CODEWITHSUNDEEP
c++c
jmasterx
jmasterx

Reputation: 54123

Will function pointers always initialize to NULL?

I'm using MSVC and it seems like the code below does not crash and the function pointer is initialized to NULL by the compiler.

int (*operate)(int a, int b);
int add(int a, int b)
{
    return a + b;
}

int subtract(int a, int b)
{
    return a - b;
}

int main()
{


    if(operate) //would crash here if not NULL
    {
        cout << operate(5,5);
    }

    operate = add;
    if(operate)
    {
        cout << operate(5,5);
    }

    operate = subtract;
    if(operate)
    {
        cout << operate(5,5);
    }
    return 0;
}

So it seems MSVC initializes function pointers to NULL, but if I build this on gcc in Linux would it also be NULL? Is it conventional or MSVC specific, can I rely on it being NULL wherever I go?

Thanks

Upvotes: 5

Views: 2583

Answers (1)

j_random_hacker
j_random_hacker

Reputation: 51226

operate is initialised to NULL because it is a global variable, not because it is a function pointer. All objects with static storage duration (which includes global variables, file-level static variables and static variables in functions) are initialised to 0 or NULL if no initialiser is given.

[EDIT in response to Jim Buck's comment:] In C++, this is guaranteed by clause 3.6.2/1 of the language standard, which begins:

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization.

I expect the same behaviour is true of C, since C++ is designed to be compatible with it on most things, although I don't have the standard for it.

[EDIT #2] As Jeff M points out in a comment, it's important to realise that variables of automatic storage duration (that is, "ordinary" local variables) are not automatically zero-initialised: unless an initialiser is given, or they are assigned values by a constructor, they will initially contain random garbage (whatever was already sitting in memory at that location). So it's a good habit to initialise all variables -- it can't hurt but can help.

Upvotes: 18

Related Questions