Cast specific types in variadic argument

问题

I have a template function that accepts variadic arguments.

template<typename... Params>
void foo(Params... p);

I want to find all occurences of a given type (const char*) in Params to replace them with another type, that these values can be cast to (my own Path class with constructor Path(const char*)). The idea is to have something like

template<typename... Params>
void foo(Params... p) {
    bar<convertCharPointerToPath<Params>...>(p...);
}

How can this conversion be done?

回答1:

If you want to convert types only, it's just a meta-function away:

template<typename T> struct convert {
  using type = T;
};

template<> struct convert<char const*> {
  using type = Path;
};

template<typename T>
using convertCharPointerToPath = typename convert<T>::type;

Now use it in your parameter pack expansion, as you do in your original post.

回答2:

Without a traits helper:

template<typename... Params>
void foo(Params... p) {
  bar<std::conditional_t<std::is_same<char const*,Params>{},Path,Params>...>(p...);
}

or, with overload<Fs...>:

auto identity=[](auto&&x)->decltype(x){return decltype(x)(x);};

template<typename... Params>
void foo(Params... p) {
  auto convert = overload([](const char* p){return Path(p);},identity);
  bar(convert(p)...);
}

overload can be found everywhere; it takes a set of lambdas and returns their overload set.

Live example.

来源：https://stackoverflow.com/questions/48131712/cast-specific-types-in-variadic-argument