C++: error: redefinition of 'class X' / multiple definition — missing include guard

Meaning

When two headers both include shape.h, the compiler sees the class definition twice and reports a redefinition. Add #pragma once or an #ifndef guard to every header, and never define non-inline functions or variables in headers.

Common causes

• Header without #pragma once or #ifndef guard
• Two guards with the same macro name in different headers
• Including a .cpp file instead of its header
• Function or variable defined (not just declared) in a header

Detailed fix by platform

Code examples

// shape.h
#pragma once            // or: #ifndef APP_SHAPE_H / #define APP_SHAPE_H ... #endif

class Shape {
public:
    virtual double area() const = 0;
    virtual ~Shape() = default;
};

How to diagnose

1. Include chain — Is the header reachable through two paths?
2. Guard — Does every header have a unique guard?
3. Source — Did something #include a .cpp?

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