Using the Inline Keyword

The inline keyword in C++ helps manage the One Definition Rule (ODR) by allowing functions and variables to be defined in header files without causing multiple definition errors.

The ODR states that there should be only one definition of any variable, function, class, or type in the entire program. However, multiple declarations are allowed.

The inline keyword tells the compiler that it can include the function or variable definition in multiple translation units but treat it as a single definition.

Example with Functions

Without inline, defining a function in a header file would cause multiple definitions:

1// utils.h
2#pragma once
3
4void PrintHello() {
5  std::cout << "Hello\n";
6}

Including utils.h in multiple source files causes a linker error:

1main.obj : error LNK2005: "void PrintHello(void)" already defined in other.obj

Using inline solves this:

1// utils.h
2#pragma once
3
4inline void PrintHello() {
5  std::cout << "Hello\n";
6}

Now, PrintHello() can be included in multiple files without causing multiple definitions.

Example with Variables

For variables, the inline keyword is similarly useful. Without inline, defining a variable in a header file causes errors:

1// globals.h
2#pragma once
3
4int GlobalVar{42};

Using inline allows it to be defined in multiple files:

1// globals.h
2#pragma once
3
4inline int GlobalVar{42};

Here's a complete example:

1// globals.h
2#pragma once
3
4inline int GlobalVar{42};
5
6inline void PrintGlobalVar() {
7  std::cout << "GlobalVar: " << GlobalVar << '\n';
8}

1// main.cpp
2#include <iostream>
3#include "globals.h"
4
5int main() {
6  PrintGlobalVar();
7}

1// other.cpp
2#include "globals.h"
3
4// Additional code can use GlobalVar
5// and PrintGlobalVar here

1g++ main.cpp other.cpp -o myProgram
2./myProgram

1GlobalVar: 42

• Single Definition: The inline keyword ensures the compiler treats multiple instances of a function or variable definition as a single definition.
• Modularity: It allows functions and variables to be defined in header files, promoting code reuse and modularity.
• Avoids Linker Errors: Prevents multiple definition errors during the linking phase.

Using the inline keyword effectively helps manage the ODR, allowing for more flexible and modular code organization in C++ projects.