Why Can\'t I Forward Declare a Typedef in C ?

Forward Declaration of a Typedef in C

It might seem intuitive that a forward declaration of a typedef would be possible, as it is with a class. However, the compiler will not allow it. Instead, it is necessary to forward declare the type that the typedef references.

For example, let's say we want to create a typedef for a class called A:

typedef A B; // error: 'A' was not declared in this scope

To fix this, we must first forward declare class A:

class A;

typedef A B; // valid

This allows us to reference the type B later in our code, even though the full definition of A is not yet known.

Best Practice for Minimizing Inclusion Tree

Since we cannot forward declare typedefs, the best practice for keeping the inclusion tree small is to include only the headers that are necessary for the current compilation unit. This can be achieved by using include guards and carefully managing dependencies between headers.

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