict aliasing is concerned with aliases of different types. It doesn’t apply here.

There’s no rule that forbids this code. It’s the moral equivalent of the following code:

int x = 42;
int& y = x;
int& z = x;

Or, more relevantly, it’s equivalent to having several child nodes refer to the same parent node in a tree data structure.

edited May 23 '17 at 12:12

Community

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answered Apr 18 '13 at 15:43

Konrad Rudolph

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nice example with a tree. :) – NoSenseEtAl Apr 19 '13 at 08:22

score 1 · Answer 2 · answered Apr 18 '13 at 15:42

1

Yes, it is legal.

I could formally prove it only by quoting the majority of the C++ standard text.

You are passing two references, both of which happen to refer to the same object, which is perfectly fine. You then assign new values to that single object, in turn. Also fine.

answered Apr 18 '13 at 15:42

Lightness Races in Orbit

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No, there *is* aliasing involved. A reference (actually, a pointer) is an alias. – Konrad Rudolph Apr 18 '13 at 15:44
@KonradRudolph: You lost me at "a reference (actually, a pointer)". There are no pointers here. – Lightness Races in Orbit Apr 18 '13 at 15:45
@KonradRudolph: Further, I cannot find anything in the standard that refers to either references or pointers as an "alias". Please assist me in my search. – Lightness Races in Orbit Apr 18 '13 at 15:47
Aliases concern a different level of abstraction than pointers and references: on the syntax level aliases don’t exist but they do underly the semantics – and they matter on the same level where the compiler has to decide whether to implement a reference as a pointer or whether it can reuse the same variable. – Konrad Rudolph Apr 18 '13 at 15:47
So, basically, you just made it up... – Lightness Races in Orbit Apr 18 '13 at 15:48
Nonsense. Let’s discuss this in the C++ lounge. – Konrad Rudolph Apr 18 '13 at 15:48

Is this short program legal C++?

2 Answers2