Why wouldn't gcc compile this trivial code that calls free() function? [closed]

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I tried the following code on both codepad.org and ideone.com:

char* ptr = new char;
free( ptr );

Yes, I know it's undefined behavior, but I want to compile it and run it to see what happens.

Visual C++ 10 compiles it, but gcc which is used on the abovementioned sites says

error: expected constructor, destructor, or type conversion before ‘(’ token

Why wouldn't this code compile with gcc and how开发者_开发知识库 could I make it compile?

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Here is your code (from your link):

#include <stdlib.h>

char* ptr = new char;
free( ptr );

free() is a function, and at the namespace scope,you cannot write function-call-statement.

At namespace scope, you can only declare and define variables, and types. You can call a function in the initialization of a variable, however. So this is okay:

char* ptr = new char; //Note new is a function call!

But this is NOT okay:

free(ptr); 

If you want to do that, then here is one trick:

const int ignore = (free(ptr), 0); //at namespace scope!

This is okay. Demo : http://ideone.com/8yymM

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This code indeed will not compile. You have to add main() call also.

#include <stdlib.h>

int main()
{
    char* ptr = new char;
    free( ptr );
    return 0;
}

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You can only call a function without a visible declaration in C. In C++ it is an error that must be diagnosed at compile time, it's not undefined behavior.

To call free you must include the stdlib.h header.

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The proper code should be this one:

#include <stdlib.h>

int main()
{
   char* ptr = new char;
   free( ptr );
}

The error depends on the fact that free(ptr) at global scope is not seen as a function call, but as a declaration of an object of class free :-o .

By the way, consider also that the opposite of ptr = new char is not free(ptr), but delete ptr. (free is the opposite of malloc. new calls malloc, but does a bit more. As delete calls free but does something more).

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Did you include <stdlib.h>? It could be that the compiler doesn't recognize free

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Because it's c++, so you need to use g++ instead of gcc.