Define global variable cpp

C how to define global variable in cpp

Another type of scope variable is global variables that are the variables declared or defined in the beginning or at the top of the program and are available to the entire program such that they can be accessed through any part of the program. Therefore there are 2 types of scope variables in C++, such as local variables that are the variables declared within the block or function of the program and are only available to those blocks and functions of the program.

How to declare a global variable in C++

I have read that any variable declared outside a function is a global variable. I have done so, but in another *.cpp File that variable could not be found. So it was not realy global.

According to the concept of scope , your variable is global. However, what you’ve read/understood is overly-simplified.

Possibility 1

Perhaps you forgot to declare the variable in the other translation unit (TU). Here’s an example:

a.cpp

int x = 5; // declaration and definition of my global variable 

b.cpp

// I want to use `x` here, too. // But I need b.cpp to know that it exists, first: extern int x; // declaration (not definition) void foo() < cout 

Typically you'd place extern int x; in a header file that gets included into b.cpp , and also into any other TU that ends up needing to use x .

Possibility 2

Additionally, it's possible that the variable has internal linkage , meaning that it's not exposed across translation units. This will be the case by default if the variable is marked const ( [C++11: 3.5/3] ):

a.cpp

const int x = 5; // file-`static` by default, because `const` 

b.cpp

extern const int x; // says there's a `x` that we can use somewhere. void foo() < cout 

You could fix this by applying extern to the definition , too:

a.cpp

This whole malarky is roughly equivalent to the mess you go through making functions visible/usable across TU boundaries, but with some differences in how you go about it.

You declare the variable as extern in a common header:

And define it in an implementation file.

You can then include the header everywhere you need access to it.

I suggest you also wrap the variable inside a namespace .

In addition to other answers here, if the value is an integral constant, a public enum in a class or struct will work. A variable - constant or otherwise - at the root of a namespace is another option, or a static public member of a class or struct is a third option.

MyClass::eSomeConst (enum) MyNamespace::nSomeValue MyStruct::nSomeValue (static) 

C vs C++ global variable in header [duplicate], In C++, int i; is a definition of an (uninitialized) object. The One Definition Rule (ODR) does not allow you to define several time the same

Global Variables in C++

C++ Allows you to use global variables, which means that they can be accessed from any Duration: 5:15

Global variables in a multi-file project in C

Source code can be found here:https://code-vault.net/lesson/fjri9hcdte:1642359047479
Duration: 8:42

C++ Global Variable

Definition of C++ Global Variable

In C++, a global variable is defined as a variable that can be used or accessed from anywhere in the entire program, which is one of the scope types in any programming language. Where the global variable scope is the extent of the program code within which the variables can be accessed or defined or declared, or used, in general, the global variable is defined as a variable that is allowed to be used by any part of the program without any restriction or error and are available to any part of the program or throughout the entire program, but they can be declared or defined usually at the top or start of the program which would be outside of all the blocks and functions of the program.

Working of Global Variable in C++

In this article, we will discuss a global variable in C++. There is a concept in C++ known as scope of variables in the programs, which means the variable’s accessibility within the program, within the function or block in the program. Therefore there are 2 types of scope variables in C++, such as local variables that are the variables declared within the block or function of the program and are only available to those blocks and functions of the program. Another type of scope variable is global variables that are the variables declared or defined in the beginning or at the top of the program and are available to the entire program such that they can be accessed through any part of the program. So, in general, it means that the global variables can be defined at the top of the program and can be seen and modified by any functions (including main) or blocks in the program that are referencing this global variable.

Examples of C++ Global Variable

Let us see in the below example how to define and declare such global variables in C++ programs.

Example #1

In the above program, we can see we have declared and defined global variable “g” with a value as 20 outside the main() function and local variable “p” with a value as 10 within the program. So when we are printing a local variable, we can see we can print or use it within the function, but the global variable value can be modified even within the function wherein in this above code, we are incrementing the g value by 1. So the output will be “21” for the value of the global variable. So in the above screenshot, we can see the output.

In C++, the variables are classified into global, local, static, etc., based on the storage class of variables. So the variables which are defined outside all the functions and blocks but within the program are known as global variables. In C++, there might be a situation where both global and local variable has the same name which is confusing to the compiler and may throw an error, but if there are in different scopes, then the compiler will print the value of the local variable name’s value as it gives first preference to local variables than global.

Hence sometimes global variables are considered as dangerous to use but as this one of the best way to have accessibility to any of the functions or blocks in the entire program which is very lengthy and it’s useful when it is difficult to declare the variables each time in the functions or blocks. So in such cases, to get access to global variables that have the same name as that of the local variable, then C++ provides an operator known as scope resolution operator (::). So to understand this, we will consider a simple example below.

