Why Do I Get \'Multiple Definition\' Errors in Header Files, and How Can I Fix Them?

Multiple Definition Errors in Header Files: Why and How to Resolve Them

Compilers encounter "multiple definition" errors when they are presented with multiple definitions of a symbol in the same program. This can occur when multiple source files include the same header file, each of which defines the same symbol.

Let's examine the provided code sample:

// complex.h
#include <iostream>

class Complex {
public:
   Complex(float Real, float Imaginary);
   float real() const { return m_Real; };
private:
   friend std::ostream& operator<p>The operator</p>
<pre class="brush:php;toolbar:false">// complex.cpp
#include "complex.h"
Complex::Complex(float Real, float Imaginary) {
   m_Real = Real;
   m_Imaginary = Imaginary;
}

// main.cpp
#include "complex.h"
#include <iostream>
int main() {
   Complex Foo(3.4, 4.5);
   std::cout <p>When compiling this code, the compiler encounters a "multiple definition" error for the operator</p>
<p><strong>Why Not for real()?</strong></p>
<p>The real() member function is implicitly inlined, meaning the compiler treats it as if it were declared inline even though it is not explicitly specified in the .h file.</p>
<p><strong>Resolutions</strong></p>
<p>To resolve the multiple definition issue for the operator</p>
<p><strong>Inline the Function:</strong><br>Add the inline keyword to the function definition to instruct the compiler to inline it:</p>
<pre class="brush:php;toolbar:false">inline std::ostream& operator<p><strong>Move the Definition to the .cpp File:</strong><br>Remove the function definition from the .h file and place it in the corresponding .cpp file:</p><pre class="brush:php;toolbar:false">// complex.cpp
std::ostream& operator<p><strong>Other Solutions</strong></p><p>Additional solutions include:</p>

• Pragma once: Using a preprocessor directive in the .h file to suppress multiple inclusions.
• Module System: Using a module system (C 20 and above) to handle dependencies between source files.
• Header Guards: Ensuring that the .h file defines a unique macro to prevent multiple inclusions.

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