Quick Answer

Multiple definitions of the same symbol

Understanding the Issue

This error occurs when the same variable or function is defined multiple times, violating the One Definition Rule. Common causes include forgetting header guards or defining variables in headers.

The Problem

This code demonstrates the issue:

Cpp Error 
// file.h
int global = 0; // Definition
// Included in multiple .cpp files

The Solution

Here's the corrected code:

Cpp Fixed 
// Solution: Header guards and extern
#ifndef FILE_H
#define FILE_H
extern int global; // Declaration
#endif

// file.cpp
int global = 0; // Single definition

Key Takeaways

Always use include guards and only declare (not define) in headers.