multiple definition of '...'

The multiple definition of '...' error means a symbol is defined more than once across translation units. The linker sees multiple definitions and cannot choose.

What It Means

You violated the One Definition Rule (ODR). In C++, every non-inline function and every object can have exactly one definition across the entire program. When you compile multiple source files that each contain a definition, the linker reports a conflict.

Why It Happens

• A function is defined in a header that is included in multiple .cpp files
• A global variable is defined in a header without extern
• The same source file is compiled twice and both object files are linked
• Missing header guards combined with multiple includes in the same translation unit
• A non-inline function is defined inside a class definition

How to Fix It

Step 1: Separate declarations from definitions

// math.h — declaration only
#ifndef MATH_H
#define MATH_H
int add(int a, int b);
#endif

// math.cpp — single definition
#include "math.h"
int add(int a, int b) {
    return a + b;
}

// main.cpp
#include "math.h"

int main() {
    return add(2, 3);
}

g++ -c math.cpp -o math.o
g++ -c main.cpp -o main.o
g++ main.o math.o -o program

Step 2: Use header guards

// utils.h
#ifndef UTILS_H
#define UTILS_H

int process(int x);

#endif

Without the guard, including utils.h twice in the same file causes redefinition errors.

Step 3: Use inline for functions in headers

// config.h
#ifndef CONFIG_H
#define CONFIG_H

inline int getDefaultPort() {
    return 8080;
}

#endif

inline allows the same function definition to appear in multiple translation units without violating ODR.

Step 4: Use static or anonymous namespaces

// helper.h — internal linkage, each TU gets its own copy
static int helperFunction(int x) {
    return x * 2;
}

Or with anonymous namespace (preferred in C++):

namespace {
    int helperFunction(int x) {
        return x * 2;
    }
}

Step 5: Use extern for global variables

// globals.h
#ifndef GLOBALS_H
#define GLOBALS_H
extern int globalCounter; // declaration only
#endif

// globals.cpp
#include "globals.h"
int globalCounter = 0; // single definition