C++ created multiple struct with class template

Question

I want to create typesafe structures that are basically identical but have different types so that they require different function signatures.

struct A {
    Time t;
    void doStuff(const A&);
    A getStuff();
};

struct B {
    Time t;
    void doStuff(const B&);
    B getStuff();
};

If I sue a template for the class

template<class T>
struct X {
    Time t;
    void doStuff(const X&);
    X getStuff();
};

how can I make functions typesafe and define function signatures differently for a struct X of type A and a struct X of type B?

Your template looks wrong. You should write `doStuff(const X&);` and `X getStuff();`. — iBug, Oct 05 '17 at 01:22
It's not at all clear what you're asking. The code you have is almost valid (except for the missing return type already mentioned, and some missing semicolons). How do you want to use the types and/or the template, or what is it you want to add or change about them that you haven't shown? — aschepler, Oct 05 '17 at 01:30
@aschepler I think it's clear enough. The OP mentioned "typesafe" several times. — iBug, Oct 05 '17 at 01:39

iBug · Accepted Answer · 2017-10-05T01:42:37.870

1

Try adding some unused template parameters.

template <int>
struct X{
    Time t;
    void doStuff(const X&); // You missed return type
    X getStuff();
}; // You missed a semicolon

// Great thanks to "aschepler"

And now you can (C++11 syntax)

using A = X<1>;
using B = X<2>;
// typedef X<1> A;
// typedef X<2> B;

The following code will fail, which is what you want:

A a; B b;
a.doStuff(b); // Fail
a = b.getStuff(); // Fail

edited Oct 05 '17 at 01:42

answered Oct 05 '17 at 01:25

iBug

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let me try that. curious, what do I need the second template for? – chrise Oct 05 '17 at 01:35
Ok, thanks, looking forward to give this a try later – chrise Oct 05 '17 at 01:41
It's entirely legal for a template definition to ignore any or all template parameters. (In fact, in that case you can even omit the parameter name, like `template struct X {`...) – aschepler Oct 05 '17 at 01:41
Good job, @aschepler . Now it's really type-safe. – iBug Oct 05 '17 at 01:43
@aschepler Now how can I define functions? `template void X??>::doStuff()` doesn't look so well. – iBug Oct 05 '17 at 01:46

score 0 · Answer 2 · answered Oct 05 '17 at 02:00

You could use a combination of Inheritance and template for that.
Following your line of thought, a possible code is:

template <class T> struct X {
    int time = 10;
    virtual void doStuff(const T&) = 0;
    virtual T getStuff() = 0;
};

struct A : X<A>
{
    void doStuff(const A& a) {
        cout << "Do A stuff" << endl;
    }

    A getStuff() {
        return *this;
    }
};

struct B : X<B>
{
    void doStuff(const B& value) {
        cout << "Do B stuff" << endl;
    }

    B getStuff() {
        return *this;
    }
};

Then, testing we have:

int main()
{
    A a; B b;

    a.doStuff(a);  // Ok
    b.doStuff(b);  // Ok
    a.doStuff(b);  // fail
    b.doStuff(a);  // fail
    b = b.getStuff();  // Ok
    b = a.getStuff();  // fail

    return 0;
}

C++ created multiple struct with class template

2 Answers2