对可变参数模板类型列表的每种类型进行操作的函数
[英]Function operating on each type of a variadic template typelist
问题描述
I have defined a typelist like this:
我已经定义了一个这样的类型列表：
template <typename ... Types> struct typelist {};
using my_list = typelist<int, double, bool, float>;
Now I have a function template, eg现在我有一个函数模板，例如
template<typename T>
void foo() {
std::cout << typeid(T).name() << std::endl;
}
and want to call this for every type in the typelist:并想为类型列表中的每个类型调用它：
foo<int>();
foo<double>();
foo<bool>();
foo<float>();
I tried to find a recursive way to solve this, but I am having trouble to define the correct, probably nested, variadic templates for the required foo functions.我试图找到一种递归方法来解决这个问题，但是我无法为所需的 foo 函数定义正确的、可能是嵌套的、可变参数模板。 Do you have any hints for a neat solution to this problem?你有什么关于这个问题的巧妙解决方案的提示吗？
3 个解决方案
解决方案1
3 已采纳 2020-08-26 09:03:02
template<class... Types> auto foo_foreach(typelist<Types...>) {
return (foo<Types>(), ...);
}
int main() {
foo_foreach(my_list{});
}
For a real oldschool, well, use template recursion you've attempted before:对于真正的老派，使用您之前尝试过的模板递归：
void foo_foreach(typelist<>) {}
template<class Head, class... Tail> void foo_foreach(typelist<Head, Tail...>);
template<class Head, class... Tail> void foo_foreach(typelist<Head, Tail...>) {
foo<Head>();
foo_foreach(typelist<Tail...>{});
}
解决方案2
0 2020-08-26 09:29:35
Here is a c++20 answer only that uses lambda template.这是一个仅使用 lambda 模板的c++20答案。
template <typename... Ts, typename C>
constexpr void for_types(C&& c) {
(c.template operator()<Ts>(), ...);
}
for_types<int, float, char>([]<typename T>()
{
std::cout << typeid(T).name() << std::endl;
});
解决方案3
0 2021-12-03 22:22:53
If you need this to work with C++14 then you can use the initializer_list trick to avoid a C++17 fold expression.如果您需要它与 C++14 一起使用，那么您可以使用initializer_list技巧来避免 C++17 折叠表达式。 This will hopefully be cheaper to compile than a recursive approach:这有望比递归方法更便宜：
template<class... Ts> void foo_foreach(typelist<Ts...>) {
(void) std::initializer_list<int>{(foo<Ts>(), 0)...};
}
int main() {
foo_foreach(my_list{});
}
Godbolt example: https://godbolt.org/z/o91Th466s Godbolt 示例： https ://godbolt.org/z/o91Th466s
There is a good explanation of the initializer_list trick athttps://blog.tartanllama.xyz/exploding-tuples-fold-expressions/在https://blog.tartanllama.xyz/exploding-tuples-fold-expressions/ 上有关于initializer_list技巧的很好解释
From that article:从那篇文章：
[...] parameter packs can only be expanded in contexts which expect a syntactic list, such as initializers and function call arguments.
std::initializer_listto create a context in which the parameter pack can be expanded.std::initializer_list创建一个可以扩展参数包的上下文。The trick is the
, 0inside theinitializer_listinitializer, which evaluates the function call, and uses 0 as the initializer value., 0在initializer_list程序中，它评估函数调用，并使用 0 作为初始化程序值。
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