Global Variable Within Multiple Files

Global Variable within Multiple Files

The global variable should be declared extern in a header file included by both source files, and then defined in only one of those source files:

common.h

extern int global;

source1.cpp

#include "common.h"

int global;

int function(); 

int main()
{
    global=42;
    function();
    return 0;
}

source2.cpp

#include "common.h"

int function()
{
    if(global==42)
        return 42;
    return 0;
}

How can I access and modify a global variable across multiple files?

In C, global variables are global to the execution unit (process), not global to the execution environment (system). Standard C does not provide a mechanism to share memory between processes. However, your platform (Linux, Windows, MacOS, etc.) probably does. Look up "shared memory".

Use and set global variables across multiple files in Python

The line from file1 import * does two things:

1. It imports the file file1.py into the module cache.
2. It creates new references of all the defined symbols in your local namespace.

Because you have separate references, you can't update the ones in the module. If those are mutable variables, you can update them and see those updates in the module itself. For example, if you had ABC = [111] you could do ABC[0] = 222 and you would see the change.

If you want to change the module itself, you need to use a reference to the module; modules are mutable!

# file2.py
import file1

def set_value():
    file1.ABC = 222

.

# file3.py

import file1
from file2 import *

set_value()
print file1.ABC

c++ Global Variables Across Multiple Files

You could simply use M_PI from the include (there are other constants too).

Edit: your setup is correct. I got a working minmal example:

globals.h

extern double g_tst;

globals.cpp

#include "globals.h"
double g_tst = 4.0;

main.cpp

#include "globals.h"
#include <stdio.h>
#include <stdlib.h>

int main()
{
fprintf (stderr, "g_tst  = %lf \n", g_tst);
return 0;
}

The problem is within your buildsystem

See wikipedia

How to use global variables in multiple .cpp files?

The compiler is compiling print.cpp. It knows nothing about source.cpp while it is compiling print.cpp. Therefore that g_x that you placed in source.cpp does you absolutely no good when print.cpp is being compiled, that's why you get the error.

What you probably want to do is

1) place extern int g_x; inside of print.h. Then the compiler will see g_x when compiling print.cpp.

2) in source.cpp, remove the extern from the declaration of g_x:

int g_x = 5;

sharing global variable scope in multiple files C++

Either
extern settings_t ESPdata in the second file, or declare the variable inline and include the header file in both cpps.

Global static variables shared in multiple source files

Your assessment is correct.

Because global_foo is declared static, each source file has its own distinct variable by that same name, and a change to one does not affect the other.

Because of this, the program will print -1, since the global_foo in main.c is unchanged.

using a global variable in multiple files

You should have a design like the following:

shared.cpp

vector<int> a;

shared.h

extern vector<int> a;

somewhere.cpp

#include "shared.h"
void code() {
  a.push_back(10);
}

Mind that, since you are using C++, you can uses classes as namespaces to avoid cluttering the global namespace, eg:

shared.cpp

vector<int> Common::a;

shared.h

class Common {
  public:
    static vector<int> a;
}

somewhere.cpp

#include "shared.h"
void code() {
  Common::a.push_back(10);
}

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