CODEWITHSUNDEEP
cpointersconstants
user1002430
user1002430

Reputation:

Does the second const matter in void f(char const * const p)?

A function declared

void f(const char * const p) { ... }

means that it takes a constant pointer to a constant character. But, with the variable p scoped only within the function itself and its usage hidden from the callee, does it matter if the second const is there?

In other words, wouldn't the following be semantically identical to the first from the perspective of the callee?

void f(const char *p) { ... }

Upvotes: 2

Views: 201

Answers (4)

Jens
Jens

Reputation: 72697

Yes. You tell the compiler that you won't modify p in that function. This allows the compiler certain optimizations, e.g. if p is already in a register, there's no need to save it and no need to allocate a new register.

It also causes the compiler to issue a diagnostic should you attempt to modify p, eg. ++p or p = ... because that violates the promise you made.

Upvotes: 1

JVApen
JVApen

Reputation: 11317

The first const indicates that you can't change the data is pointing to, the second indicates that this pointer can't be overwritten.

So, for a caller of this function, it doesn't matter. For the implementation of the function, it can be useful, though it will only have effect locally.

Upvotes: 2

Alex Shesterov
Alex Shesterov

Reputation: 27565

The second const (the one after the *) doesn't matter from the callee point of view.

But it does matter from the point of view of the function body. The second const makes sure that you can't make the pointer change its value (i.e. point to a different memory location).

It's comparable to declaring a simple primitive parameter as const, like in void f(const int value) {}

Upvotes: 2

templatetypedef
templatetypedef

Reputation: 372982

Yes, I believe so. The additional const here only means that in the implementation of f, the implementer is not allowed to reassign p to point to some other character or C-style string.

Upvotes: 0

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