template variadic function: arbitrary number of classes with arbitrary number of constructor parameters

问题

I am unsure of what the proper name is, so far I comprehend the notion of using variadic template arguments (e.g., in comparison to std::initializer_list).

So let's assume there is an arbitrary number of classes k and an arbitrary number of parameters i which is dependant on each class k constructor.

I know that I may construct any class k using a variadic template:

template <typename T, typename... Args>
void make_class(Args... args)
{
    auto k = T(args...);
}

However I want to allow creation of multiple classes which are not of the same type and most likely have different Args. Furthermore, I want to iterate the classes k being constructed. Something along the lines:

template <typename T, typename... Classes, typename Args>
void make_classes(Classes... classes)
{
    for (auto type : classes) {
        make_class<T>(args...); //???
    }   
}

Is this possible?
If not possible, are there any potential work-arounds?
Is there a name for such a pattern/approach? Is this a parameter_pack?

Apologies if this is a duplicate.

回答1:

You'll need to delineate the arguments for each class by passing them in a tuple, since otherwise the result could be ambiguous for classes with more than one constructor (and note that most classes have at least a default constructor and copy constructor).

That done, this is easy to write:

template<class... Classes, class... Args>
void make_classes(Args&&... args) {
    auto k = std::tuple<Classes...>{
        std::make_from_tuple<Classes>(std::forward<Args>(args))...};
}

Usage:

make_classes<std::pair<int, float>, std::string, std::vector<double>>(
    std::forward_as_tuple(42, 3.14f),
    std::forward_as_tuple("hello"),
    std::forward_as_tuple(5, 99.99));

Example.

Note that make_from_tuple is a C++17 library addition; if your library doesn't have it you can copy the example implementation.

来源：https://stackoverflow.com/questions/39168220/template-variadic-function-arbitrary-number-of-classes-with-arbitrary-number-of

标签

c++

variadic-templates

variadic-functions