如何为多种C ++类型创建编译时检查？

Question

I have a template <class Class> generic function that includes:

std::ostringsteam objectStream;

objectStream << std::forward<Class>(object);

return objectStream.str();

For efficiency, I wish to optimize the case of Class strings.

Therefore, I'm doing tags dispatch on templates for overloading the template function per item 27 is Scott Myers' Effective Modern C++ book.

Therefore I need to generate at compile time either a std::true_type or std::false_type .

Given a template<class Class> , I need a std::true_type if any of these expressions are true:

std::is_same<typename std::decay<Class>::type,       char   * >()
std::is_same<typename std::decay<Class>::type, const char   * >()
std::is_same<typename std::decay<Class>::type,       string   >()
std::is_same<typename std::decay<Class>::type,       string * >()
std::is_same<typename std::decay<Class>::type, const string * >()

I'm not sure how to do the OR so the compiler can dispatch on the tags at compile time correctly.

Related question, is there a way to drop the const in a const char * to make it char * ?

Answer 1

Therefore I need to generate at compile time either a std::true_type or std::false_type .

Given a template<class Class> , I need a std::true_type if any of these expressions are true [...]

Remembering that std::true_type and std::false_type are alias, respectively, for

std::integral_constant<bool, true>; // aka std::true_type
std::integral_constant<bool, false>; // aka std::false_type

if I understand correctly, you want this or something similar

using tct = typename std::decay<Class>::type; // to make shorter

using yourType = std::integral_constant<bool, std::is_same<tct, char * >::value
                                           || std::is_same<tct, const char * >::value
                                           || std::is_same<tct, string >::value
                                           || std::is_same<tct, string *>::value
                                           || std::is_same<tct, const string *>::value>;

Related question, is there a way to drop the const in a const char * to make it char * ?

I suppose you can create a custom template as follows

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

template <typename T>
struct foo<T const *>
 { using type = T *; };

So you can simplify the preceding code as follows

using tct = typename std::decay<Class>::type; // to make shorter
using tft = typename foo<tct>::type

using yourType = std::integral_constant<bool, std::is_same<tft, char * >::value
                                           || std::is_same<tft, string >::value
                                           || std::is_same<tft, string *>::value>;

Answer 2

For your first question,

If you're using c++17 you can do this in a few lines using a fold expression

#include <iostream>
using namespace std;

template <typename C, typename... Ts> 
using matches_my_types = std::bool_constant< ( ... | std::is_same<C,Ts>::value)>;

//or with a predefined set of types..
template <typename C>
using matches_my_predefined_set_of_types = matches_my_types<C,bool,double,int>;

int main() {

    using Class = int; 
    std::cout << matches_my_types<Class,bool,double,int>::value << std::endl;
    std::cout << matches_my_predefined_set_of_types<Class>::value << std::endl;   
    return 0;
}

Demo

For c++11 you can do something similar, but using recursion in place of the fold.

#include <iostream>
using namespace std;

template<typename B, typename...Bs> struct any_true
    : std::conditional_t<bool(B::value), B, any_true<Bs...>>{};
template<typename B> struct any_true<B> : B {};

template <typename C, typename... Ts> 
using matches_my_types = any_true<std::is_same<C,Ts>...>;


int main() {

    using Class = int;
    std::cout << matches_my_types<Class,bool,double,int>::value << std::endl;
    return 0;
}

Demo

For your second question, if you want a general remove const on pointer to const T, you can use the built in type_traits and conditional,

#include <iostream>
#include <typeinfo>

using namespace std;

template <typename T>
using remove_const_if_pointer_to_const = 
    std::conditional_t<std::is_pointer<T>::value,
        std::add_pointer_t<std::remove_const_t<std::remove_pointer_t<T>>>,
        T>;

int main() {

    using A = int;
    using B = int*;
    using C = const int*;

    std::cout << typeid(remove_const_if_pointer_to_const<A>).name() << std::endl;
    std::cout << typeid(remove_const_if_pointer_to_const<B>).name() << std::endl;
    std::cout << typeid(remove_const_if_pointer_to_const<C>).name() << std::endl;

    return 0;
}

Demo

Answer 3

I like max66's answer. Simple and elegant.

If you need a more classic metafunction solution (ie something like check_type_t<T> ) and you don't want to use any meta-programming library like boost::mpl or boost::hana etc... You can simply do the following:

template <class Class, class = void>
struct check_type_ {
   using type = std::false_type;
};

template <class Class>
struct check_type_<Class,
   std::enable_if_t<
      std::is_same<typename std::decay<std::remove_const_t<Class>>::type, char*>::value
>> {
   using type = std::true_type;
};

template <class Class>
struct check_type_<Class,
   std::enable_if_t<
      std::is_same<typename std::decay<std::remove_const_t<Class>>::type, string*>::value
>> {
   using type = std::true_type;
};
template <class Class>
struct check_type_<Class,
   std::enable_if_t<
      std::is_same<typename std::decay<Class>::type, string>::value
>> {
   using type = std::true_type;
};

template <class Class>
using check_type_t = typename check_type_<Class>::type;

static_assert(std::is_same<check_type_t<char*>, std::true_type>::value, "");
static_assert(!std::is_same<check_type_t<int>, std::true_type>::value, "");

C++ will try to select the most specialized template so when any of the type you want(for example string) is passed to Class in check_type_t<> then the

std::enable_if_t<
   std::is_same<typename std::decay<Class>::type, string>::value
>

is not ill formed and resulting to void . So this specialization is selected and the type inside is std::true_type . If none of the specializations is well formed then the main template is selected and the result is std::false_type .

I hope it helped.