For each type, call a template function with each of these types

Question

As the title says, I'm trying to create something to be used as followed :

template <typename T>
void testFunc(int& i)
{
    ...
}

int i { 0 };
ForEach<int, float>::run<testFunc>(i);

I've already tried some things, but I'm hitting some problems:

template<typename CurrentComponentType, typename... ComponentTypes>
struct ForEach
{
    template<void (&func)(auto&&... args)>
    static constexpr void run(auto&&... args)
    {
        func<CurrentComponentType>(std::forward<decltype(args)>(args)...);
        ForEach<ComponentTypes...>::run<func>(std::forward<decltype(args)>(args)...);
    }
};

template<typename CurrentComponentType>
struct ForEach<CurrentComponentType>
{
    template<void (&func)(auto&&... args)>
    static constexpr void run(auto&&... args)
    {
        func<CurrentComponentType>(std::forward<decltype(args)>(args)...);
    }
};

I don't know how to take a template function as a (template but not necessarily) argument.
For some reasons that I don't understand, I cannot call again the run() function as so: run<func>(. It says '<unresolved overloaded function type>'.

I think there are multiple things I don't understand.

How can I fix it, and why doesn't it work this way? What am I misunderstanding?

`testFunc` isn't a function, it's a template function, but the template parameter for `run()` is a function rather than a template function. — Patrick Roberts, Apr 08 '21 at 22:58
@PatrickRoberts that indeed explain the 2nd problem... but I dont have any idea how to resolve it... — DiantArts, Apr 08 '21 at 23:02
In C++20 you can do [this](https://godbolt.org/z/s8hE7zaMP), are you okay with using lambdas instead of template functions? — Patrick Roberts, Apr 08 '21 at 23:10
@PatrickRoberts I am, I just didn't know there is a difference — DiantArts, Apr 08 '21 at 23:13
A lambda (even with template parameters) is a value, whereas a template function is a template value. C++ doesn't support non-type template template parameters. — Patrick Roberts, Apr 08 '21 at 23:14
Possible duplicate: https://stackoverflow.com/questions/15651488/how-to-pass-a-template-function-in-a-template-argument-list — Jerry Jeremiah, Apr 08 '21 at 23:23
Here's another approach in C++17 if you make `testFunc` a template class with an `operator()`: https://godbolt.org/z/fh58noMWs — Patrick Roberts, Apr 08 '21 at 23:25

Patrick Roberts · Answer 1 · 2021-04-08T23:50:20.323

Each of the solutions below can be used like so:

int main(void)
{
    int i { 0 };
    ForEach<int, float>::run<testFunc>(i);
}

Because you can't pass an uninstantiated template function as a (template) parameter, you need to make it a template class with an operator():

#include <utility>

template<typename T>
struct testFunc
{
    void operator()(int& i) {}
};

template<typename T, typename... Ts>
struct ForEach
{
    template<template<typename> typename F, typename... Args>
    static constexpr void run(Args&&... args)
    {
        ForEach<T>::template run<F>(std::forward<Args>(args)...);
        ForEach<Ts...>::template run<F>(std::forward<Args>(args)...);
    }
};

template<typename T>
struct ForEach<T>
{
    template<template<typename> typename F, typename... Args>
    static constexpr void run(Args&&... args)
    {
        F<T>{}(std::forward<Args>(args)...);
    }
};

Starting in C++17, you can use fold expressions to avoid recursion:

#include <utility>

template<typename T>
struct testFunc
{
    void operator()(int& i) {}
};

template<typename... Ts>
struct ForEach
{
    template<template<typename> typename F, typename... Args>
    static constexpr void run(Args&&... args)
    {
        (F<Ts>{}(std::forward<Args>(args)...), ...);
    }
};

Finally, in C++20 you also have the option to use a lambda with a template parameter list instead:

#include <utility>

auto testFunc = []<typename T>(int& i) {};

template<typename... Ts>
struct ForEach
{
    template<auto F, typename... Args>
    static constexpr void run(Args&&... args)
    {
        (F.template operator()<Ts>(std::forward<Args>(args)...), ...);
    }
};

For each type, call a template function with each of these types

1 Answers1