What is the effect of typedef'ing a struct to

Question

This question already has an answer here:

• Difference between 'struct' and 'typedef struct' in C++? 8 answers

I've seen this code written at the top of an API:

typedef struct SYSTEM SYSTEM;

The type SYSTEM was previously undefined. Does anyone know what this does? What does the compiler think a SYSTEM after this line?

Thanks for the answers! My question is that SYSTEM was not previously defined in the file, so I have no idea what it is. For example, that line will compile by itself, but struct SYSTEM was not defined anywhere.

Answer 1

It allows you in C to use the name SYSTEM instead of using struct SYSTEM. The typedef creates a synonym SYSTEM to struct SYSTEM.

In your case as struct SYSTEM is not declared, the declaration also declares struct SYSTEM as an incomplete type like in this declaration: struct SYSTEM;.

The complete object type struct SYSTEM can then be declared in another translation unit.

Answer 2

This is really only effective and useful in C.

In C, you have to type struct type, while C++ allows you to omit the struct part of the type and just write type.

typedef struct type type allows you to use type in C without having to use the struct keyword before the struct name.

Some C programmers regard this bad practice though, and prefer to write struct type. Just trying to put that out there.

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The reason you're not getting any errors is because that line acts as a forward declaration for the struct SYSTEM that may or may not exist. All the typedef does is tell the compiler that when you say X, you mean Y.