Defining variables in a header file c++

Question

So I am really new to c++. I want to define a variable in a header file:

I have it like this in my .h file:

#pragma once
#define x = 0x1;

But when I do this in my .cpp file it doesn't work:

#include "x.h"

int main(){
std::cout << x << std::endl;
}

`#define` is a text replacement tool (so you got `std::cout << = 0x1 << std::endl;`). `constexpr static auto x = 0x01;` would define a variable. — Jarod42, Sep 23 '21 at 11:16
whatever resource you are using to learn you should stop using it. `#define x = 0x1;` is not defining a variable. Seems like you could use a [book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — 463035818_is_not_an_ai, Sep 23 '21 at 11:16
You can use `#define x 0x1` but that's a very bad idea. `#define` is used in special cases, for example different builds. At this level you should stay away from it. — Barmak Shemirani, Sep 23 '21 at 11:23
Why do you think that its being in a header file is part of the problem? — molbdnilo, Sep 23 '21 at 11:27
"doesn't work" needs elaboration. In what way? What is the error message? — L. Scott Johnson, Sep 23 '21 at 11:33
You've defined a preprocessor macro. You did not define a variable, which would look like: `int x = 0x01;`. In general, headers are used for **declarations**, not for **definitions**. In this case, your header would **declare** `extern int x;` and elsewhere in some source file you'd **define** `int x = 0x01;`. — Eljay, Sep 23 '21 at 11:59

score 0 · Answer 1 · edited Sep 24 '21 at 08:05

0

always defines variables that are global or const like

const int VARIABLE = 1;

then you can easily do

std::cout << VARIABLE  << std::endl;

edited Sep 24 '21 at 08:05

Jarod42

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answered Sep 23 '21 at 12:51

score 0 · Answer 2 · answered Sep 23 '21 at 15:52

They are not variables defined in the header file. they are macros.

Macros: Macros are a piece of code in a program that is given some name. Whenever this name is encountered by the compiler the compiler replaces the name with the actual piece of code. The ‘#define’ directive is used to define a macro. Lets understand the macro definition with the help of a program:

#include <iostream>

// macro definition
#define size 5
int main()
{
    for (int i = 0; i < size; i++) {
        std::cout << i << "\n";
    }

    return 0;
}

whenever you use size compiler replaces it with 5

#define x 0x1

Defining variables in a header file c++

2 Answers2