Declare value in c++ header

Question

Well, i'm a newbie to C++ from Python and not familiar with declaring variable in header file. I am trying to create several .cpp file in which calculate some values from some the same constant value set. My code something like:

/Global_input.h/
extern int y = 1;
extern int x = 2;

/Properties_1.cpp/
//I wanna calculate z = x + y
include<Global_input.h>
int z;

/Properties_2.cpp/
//I wanna calculate g = x*y
include<Global_input.h>
int g;

I got stuck here, the way I search is create a new class or another .cpp file. Can I directly call x,y for such simple cases. Thanks in advance

score 1 · Answer 1 · edited May 23 '17 at 12:17

1

Use static const variable for this purpose:

static const int x = 1;

The const keyword here ensures that your code will not change x at any point (you stated that its value is supposed to be constant). I recommend reading the following SO thread to get an idea what the purpose of the static keyword is:

Variable declarations in header files - static or not?

edited May 23 '17 at 12:17

Community

1
1

answered Feb 18 '17 at 02:55

Nejc

927
6
15

score 1 · Answer 2 · answered Feb 18 '17 at 05:14

1

In addition to creating Global_input.h also create a Global_input.cpp file as follows -

/Global_input.h/
extern int y;
extern int x;

/Global_input.cpp/
#include "Global_input.h"

int y = 1;
int x = 2;

extern just declare the variable not define it. You must define it somewhere else.

answered Feb 18 '17 at 05:14

army007

551
4
20

`extern int y = 1;` defines `y` because it has an initializer. – Pete Becker Feb 18 '17 at 12:48
I guess that is not standard behavior (or at least good practice). Coz both `clang` and `gcc` give warning about it when the header is included in a single translation unit. And if the header is included in multiple translation unit (which is the case here) it complains about multiple references of `x` and `y`. – army007 Feb 18 '17 at 13:47
That **is** standard behavior. The reason that compilers complain about multiple definitions when you include that header in more than one translation unit is that each translation unit gets a definition. – Pete Becker Feb 18 '17 at 15:16

score 0 · Answer 3 · answered Feb 18 '17 at 03:17

0

your my.h files should have this format:

//Put before anything else
#ifndef MY_H
#define MY_H

//code goes here

//at the end of your code
#endif

your my.cpp files should look like this:

//include header file you wish to use
#include "my.h"

answered Feb 18 '17 at 03:17

captainUnderpants

11
1

or use `#pragma once` and don't worry about things like typos and whatnot. – xaxxon Feb 18 '17 at 04:05

score 0 · Answer 4 · answered Mar 09 '17 at 19:28

You need to tell the compiler to only read once the header, e.g. like this:

/* Global_input.h */
#ifndef GLOBAL_INPUT_H
#define GLOBAL_INPUT_H
    static const int y = 1;
    static const int x = 2;
#endif

/* Properties_1.cpp */
//I wanna calculate z = x + y
#include<Global_input.h>
int z = x + y;

/* Properties_2.cpp */
//I wanna calculate g = x*y
#include<Global_input.h>
int g = x * y;

Declare value in c++ header

4 Answers4

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