Example #2

In the above program, we can see we have both global and local variables with the same name as “g”, but when we are trying to print both the values, it will print “9.3” only for both as the compiler will give more preference to a local variable over a global variable. Therefore, in the above screenshot, the output gives 9.3 only when we are printing both global and local variable values. So to print the global variable value, we need to use scope resolution operator so the above code will be modified inline 11 where when we are printing g value, we have to write it as::g to access the value of g as a global variable which we can see in the below with the modified code and output showing the values of both global and local variable values.

In the above program, we can see that we have declared g as a global variable at the top of the program before the main() function which holds the “5.8” value. And we have declared local variable g within the main() function, which holds a “9.3” value. So in the above screenshot, we can see we are able to print both values of a global variable using scope resolution operator (::) in line 11 and local variables in line 13.

Conclusion

In this article, we conclude that a global variable in C++ is a variable defined and declared at the top of the program, which can be available to the entire program even within the functions or blocks compared to a local variable. This article saw how we can declare global variables and use them in the program with a simple example. In this, we also saw if there is an ambiguity in the names of the variables, which means both local and global variables having the same name, then the global variable can be accessed using the scope resolution operator with an example in the above article.

Final thoughts

This is a guide to C++ Global Variable. Here we discuss the working of Global Variable in C++ along with the different examples and its code implementation. You may also have a look at the following articles to learn more –

C++ Storage Class: Local, Global, Static, Register and Thread Local, The scope of a global variable is the whole program. This means, It can be used and changed at any part of the program after its

C++ Global variable declaration

You must use extern , otherwise you will have separated bShouldRegister variables in each translation unit with probably different values.

Put this in a header file (.h):

extern bool bShouldRegister; 

Put this in one of implementation files (.cpp):

If you can use C++17, consider using an inline variable:

// in a header file inline bool bShouldRegister = true; 

See How do inline variables work? for more information.

A more C++-like way would be using a class member, syntactically indicated by the static keyword. Class member variables have implicit external linkage.

#ifndef VARIABLES_H #define VARIABLES_H class RegUtil < public: static bool bShouldRegister; >; #endif 

in one of your cpp files (maybe variables.cpp ), you have to define this class member:

#include "variables.h" bool RegUtil::bShouldRegister; 

How to declare a global variable in C++, To declare global variables in C++, we can declare variables after starting the program. Not inside any function or block.

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Declare a Global Variable in C++

1. Declare a Global Variable in a Single Source File in C++
2. Declare a Global Variable in Multiple Source Files in C++

This article will explain several methods of how to declare a global variable in C++.

Declare a Global Variable in a Single Source File in C++

We can declare a global variable with the statement that is placed outside of every function. In this example, we assume the int type variable and initialize it to an arbitrary 123 value. The global variable can be accessed from the scope of the main function as well as any inner construct (loop or if statement) inside it. Modifications to the global_var are also visible to each part of the main routine, as demonstrated in the following example.

#include  using std::cout; using std::endl;  int global_var = 123;  int main()   global_var += 1;  cout      for (int i = 0; i  4; ++i)   global_var += i;  cout   <" ";  >  cout     return EXIT_SUCCESS; > 

On the other hand, if additional functions are also defined in the same source file, they can directly access the global_var value and modify it in the function scope. The global variables can also be declared with a const specifier, which forces them to only be accessible through the current translation unit (source file with all included headers).

#include  using std::cout; using std::endl;  int global_var = 123;  void tmpFunc()  global_var += 1;  cout    >  int main()   tmpFunc();   return EXIT_SUCCESS; > 

Declare a Global Variable in Multiple Source Files in C++

Alternatively, there may be global variables declared in different source files, and needed to be accessed or modified. In this case, to access the global variable, it needs to be declared with an extern specifier, which tells the compiler (more precisely the linker) where to look for the glob_var1 definition.

#include  using std::cout; using std::endl;  int global_var = 123; extern int glob_var1; // Defined - int glob_var1 = 44; in other source file  void tmpFunc()  glob_var1 += 1;  cout    >  int main()   tmpFunc();   return EXIT_SUCCESS; > 

There might be global variables declared with a static specifier in the different source files in some cases. These variables are accessible only within the file where they are defined and can’t be reached from other files. If you still try to declare them with extern in the current file, the compiler error will occur.

#include  using std::cout; using std::endl;  int global_var = 123; extern int glob_var2; // Defined - static int glob_var2 = 55; in other source file  void tmpFunc()  glob_var2 += 1;  cout    >  int main()   tmpFunc();   return EXIT_SUCCESS; > 

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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