Having trouble checking whether template parameter is something you get when you pass a function's identifier

问题

I want to implement the following function:

template<typename Function, typename... Parameters>
inline void foo(
    const Function&   kernel_function,
    bar_t             bar
    Parameters...     parameters)

{
    static_assert(/* magic */,
        "You can only foo() a function, not values of any other type");
    / * etc. etc. */
}

and I need it to only be called with the identifiers of functions, or with pointers to functions: No lambads or methods or classes with operator() or std::functions. What should I replace /* magic */ with? Just using std::is_function doesn't seem to work.

回答1:

In <type_traits> we have std::is_function which returns true if you pass it an actual function and false with lambdas, classes with overloaded operator() and pointers to functions. We also have std::is_pointer and std::remove_pointer which can be used to check for and remove the pointer type from a function pointer to test the pointer with std::is_function. Using those your assert would look like

static_assert(std::is_function<Function>::value || 
    (std::is_pointer<Function>::value &&
    std::is_function<std::remove_pointer<Function>::type>::value),
    "You can only foo() a function, not values of any other type");

回答2:

This is based on @DietmarKuhl' answer in the sort-of-duplicate question:

#include <type_traits>
#include <utility>

template<typename Fun>
struct is_function_ptr: std::integral_constant<bool,
    std::is_pointer<Fun>::value and
    std::is_function<typename std::remove_pointer<Fun>::type>::value> { };


template<typename Function, typename... Parameters>
inline void foo(
    const Function&   kernel_function,
    bar_t             bar
    Parameters...     parameters)
{
    static_assert(
        std::is_function<Function>::value or is_function_ptr<Function>::value,
        "You can only foo() a function, not values of any other type");

    / * etc. etc. */
}

Although, to be honest, I have not tested extensively.

来源：https://stackoverflow.com/questions/43348943/having-trouble-checking-whether-template-parameter-is-something-you-get-when-you

标签

c++

metaprogramming

typetraits