Question

I have an opaque type in my library defined as:

typedef struct MyOpaqueType* MyType;  // easier to type for client code

I can’t pass a pointer-to-const struct around using the typedef, so some functions look like:

void UsePointerToConst ( const struct MyOpaqueType * )

instead of:

void UserPointerToConst( const MyType ) // can't use, is really constant pointer

So, given this, I have two questions:
Is the struct keyword in the parameter list only necessary in C?
Is there a better way to do this? Should I create a typedef such as:

typedef const struct MyOpaqueType* ConstantMyType; ?

Answer 1

Is the struct keyword in the parameter list only necessary in C?

Yes. See Jens Gustedt’s answer.

Is there a better way to do this?

Just typedef the struct, not the pointer. This is better because

• you only need one typedef instead of one for each of {MyOpaqueType, MyOpaqueType *, MyOpaqueType const *, MyOpaqueType *const and MyOpaqueType const *const} and all variants involving restrict (which doesn’t exist in C++),
• it’s clear to the user that pointer semantics apply, i.e., passing the datatype around is really a matter of pointer copying (no performance worries), users are less likely to forget cleaning up after use, C++ users may use smart pointers, and
• it’s a common C convention (think FILE *).

There’s also no danger; when someone forgets the *, they get a compiler error.