Why is possible to call nonstatic member-function without an object instance?

Question

Possible Duplicate:
What will happen when I call a member function on a NULL object pointer?

Well I think this code and program output explain it self:

#include <iostream>
#include <string>
using namespace std;

class Test
{
public:
    void Not_Static(string args)
    {
        cout << args << endl;
    }
};

int main()
{
    Test* Not_An_instance = nullptr;
    Not_An_instance->Not_Static("Non-static function called with no object?");
    cin.ignore();
    return 0;
}

program output:

Non-static function called with no object?

why is that possible?

@FredLarson, though true, it doesn't really *explain* why it's possible. — Michael Krelin - hacker, Jan 23 '12 at 19:48
It's possible because it is not C++'s job to stop you from shooting yourself in the face. — Nicol Bolas, Jan 23 '12 at 20:32
@NicolBolas, I think the OP asked why there was no shooting. — Michael Krelin - hacker, Jan 24 '12 at 00:32
@MichaelKrelin-hacker: No *apparent* shooting. Undefined behavior means *undefined*. This includes the possibility of everything appearing to be fine. — Nicol Bolas, Jan 24 '12 at 00:34
@NicolBolas, yes, but unless I misunderstood the question the OP did know what's wrong with it, what he did not know is why it's worked. But well, know that the question is dead... :) — Michael Krelin - hacker, Jan 24 '12 at 01:08

score 9 · Accepted Answer · answered Jan 23 '12 at 19:46

Undefined behaviour. Your program invokes undefined behaviour by invoking a method on a null pointer, so everything is allowed, including your output.

Remember: C++ language's specification don't specify the output of every possible program to leave room for optimizations. Many things are not checked explicitly and can result in behaviour that seems incorrect or illogical, but is simply unspecified.

score 5 · Answer 2 · answered Jan 23 '12 at 19:46

5

This behavior is undefined - so it is quite possible it will print that output. The issue is undefined behavior can easily bite you, so you shouldn't do such a thing.

answered Jan 23 '12 at 19:46

JonH

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score 2 · Answer 3 · answered Jan 23 '12 at 19:45

2

Because it doesn't use this and therefore doesn't dereference null pointer. Make it virtual and it will likely fail.

answered Jan 23 '12 at 19:45

Michael Krelin - hacker

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Why is possible to call nonstatic member-function without an object instance?

3 Answers3