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.