C++ - Making a structure from header file invisible to outside

Question

I'm currently writing a program, which shares a lot of template structures in one of the header files. To evaluate those templates, I use some other helper structures, like here:

#define MAX 10

struct function1 {
    static constexpr int f(int x) {
        return x*(x*x+20);
    }
};

struct function2 {
    static constexpr int f(int x) {
        return (x*x+20);
    }
};

// This structure contains supremum of function contained in
// type T, on interval (0, MAX). We're assuming that f(0) is 0
// for each function class passed in T.
template <typename T>
struct supremum {
    static constexpr int valueF(int n) {
        return n == 0 ? 0 : (T::f(n) > T::f(n-1) ? n : valueF(n-1));
    }
    static constexpr int value = valueF(MAX);
};

template <typename T1, typename T2>
struct supremums {
    static constexpr int s1 = supremum<T1>::value;
    static constexpr int s2 = supremum<T2>::value;
};

// Sample struct initialization
supremums<function1, function2> s;

So this is my header file - the problem is that I want to share structure supremums to users using it , but I don't want to do that with helper structure supremum. Is there a way to hide supremum from "the outside"?

Sorry, I just noticed these supremum structures are not valid, but these are only examples on which I wanted to show a specific problem, so their validity is not an issue.

put it into a namespace called `details` and document everything inside this namespace are implementation details — Bryan Chen, Dec 01 '15 at 21:50
@BryanChen: Answer section: ↓↓↓↓↓↓↓↓↓↓↓↓↓ — Lightness Races in Orbit, Dec 02 '15 at 00:03

score 4 · Accepted Answer · edited May 23 '17 at 12:04

4

Since templates are usually implemented in header files, you cannot normally hide them from a user if they want to snoop.

The best you can do is put them in a namespace, such as detail or impl_detail and document them as being implementation details. Hopefully users of your header file will pay heed to the documentation and not use anything under that namespace.

edited May 23 '17 at 12:04

Community

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answered Dec 01 '15 at 21:58

R Sahu

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Well, from your link it appears that *must* is a bit exaggerated, even though I agree that that's the most common approach and a widely used one. – skypjack Dec 02 '15 at 00:00
@skypjack, can you provide counter examples that refute the *must* claim? – R Sahu Dec 02 '15 at 02:39
1

@R Sahu see the first answer from the question you pointed out and also this [link](https://isocpp.org/wiki/faq/templates) for further details. – skypjack Dec 02 '15 at 06:51
2

@R Sahu Please, note also that I agree with you on the fact that he has no much chances in this case, for he wants to export them in a public API. Anyway, to say *templates must be implemented in header files* is some how wrong, for you can work around it in some cases. Can you? Yes. Would I do that? No. That's for the sake of clarity anyway. :-) – skypjack Dec 02 '15 at 07:03
@skypjack, I read the C++ FAQ link. I see what you are saying. Let me rephrase the answer. – R Sahu Dec 02 '15 at 07:04
1

@skypjack, thanks for being patient with me while I educated myself :) – R Sahu Dec 02 '15 at 07:09

C++ - Making a structure from header file invisible to outside

1 Answers1