c++ - 根据表达式的有效性选择要应用的函数

Question

问题如下，在 C++14 :

• 让我们有两个函数 FV&& valid_f , FI&& invalid_f , 和参数 Args&&... args
• 函数apply_on_validity应该申请valid_f在 args如果表达式 std::forward<FV>(valid_f)(std::forward<Args>(args)...)有效
• 否则，如果 std::forward<FV>(invalid_f)(std::forward<Args>(args)...)是一个有效的表达式，apply_on_validity应该申请invalid_f在 args
• 否则apply_on_validity什么都不做

我猜代码应该是这样的:

template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    // Apply valid_f by default
    std::forward<FV>(valid_f)(std::forward<Args>(args)...);
}

template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    // Apply invalid_f if valid_f does not work
    std::forward<FV>(invalid_f)(std::forward<Args>(args)...);
}

template <class FV, class FI, class... Args, /* Some template metaprog here */>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    // Do nothing when neither valid_f nor invalid_f work
}

但我真的不知道该怎么做。有什么想法吗？

---

链接到概括here .

Answer 1

拍摄:

template <int N> struct rank : rank<N-1> {};
template <> struct rank<0> {};

然后:

template <class FV, class FI, class... Args>
auto apply_on_validity_impl(rank<2>, FV&& valid_f, FI&& invalid_f, Args&&... args)
    -> decltype(std::forward<FV>(valid_f)(std::forward<Args>(args)...), void())
{
    std::forward<FV>(valid_f)(std::forward<Args>(args)...);
}

template <class FV, class FI, class... Args>
auto apply_on_validity_impl(rank<1>, FV&& valid_f, FI&& invalid_f, Args&&... args)
    -> decltype(std::forward<FI>(invalid_f)(std::forward<Args>(args)...), void())
{
    std::forward<FI>(invalid_f)(std::forward<Args>(args)...);
}

template <class FV, class FI, class... Args>
void apply_on_validity_impl(rank<0>, FV&& valid_f, FI&& invalid_f, Args&&... args)
{

}

template <class FV, class FI, class... Args>
void apply_on_validity(FV&& valid_f, FI&& invalid_f, Args&&... args)
{
    return apply_on_validity_impl(rank<2>{}, std::forward<FV>(valid_f), std::forward<FI>(invalid_f), std::forward<Args>(args)...);
}

DEMO