Creating C++ class instances globally within my main file

Question

I have a program with some classes for OpenGL stuff e.g. Window, Program etc.

I want the instances of the Window and Program classes which I use in my main file to be global for the main file, I'd been doing this for a while with Window like this

//Outside main()
Window window
(
    "test",    //title
    1366,      //width
    768,       //height
);

Then I can use this instance in the rest of the main file e.g.

//Within main()

window.create();

window.refresh();

etc.

Now when I try to make a global instance of Program I do it like this:

//Just below Window, still outside main()
Program program();    //No arguments for the constructor for Program

Then when I try to use it in my main function like this:

program.addShader(GL_VERTEX_SHADER, "vertex_shader_source.glsl");

I get the error:

request for member 'addShader' in 'program', which is of non-class type 'Program()'

Having looked into this it looks like the compiler thinks that my initialisation of the instance before main() is a function instead so it thinks I'm trying to call methods on a function which obviously doesn't work.

I just can't work out how to make this work in the way that it does for Window, the key differences that I can tell which might mean Window works and Program doesn't are:

The constructor for Window has arguments and Program doesn't.
I have other instances of the Program class in main() other than this global one, of course they have different name but still it might cause some problem?

Duplicates: [why-is-there-no-call-to-the-constructor](http://stackoverflow.com/questions/3810570/why-is-there-no-call-to-the-constructor) — Nicolas, Aug 01 '14 at 09:48
Related: [Default constructor with empty brackets](http://stackoverflow.com/q/180172/873025) — Felix Glas, Aug 01 '14 at 09:53

score 2 · Answer 1 · answered Aug 01 '14 at 09:47

You've discovered what's called the most vexing parse.

Indeed, when you do that:

Program program();

it is ambiguous, as you could be doing two things:

Creating a variable program which ctor takes no argument
Declaring a function called program which returns a Program and takes no argument

And the C++ standard imposes that a compiler choose the second possibility when it encounters this code.

To avoid this problem, just remove the parens

//Declares and defines a new program variable by invoking the default ctor
Program program;

Wojtek Surowka · Accepted Answer · 2014-08-01T10:13:22.957

0

Use

Program program;

instead of

Program program();

You do not need to use parens for constructor without parameters. Actually the second construct is interpreted as function declaration (function without parameters returning Program).

edited Aug 01 '14 at 10:13

answered Aug 01 '14 at 09:43

Wojtek Surowka

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The compiler would interpret it as a function declaration (just a precision as this is the accepted answer). – teh internets is made of catz Aug 01 '14 at 10:11

score 0 · Answer 3 · answered Aug 01 '14 at 09:45

Don't write it like this:

Program program();

because the parser thinks you just declared a function called "program" with no arguments, which return Program instance.

This is the correct way to define an object using its default constructor:

Program program;

Creating C++ class instances globally within my main file

3 Answers3