C++: error: cannot declare variable 'x' to be of abstract type 'Shape' (pure virtual functions)

Meaning

A class with virtual void f() = 0; is abstract. The note under the error lists the pure virtual function that remains unimplemented, which often means a typo or a mismatch (missing const, different parameter type) so the derived function does not override.

Common causes

• Instantiating the base class directly
• Derived class forgot to override one pure virtual function
• Override signature differs (const, parameter type, reference qualifier)
• Returning or passing an abstract class by value (object slicing)

Detailed fix by platform

Code examples

struct Shape {
    virtual double area() const = 0;
    virtual ~Shape() = default;
};

struct Circle : Shape {
    double r = 1;
    double area() const override { return 3.14159 * r * r; }
};

int main() {
    // Shape s;                       // error: cannot declare variable 's' to be of abstract type 'Shape'
    Circle c;                         // OK
    const Shape& s = c;
}

How to diagnose

1. Note — Which pure virtual function is listed?
2. Signature — Does the override match exactly?
3. Storage — Is a base class stored by value?

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