Why can I call a function on a Class that has not been instantiated?

Question

Can someone please explain why the below code is able to call the SayHello function and print out "Hello".

The constructor and destructor never get called as there is no object actually created, so why then can I call the SayHello function??

class A
{
public:
    A()
    {
        std::cout<<"In Constructor"<<std::endl;
    };
    ~A()
    {
        std::cout<<"In Destructor"<<std::endl;
    };

    void SayHello()
    {
        std::cout<<"Hello"<<std::endl;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{       
    A* a = nullptr;
    a->SayHello();

    return 0;
}

I'm certain this has been asked like this before. – crashmstr Mar 05 '15 at 12:47 — crashmstr, Mar 05 '15 at 12:47

score 3 · Answer 1 · answered Mar 05 '15 at 12:46

3

This is undefined behaviour. It happens to work because SayHello doesn't access any member data. Don't do this.

answered Mar 05 '15 at 12:46

TartanLlama

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Yes I know not to do it, I just want to know why it happens – Harry Boy Mar 05 '15 at 12:47
4

@HarryBoy: Because undefined behaviour is undefined. Anything can happen; anything includes the function working as it would if called properly. – Mike Seymour Mar 05 '15 at 12:48
when you say it's an undefined behaviour, could you quote where this knowledge come from ? – Guiroux Mar 05 '15 at 12:48
1

@Guiroux have a look at the question I marked this as a duplicate of, it has the necessary references. – TartanLlama Mar 05 '15 at 12:52
thank you ofr the precision – Guiroux Mar 05 '15 at 12:58

Why can I call a function on a Class that has not been instantiated?

1 Answers1