Is There Anything Like a Templatized Case-Statemen

问题:

So I have this really ugly code:

template <typename T>
std::conditional_t<sizeof(T) == sizeof(char),
                   char,
                   conditional_t<sizeof(T) == sizeof(short),
                                 short,
                                 conditional_t<sizeof(T) == sizeof(long),
                                               long,
                                               enable_if_t<sizeof(T) == sizeof(long long),
                                                           long long>>>> foo(T bar){return reinterpret_cast<decltype(foo(bar))>(bar);}

I'm using nested conditional_ts to make a case-statement of sorts. Is there something out there that accomplishes this more elegantly or do I need to cook up my own templatized-case-statement?

_{Note: I am actually aware that this use of reinterpret_cast is bad: Why Doesn't reinterpret_cast Force copy_n for Casts between Same-Sized Types?}

回答1:

I had to do something like this once so I wrote a small wrapper to acheive the result neatly. You could use it as follows (see here for a test)

template<class T>
typename static_switch<sizeof(T)
            ,int // default case
            ,static_case<sizeof(char),char>
            ,static_case<sizeof(short),short>
            ,static_case<sizeof(long),long>
            >::type foo(T bar){ ... }

Behind the scenes it pretty much does what you already have but by wrapping it we keep it (more) readable. There is also a version to allow you to switch direclty on the type T if you needed that.

Edit: At @Deduplicator's suggestion here is the code behind it

#include <type_traits>  

/* 
 * Select a type based on the value of a compile-time constant such as a 
 * constexpr or #define using static_switch. 
 */ 

template<int I,class T> 
struct static_case { 
    static constexpr int value = I; 
    using type = T; 
}; 

template<int I, class DefaultType, class Case1, class... OtherCases> 
struct static_switch{ 
    using type = typename std::conditional< I==Case1::value ,  
                    typename Case1::type, 
                    typename static_switch<I,DefaultType,OtherCases...>::type 
                     >::type; 
}; 

struct fail_on_default {};

template<int I, class DefaultType, class LastCase> 
struct static_switch<I,DefaultType,LastCase> { 
    using type = typename std::conditional< I==LastCase::value ,  
                    typename LastCase::type, 
                    DefaultType 
                     >::type; 

    static_assert(!(std::is_same<type, fail_on_default>::value),
                  "Default case reached in static_switch!");
}; 

回答2:

The template version of a switch statement is a specialized template.

template<size_t n> struct matching_type;
template<> struct matching_type<sizeof(char)> { typedef char type; };
template<> struct matching_type<sizeof(short)> { typedef short type; };
template<> struct matching_type<sizeof(int)> { typedef int type; };
template<> struct matching_type<sizeof(long)> { typedef long type; };
template<> struct matching_type<sizeof(long long)> { typedef long long type; };

template<typename T>
matching_type<sizeof(T)>::type foo(T bar)
{
    return reinterpret_cast<decltype(foo(bar))>(bar);
}

回答3:

As long as you understand the risk that the same sized type may not be the convertible, you could simply plugin a mpl::map..

typedef map<
      pair<int_<sizeof(char)>, char>,
      pair<int_<sizeof(short)>, short>,
      pair<int_<sizeof(int)>, int>,
      pair<int_<sizeof(long long)>, long long>
    > m;

e.g.

#include <algorithm>
#include <iostream>

#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>

using namespace boost::mpl;

typedef map<
      pair<int_<sizeof(char)>, char>,
      pair<int_<sizeof(short)>, short>,
      pair<int_<sizeof(int)>, int>,
      pair<int_<sizeof(long long)>, long long>
    > m;

template <typename T>
typename at<m, int_<sizeof(T)>>::type foo(T bar)
{ return reinterpret_cast<decltype(foo(bar))>(bar); }


struct doh
{
    std::string a, b, c;
};

int main()
{
    {
      char c;
      static_assert(std::is_same<decltype(foo(c)), char>::value, "error");
    }
    {
      short c;
      static_assert(std::is_same<decltype(foo(c)), short>::value, "error");
    }
    {
      int c;
      static_assert(std::is_same<decltype(foo(c)), int>::value, "error");
    }
    {
      long long c;
      static_assert(std::is_same<decltype(foo(c)), long long>::value, "error");
    }
    {
      double c;
      static_assert(std::is_same<decltype(foo(c)), long long>::value, "error");
    }    
    {
      doh c;
      static_assert(std::is_same<decltype(foo(c)), void_>::value, "error");
    }    
}

回答4:

Something like this perhaps:

template <size_t N> struct SuitablySized;

template<> struct SuitablySized<sizeof(char)> {
  typedef char type;
};
template<> struct SuitablySized<sizeof(short)> {
  typedef short type;
};
// Add more cases to taste

template <typename T>
typename SuitablySized<sizeof(T)>::type foo(T bar);

回答5:

A type tag:

template<class T>struct tag{using type=T;};

void_t (coming in C++17 to a compiler near you):

template<class...>struct voider{using type=void;};
template<class...Ts>using void_t=typename voider<Ts...>::type;

enable_first_t takes a pack of std::enable_if (note the lack of _t), and returns the first that passes the test. You can use tag<X> to replace std::enable_if<true, X>:

template<class T,class=void>struct has_type:std::false_type{};
template<class T>struct has_type<T, void_t<typename T::type>>:std::true_type{};

namespace details {
  template<class, class...Ts>
  struct enable_first {};
  template<class T0, class...Ts>
  struct enable_first<std::enable_if_t< !has_type<T0>{} >, T0, Ts... >:enable_first<void, Ts...> {};
  template<class T0, class...Ts>
  struct enable_first<std::enable_if_t<  has_type<T0>{} >, T0, Ts...>:T0 {};
}

template<class...Ts>using enable_first_t=typename details::enable_first<void, Ts...>::type;

template<class T>
using result = enable_first_t<
  std::enable_if<sizeof(T)==sizeof(char), char>,
  std::enable_if<sizeof(T)==sizeof(short), short>,
  std::enable_if<sizeof(T)==sizeof(long), long>,
  tag<int> // default
>;

this behaves a lot like a switch, but the statements are full boolean expressions.

live example