How Can I Forward Declare Nested C Classes?

Forward Declaring Inner Classes

It can be desirable to reference nested classes without including their defining header file. However, attempting to forward declare inner classes outside of their containing class can lead to compilation errors.

Consider the following example:

class Container {
public:
    class Iterator {
        ...
    };

    ...
};

class Foo {
    void Read(Container::Iterator& it);
};

Compiling this code will result in errors due to the incomplete type of Container and the undeclared it variable.

Solution

Unfortunately, it is not possible to forward declare inner classes outside of their containing class. The C standard does not allow for this syntax. Therefore, addressing this issue requires alternative solutions:

• Move the Inner Class Outside: Declare the inner class as a non-nested class. This allows it to be forward declared and included separately.
• Reorder Declarations: If possible, reorder the class declarations so that the inner class is fully defined before references are made to it.
• Use a Base Class: Create a common base class that the inner class can inherit from and that the function can pass as a parameter. This allows the function to accept a reference to the base class, even though the concrete inner class type is not known.

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