Code Blocks incorrect c++ output

Question

I have the following code:

#include <iostream>

using namespace std;

Sum (int a, int b)
{
    int x = a - b;
    //cout << x << " \n";
    return x;
}

int main()
{
    int s1 = Sum(3, 6);
    cout << s1;

    return 0;
}

System info: Win 7 Sp1 x64 Ultimate/Professional or Win 8.1 x64 Code Blocks 16.01 MinGW Debugger name and version: GNU gdb (GDB) 7.6.1 compiler: GNU GCC Compiler

This code compiles with no problems, but this IS the problem, there should be errors.

1) Function Sum, has no return value, on http://cpp.sh/ it doesn't let me compile because of this.

2) Variable's s1 value is -3 whether I write "return x" or not.

It somehow passes the value of x everytime BUT if I uncomment the cout statement above the "return x" everything starts to work out as expected, what the hell :) --> s1 will have a random value when no return statement is in place (because it was not initialized prior to being used for the function call) and -3 when the return is there.

I've tried this on 3 separate computers and they all exhibit the same behaviour. So I don't think the machine is the problem. I also tried using a different compiler but I don't know if I configured them correctly and they don't have a debugger right ? I tried Borland c++ and Digital Mars. Borland has a new version 10.1 instead of the 5.5 that codeblocks supports and I couldn't make the new one work. I don't even know if this is a compiler or program issue ?

I'm trying to learn C++ and this is very annoying. Our teacher is using the same software in class but on Linux and it works perfectly.

Off topic: Is there a way to insert code with line numbers here ? First post here so i'm still new at this :).

Thank you !

Which version of GCC are you using? Because like you said that should not compile with any version of any C++ compiler. — Some programmer dude
– Some programmer dude, Commented Dec 17, 2016 at 12:51
What is THE question? Why an incorrect code behaves undefinedly? — Fureeish
– Fureeish, Commented Dec 17, 2016 at 12:51
To fill things in a bit: C, back in the olden days, had "implicit int": function declarations with no return type indicated that the function returned int. That was removed for C99, but many compilers continue to support it so the old code doesn't break. — Pete Becker
– Pete Becker, Commented Dec 17, 2016 at 13:15

Rama · Accepted Answer · 2016-12-17 14:53:34Z

1

Go to "Project" -> "Build Options" -> "Compiler Settings" tab -> "Compiler Flags"

And disable -fpermissive

-fpermissive Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.

Or disable it using pragma on the top of your code:

#pragma GCC diagnostic ignored "-fpermissive"

Also you could try to add the flag "-pedantic" in "Compiler Flags" tab

BTW:

If you try online:

#pragma GCC diagnostic error "-fpermissive"

using namespace std;

Sum (int a, int b)
{
    int x = a - b;
    //cout << x << " \n";
    return x;
}

int main()
{
    int s1 = Sum(3, 6);
    cout << s1;

    return 0;
}

You got exactly same behavior you described!

edited Dec 17, 2016 at 14:53

answered Dec 17, 2016 at 13:11

Rama

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3 Comments

Mihai Nicolau Over a year ago

I don't think -fpermissive is enabled on my machine. I can't find it in the list of options. postimg.org/image/qavbl3a6n here is a screenshot

Rama Over a year ago

Maybe in "Other Compiler options"?

Rama Over a year ago

[EDITED] added #pragma

pharask · Accepted Answer · 2018-01-28 06:53:06Z

0

As Rama said, you might have enabled -fpermissive in your codeblock. Go to "Project" -> "Build Options" -> "Compiler Settings" tab -> "Other options" and delete -fpermissive.

answered Jan 28, 2018 at 6:53

pharask

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