What does it mean to declare a variable as a function?

Question

In the example below I do

MyClass a ();

I've been told that a is actually a function that returns a MyClass but neither of the following lines work.

MyClass b = a();
a.a = 1;

So what is a and what can I do with it?

#include "stdafx.h"
#include "iostream"
using namespace std;

class MyClass {
    public:
        int a;
};

int _tmain(int argc, _TCHAR* argv[])
{
    MyClass a ();
    // a is a function? what does that mean? what can i do with a?

    int exit; cin >> exit;
    return 0;
}

`MyClass b = a();` does work; it just won't link because you haven't provided a definition of `a`. — ecatmur, May 09 '13 at 18:12
possible duplicate of [Most vexing parse: why doesn't A a(()); work?](http://stackoverflow.com/questions/1424510/most-vexing-parse-why-doesnt-a-a-work) — SomeWittyUsername, May 09 '13 at 18:25

score 8 · Accepted Answer · edited May 09 '13 at 19:03

8

I've been told that a is actually a function that returns a MyClass [...]

That is a function declaration. It just declares a function called a and makes the compiler aware of its existence and its signature (in this case, the function takes no arguments and returns an object of type MyClass). This means you may provide the definition of that function later on:

#include "iostream"

using namespace std;

class MyClass {
    public:
        int a;
};

int _tmain()
{
    MyClass a (); // This *declares* a function called "a" that takes no
                  // arguments and returns an object of type MyClass...

    MyClass b = a(); // This is all right, as long as a definition for
                     // function a() is provided. Otherwise, the linker
                     // will issue an undefined reference error.

    int exit; cin >> exit;
    return 0;
}

MyClass a() // ...and here is the definition of that function
{
    return MyClass();
}

edited May 09 '13 at 19:03

A.H.

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answered May 09 '13 at 18:09

Andy Prowl

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Isn't that a default ctor in the context of the `main` scope? – RandyGaul May 09 '13 at 18:10
3

@RandyGaul: No, that would be `MyClass a;` or, in C++11, `MyClass a{}`. – Andy Prowl May 09 '13 at 18:11
This is mentioned in the [c++ FAQ](http://www.parashift.com/c++-faq/ctors.html), for example. – Corey May 09 '13 at 18:13
I think this can be confusing to c++ beginers because of MyClass's int a member. – Bogdan Chiţu May 09 '13 at 18:18
I understand now. Thanks. I didn't know you could declare the function inside of another function. And I was confused because in Javascript the Function/Object line is gray. – user1873073 May 09 '13 at 18:30

What does it mean to declare a variable as a function?

1 Answers1