gcc compiler reporting size of class=zero

Question

I have the small program

#include<iostream>
using namespace std;

class xyz{
   private: int xyz[];  // Why this line is not giving error.
}; 

int main(int argc, char *argv[])
{
    cout<<sizeof(xyz); //Q=Why this code is not giving error.
    return 0;
}

I am using gcc 4.3. Please tell me why am I wrong?

That does seem odd - later versions of gcc also seem to give the same result (0), and no warnings when compiled with `g++ -Wall ...`. `clang` gives a warning for the unused private field, but that's all. — Paul R, Sep 02 '13 at 07:02
GCC 4.3 is very old, and less C++ standard conforming than newer version. Newer GCC gives also better diagnostics and optimizations. Try updating your GCC to GCC 4.8.1 (or at least 4.7.3) and use `gcc -Wall -g` — Basile Starynkevitch, Sep 02 '13 at 07:04
@PaulR I tried with checking "g++ -Wall -o check check.cpp" but didn't get any warnings as such: — Santosh Sahu, Sep 02 '13 at 07:04
So is there no way to get warnings in the compiler i am using? — Santosh Sahu, Sep 02 '13 at 07:08
I've now tried various versions of gcc, clang and Intel's ICC and they all give the same result - no warnings, and sizeof(xyz) = 0. — Paul R, Sep 02 '13 at 07:08

score 3 · Answer 1 · edited May 23 '17 at 12:20

3

What you are looking at is a g++ compiler extension. You can trigger the warning

ISO C++ forbids zero-size array 'xyz'

if you compile with the -Wpedantic flag, and you can stop it from compiling using the -pedantic-errors flag. The reason your output shows 0 is that g++ will translate that into (the also not standard conforming) int[0]. Also see this answer for more information.

edited May 23 '17 at 12:20

Community

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answered Sep 02 '13 at 07:07

nikolas

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It seems to be more widespread than just gcc. – Paul R Sep 02 '13 at 07:09
@PaulR That's true, but this question is gcc specific; "this is a g++ compiler extension" does not say other compilers don't have it as well ;) – nikolas Sep 02 '13 at 07:10
-pedantic extension makes my compiler generate warining for the "ISO C++ forbids zero-size array âaâ". – Santosh Sahu Sep 02 '13 at 07:13

gcc compiler reporting size of class=zero

1 Answers1