GCC warning [Wuninitialized]

Question

Why GCC 4.7 complains when instantiate a class inside a function (with a pointer)?

Bad:

#include "foo.h"

int fn () {
    Foo *foo;
    foo->method();

   return 0;
}

main.cpp: In member function 'int foo()': main.cpp:21:52: warning: 'fn' may be used uninitialized in this function [-Wuninitialized]

Good:

#include "foo.h"

Foo *foo;

int fn () {
    foo->method();

   return 0;
}

Good:

#include "foo.h"

int fn () {
    Foo foo;
    foo.method();

   return 0;
}

Answer 1

There's a difference between Foo * foo; and Foo foo; The first declares a pointer to a Foo, the second declares & invokes the default-constructor of a Foo.

EDIT: Maybe you meant to write Foo * foo= new Foo();, in order to allocate a Foo on the heap which can outlive the function call.

Answer 2

The first (bad) one, foo is pointing to a garbage pointer. You could remove the warning by initializing it like Foo* foo = NULL; but then you'd get an error when you tried to dereference it (runtime error).

The second (good) one doesn't complain because C automatically initializes variables in the translation unit scope to NULL or 0 or the appropriate equivalent if they're not already initialized.

The last (good) one doesn't complain because you are calling a method on the object and the assignment of the function pointer is done by the compiler, similar but not the same as number 2. So the compiler already knows the address of the method method and has assigned that address to the appropriate location on the Foo struct.