CODEWITHSUNDEEP
d
Adam Ryczkowski
Adam Ryczkowski

Reputation: 8064

How to declare extern("C") const function?

I'd like to write D bindings. For the life of the class A its someStruct * member variable will never change, so I guess it is natural to declare it as immutable. But then its constructor wouldn't compile unless I manage somehow to declare the APICall function's return value as const. How to do that?

struct someStruct;

const someStruct* APICall();

class A
{
    this()
    {
        this.ptr = myfunc();
    }
    private:
    immutable someStruct* ptr;
}

Error: function app.APICall without 'this' cannot be const

Upvotes: 2

Views: 120

Answers (1)

Adam D. Ruppe
Adam D. Ruppe

Reputation: 25595

You want to use parenthesis around the return value:

const(someStruct*) APICall();

or if it never changes, immutable is better. (const is mostly useful on function parameters rather than return values)

const or immutable without parenthesis before or after a function declaration applies to the this parameter, which is why the error says what it says: you are trying to apply it to a this which isn't there.

But, before doing this, make sure it is actually immutable - that the pointer never changes AND that the data it points to never changes either. If there's any mutability through the pointer, you should just make it mutable.

Upvotes: 2

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