Resolving Multiple Definitions

Linker errors caused by multiple definitions of the same function can be frustrating, but they are essential to address for creating a reliable and maintainable program.

These errors occur when the linker finds more than one definition for a function or variable with external linkage.

Common Causes

1. Duplicate Definitions: Defining the same function in multiple source files.
2. Multiple Includes: Including a header file with definitions in multiple source files without proper guards.

Solutions

Use Header Guards: Ensure header files use guards or #pragma once to prevent multiple inclusions.

1// myheader.h
2#pragma once
3
4void SayHello(); // Declaration only

Separate Declarations and Definitions: Declare functions in header files and define them in source files.

1// myheader.h
2#pragma once
3
4void SayHello(); // Declaration

1// mysource.cpp
2#include "myheader.h"
3#include <iostream>
4
5void SayHello() {
6  std::cout << "Hello, world!\n";
7}

Inline Functions: For small functions, consider using the inline keyword in the header file.

1// myheader.h
2#pragma once
3
4inline void SayHello() {
5  std::cout << "Hello, world!\n";
6}

Avoid Defining Variables in Headers: Only declare variables in headers, define them in one source file.

1// myheader.h
2#pragma once
3
4extern int GlobalVar; // Declaration

1// mysource.cpp
2#include "myheader.h"
3int GlobalVar{42}; // Definition

Here’s an example of resolving multiple definitions:

1// greeting.h
2#pragma once
3
4void SayHello();

1// greeting.cpp
2#include "greeting.h"
3#include <iostream>
4
5void SayHello() {
6  std::cout << "Hello from greeting\n";
7}

1// main.cpp
2#include "greeting.h"
3
4int main() {
5  SayHello();
6}

Let’s compile, link and run the program:

1g++ greeting.cpp main.cpp -o myProgram

1./myProgram

1Hello from greeting

By following these practices, you ensure that each function and variable has a single definition, avoiding linker errors and creating a more maintainable codebase.