我决定让代码更有趣，这是我第一次尝试的改进版本，它具有以下好处：
- 支持
enum类型
- 明确指定应该转换多少个参数
- 复杂部分的通用实现，每个使用它的函数都有一个小助手。
编码：
#include <iostream>
#include <utility>
#include <type_traits>
// an enum we would like to support
enum class tribool { FALSE, TRUE, FILE_NOT_FOUND };
// declare basic generic template
// (independent of a specific function you'd like to call)
template< template< class > class CB, std::size_t N, typename = std::tuple<> >
struct var_to_template;
// register types that should be supported
template< template< class > class CB, std::size_t N, typename... Cs >
struct var_to_template< CB, N, std::tuple< Cs... > >
{
// bool is pretty simple, there are only two values
template< typename R, typename... Args >
static R impl( bool b, Args&&... args )
{
return b
? var_to_template< CB, N-1, std::tuple< Cs..., std::true_type > >::template impl< R >( std::forward< Args >( args )... )
: var_to_template< CB, N-1, std::tuple< Cs..., std::false_type > >::template impl< R >( std::forward< Args >( args )... );
}
// for each enum, you need to register all its values
template< typename R, typename... Args >
static R impl( tribool tb, Args&&... args )
{
switch( tb ) {
case tribool::FALSE:
return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::FALSE > > >::template impl< R >( std::forward< Args >( args )... );
case tribool::TRUE:
return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::TRUE > > >::template impl< R >( std::forward< Args >( args )... );
case tribool::FILE_NOT_FOUND:
return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::FILE_NOT_FOUND > > >::template impl< R >( std::forward< Args >( args )... );
}
throw "unreachable";
}
// in theory you could also add int, long, ... but
// you'd have to switch on every possible value that you want to support!
};
// terminate the recursion
template< template< class > class CB, typename... Cs >
struct var_to_template< CB, 0, std::tuple< Cs... > >
{
template< typename R, typename... Args >
static R impl( Args&&... args )
{
return CB< std::tuple< Cs... > >::template impl< R >( std::forward< Args >( args )... );
}
};
// here's your function with the template parameters
template< bool B, tribool TB >
int HeavyLoop_impl( int arg )
{
for( int i = 0; i < 10000000; i++ ) {
arg += B ? 1 : 2;
arg += ( TB == tribool::TRUE ) ? 10 : ( TB == tribool::FALSE ) ? 20 : 30;
}
return arg;
}
// a helper class, required once per function that you'd like to forward
template< typename > struct HeavyLoop_callback;
template< typename... Cs >
struct HeavyLoop_callback< std::tuple< Cs... > >
{
template< typename R, typename... Args >
static R impl( Args&&... args )
{
return HeavyLoop_impl< Cs::value... >( std::forward< Args >( args )... );
}
};
// and here, everything comes together:
int HeavyLoop( bool b, tribool tb, int arg )
{
// you provide the helper and the number of arguments
// that should be converted to var_to_template<>
// and you provide the return type to impl<>
return var_to_template< HeavyLoop_callback, 2 >::impl< int >( b, tb, arg );
}
int main()
{
bool b = true;
tribool tb = tribool::FALSE;
int arg = 0;
int res = HeavyLoop( b, tb, arg );
std::cout << "res: " << res << std::endl;
return 0;
}
这是一个活生生的例子，以防你想玩它。