Data Sharing [C++] [02]: Between Files

Codes in this post can be found in folder DataSharingBetweenFiles.

extern

To share data between different files within a project, the most common way is to apply the extern declaration to global variables.

The best pratctice is to define the global variables in source files and then use a header file to declare those global variables by extern. This can be demonstrated by IncDec.h, IncDec.cpp and main file DataSharingBetweenFiles.cpp below.

// IncDec.h
#pragma once

extern int globalVariable;

void function1();
void function2();

// IncDec.cpp
#include "IncDec.h"

int globalVariable = 0;

void function1() {
    globalVariable += 1;
}

void function2() {
    globalVariable -= 1;
}

// DataSharingBetweenFiles.cpp
#include <iostream>
#include "IncDec.h"
using namespace std;

int main() {
    function1();
    cout << globalVariable << endl; // 1
    function2();
    cout << globalVariable << endl; // 0
}

Alternatively, We can also define the global variables in the main file. However, it would be hard to read when we have dozens of files, with each containing a couple of global variables. Therefore, it is not recommended to define global variables in the main source file.

struct/class

The better alternative to global variables is to define a struct/class in the header file. This can be demonstrated by IncDec.h, IncDec.cpp and main file DataSharingBetweenFiles.cpp modified from the above example.

// IncDec.h
#pragma once
struct globalData {
    int data1 = 0;
    int data2 = 0;
};

extern globalData globalVariableStruct;

void function1();
void function2();

// IncDec.cpp
#include "IncDec.h"

globalData globalVariableStruct;

void function1() {
    globalVariableStruct.data1 += 1;
}

void function2() {
    globalVariableStruct.data2 -= 1;
}

// DataSharingBetweenFiles.cpp
#include <iostream>
#include "IncDec.h"
using namespace std;

int main() {
    function1();
    cout << globalVariableStruct.data1 << endl; // 1
    function2();
    cout << globalVariableStruct.data2 << endl; // -1
}

struct is especially useful when a group of variables are related. For example, in order to define dynamics of a vehicle, we don’t need to define dozens of global variables one by one. Instead, we can define a struct named vehicleDynamics, and then include all the variables that we care about into the struct. It is also very flexible in that we don’t need to change our codes much if we want to add more variables into the structure later.

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