Unexpected behaviour

Hi
The following code must give error but it does not! I use gcc 3.3.4 under Slackware Linux.

Code:

---

#include <iostream>

using namespace std;

int main()
{
if (int x = getchar())
cout << x;

x = 50; // must be an error

}

---

But the code below gives error message...

Code:

---


#include <iostream>

using namespace std;

int main()
{
if (int x = getchar())
cout << x;

cout << x;  // error
}

---

What is the reason of this?

Declare x before comparing it to something in an if statement. You can't declare a variable inside a conditional expression. Also, if you're trying to compare it, you need to use == instead of =. And shouldn't x be a char variable?

That's not the issue he was asking about. Perhaps assigning a variable a value is more sensitive than using it. Just don't do that, that's all.

Hi
Actually it is not the thing I am asking about Krak (as CodeMonkey said). Just try to cpoy and paste the codes, then compile them.In the first code, there is no error. Although, an error message is expected. In the second code, you will get the error message as expected.
In the first code, the variable "x" is assigned an integer value after the statemnt it is declared first. This shouldn't be happen!

With VC6, I get the same error and warning for both your programs, and both errors refer to the last line:

error C2065: 'x' : undeclared identifier
warning C4508: 'main' : function should return a value; 'void' return type assumed
Error executing cl.exe.

Quote:

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Originally Posted by fnoyan

Hi
The following code must give error but it does not! I use gcc 3.3.4 under Slackware Linux.

What is the reason of this?

---

There should be a compiler error in both cases. I ran your program through g++3.4.2 in my Linux box and got both errors.

I ran the program through g++ 3.3.3 (cygwin) on my Windows XP machine and got the same results as you did: no error for the first, but a properly reported error on the second. (Borland and Microsoft Windows compilers also properly reported errors for both cases.)

For g++ 3.3.3, assembly code indicated that the value outside the loop is stored in the location of the loop-local x. Apparently the compiler didn't mind.

(Of course, the example code does nothing, and when you add program statements that access the variable outside its scope, the compiler properly reports the error. However, if this is really a compiler bug, then it is cause for concern, since it may affect other things that you would actually use in a program.)

I vaguely remember that, once upon a time, there was a gcc bug report something like this: If a variable was declared extern and another variable with the name had local scope, then after the local variable went out of scope, attempts to access the extern variable caused segfault (in some cases). Perhaps when (if) the compiler writers fixed the one problem they introduced the bug that you have seen. (Then maybe they fixed both with gcc 3.4. Maybe you can upgrade gcc to 3.4.?)

Regards,

Dave