Declaration in if statement weird behaviour (C++)

Question

I have following code:

int X = 1;
if (int X = X)
{
    printf("%d\n", X);
    if (int X = X)
    {
       printf("%d\n", X);
    }
}

My expectation is X should be 1, 1. However, the output is:

1818935350
32767

Anyone know what's going on here? Compiled with clang-800.0.42.1

Edit: I tried the following code with a bit tweak, and now it behaves as I expected.

int X = 1;
if (int Y = X)
{
    printf("%d\n", Y);
    if (int Z = X)
    {
        printf("%d\n", Z);
    }
}

One guess is when you use variable on RHS of a declaration inside if statement, it might not refer to the variable with the same name declared at parent scope, instead it's referring to the variable that's being defined...

get rid of the 'int' before the X in the if statement. Also if you want to make a comparison you need to use `==` not `=`. `=` sets the value of X. However, it's tough seeing what you're trying to accomplish here. Maybe give some desired input and output — bpgeck, Nov 13 '16 at 04:44
@bpgeck That's not what I intend to do. I want to check the behaviour when you use declaration inside if statement. — A.Stone, Nov 13 '16 at 04:46
@Avi Berger Yeah I know, but believe or not. I got a problem that's kind of similar to this. I need some macro that will be expanded to the if statements, and I want them to automatically refer to the one with the same name in parent scope ... Seems that's not possible. — A.Stone, Nov 13 '16 at 05:08
See [Has C++ standard changed with respect to the use of indeterminate values and undefined behavior in C++14?](http://stackoverflow.com/q/23415661/1708801) for a more detailed explanation of the issue. — Shafik Yaghmour, Nov 13 '16 at 05:58

score 4 · Accepted Answer · answered Nov 13 '16 at 04:57

4

When you say int X = X, both X's refer to the same int. That is, the one you are declaring right there in that line. So you are initializing X with itself, which is undefined behavior, since it was (of course) not already initialized. The X that you initialized with 1 is never printed in your code, since it is declared in a parent scope and shadowed by the ones in the inner scopes.

answered Nov 13 '16 at 04:57

Benjamin Lindley

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I'm surprised the compiler allows this. – Dai Nov 13 '16 at 05:03
@benjamin Lindley. See my edit, it's same with your guess. – A.Stone Nov 13 '16 at 05:04
@A. Stone: It's not a guess. – Benjamin Lindley Nov 13 '16 at 05:04
@A.Stone - your edit is of no surprise. The real issue pointed out here is the scope of the variable. – artm Nov 13 '16 at 05:05
@BenjaminLindley. Sorry... – A.Stone Nov 13 '16 at 05:09
@A.Stone: No worries. Hope that statement didn't come off as arrogant. Just confident. – Benjamin Lindley Nov 13 '16 at 05:14
@BenjaminLindley :) – A.Stone Nov 13 '16 at 05:15
@A.Stone I just check doing `A a = a;` prevent call to default constructor instead copy constructor is called. – Logman Nov 13 '16 at 05:24

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

0

In C, this if (int X = X) is plain wrong because the if statement expects a control expression - but NOT a declaration.

6.8.4.1 The if statement

The controlling expression of an if statement shall have scalar type.

I'm not sure about clang, but gcc gives this code an obvious error (not a warning) even without any compiler flag (ie without -Wall -Wextra)

error: expected expression before ‘int’

edited Jun 20 '20 at 09:12

Community

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answered Nov 13 '16 at 04:48

artm

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Maybe it's C++ only, can you try clang++ or g++? – A.Stone Nov 13 '16 at 04:49
@A.Stone I tested with c++. No error is given but it doesn't enter the if statement – bpgeck Nov 13 '16 at 04:51
@bpgeck This is weird, since it does print out stuff on my machine. – A.Stone Nov 13 '16 at 04:52
If it's not entering the if block, it's probably because `X` is getting the value 0 in your tests, and so the expression evaluates to false. – Benjamin Lindley Nov 13 '16 at 05:04
@BenjaminLindley it actually failed to compile in `gcc`, but ok with `g++`. The reason was shadowing as you pointed out. – artm Nov 13 '16 at 05:06
@artm I checked that C doesn't allow you to define variables in if / for / while conditions. – A.Stone Nov 13 '16 at 05:11

score -1 · Answer 3 · answered Nov 13 '16 at 04:53

-1

Change all the ( Int X =X) to (X) It is creating new variables and possibly returning old data stored in that location.

answered Nov 13 '16 at 04:53

Monique Taylor

1

I'm not trying to do anything here, just to test it's behaviour. – A.Stone Nov 13 '16 at 04:55

Declaration in if statement weird behaviour (C++)

3 Answers3