Question

I’ve found this in the CGPath.h header file. I’m curious what this const thing does?

typedef struct CGPath *CGMutablePathRef;
typedef const struct CGPath *CGPathRef;

My guess: If I typedef something as const, it’s constant so immutable and can’t be changed in any way. Does that make sense?

Answer 1

Yes the const means you cannot change the path externally.

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For CoreFoundation-based libraries though, the const is more a hack to allow mutable objects to use immutable methods without casting, and not vice-versa. That means

CGMutablePathRef mpath;
...
CGPathContainsPoint(mpath, ...);

compiles fine because a Foo* can be implicitly converted to a const Foo*, but

CGPathRef path;
...
CGPathAddRect(path, ...);

will throw an error because a const Foo* cannot be safely converted to a Foo*.