结构的定义应该在哪里用C声明？ .c或.h？

Question

When implementing an ADT in C, I always thought that the definition of the struct should be in the .c file so that it was private. And in the typedef should be placed in the .h file so that other modules could use the ADT but could not modify its attributes directly. I recently ran into a problem and I had to move the struct to the .h . I also found an answer Error: In C, got the error "dereferencing pointer to incomplete type" in a struct pointer where it is said that the struct should be defined in .c file. Is this true? If it is, what is the reason?

Answer 1

As I noted in a comment :

The structure definition for an ADT should usually be in the source ( .c ) file that defines the support functions, and only a declaration of the type should exist in the header ( .h ) file.

However, that means you cannot allocate instances of the structure outside the source file defining the ADT. You can only define pointers to the ADT outside.

If you had to move the definition from source to header, that means that something was not using a pointer that should have been using a pointer. Go back and fix the broken code, and move the structure definition back into the source file for the ADT.

And, confirming what Nevado notes in a comment , it also means you cannot have an array of the ADT; you can have arrays of pointers to the ADT, but not arrays of the ADT itself.

Note that it is perfectly legitimate to expose the structure if that's what you wish, or need, to do. That then allows you to create variables of the data type, or arrays of them, or to embed the type (rather than a pointer to the type) in other structures, etc. You lose some control; the programmer writing the code that uses the structure can see the details and could access (read or modify) the structure elements directly. Cautious programmers won't do that unnecessarily, but it opens up possibilities precluded by the strict ADT idiom — at the risk of mistakes being made.