C++: undefined reference to `vtable for X' — virtual function not defined

Meaning

The compiler emits the vtable in the file that defines the class's first non-inline virtual function (the "key function"). If that function has no body, or its .cpp is missing from the build, you get undefined reference to vtable for X and often typeinfo for X too.

Common causes

• Virtual function declared but never defined
• The .cpp containing the key function is not part of the target
• Pure virtual function called through a base that has no definition
• Missing virtual destructor body (virtual ~X(); never defined)

Detailed fix by platform

Code examples

// widget.h
struct Widget {
    virtual void draw();               // declared ...
    virtual ~Widget() = default;
};

// widget.cpp
#include "widget.h"
void Widget::draw() {}                 // ... and defined here, so the vtable exists

How to diagnose

1. Class — Which class is in the message?
2. Definitions — Does each virtual function have a body?
3. Build — Is its .cpp compiled and linked?

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