編譯時獲取boost :: function arity嗎？

Question

我需要根據boost::function對象的參數（參數數量）在BOOST_PP_IF語句中做出決定。這可能嗎？

boost::function_types::function_arity做我在找什麼，但在運行時;我在編譯時需要它。

Answer 1

/其他條件出於某種原因，我包括保持打破，但不在預覽= [

#include <ostream> 
#include <iostream> 
#include <boost/function.hpp> 

// Assume that you want to print out "Function is N-arity" for general case. But "nularity" for 0 

template< int i > 
struct DarkSide 
{ 
template<class U> 
void operator()(std::ostream& out, const U& u) { out << "Function is "<<i<<"-arity"<<u; } 
void operator()(std::ostream& out, std::ostream& (*pf)(std::ostream&)) { out << "Function is "<<i<<"-arity"<<pf; } 
}; 

template<> 
struct DarkSide<0> 
{ 
template<class U> 
void operator()(std::ostream& out, const U& u) { out << "Function is nularity"<<u; } 
void operator()(std::ostream& out, std::ostream& (*pf)(std::ostream&)) { out << "Function is nularity"<<pf; } 
}; 

int main() { 
typedef boost::function< void () > vFv; 
typedef boost::function< void (int x) > vFi; 
DarkSide<vFv::arity>()(std::cout,"\n"); 
DarkSide<vFi::arity>()(std::cout,std::endl); 
} 

Answer 2

function_arity 

template<typename F> 
struct function_arity; 

Header 

#include <boost/function_types/function_arity.hpp> 

F 

    Callable builtin type 
function_arity<F> 

    Function arity as MPL - Integral Constant 
function_arity<F>::value 

    Constant value of the function arity 

注意，這是編譯時間常數

你應該從這裏開始： http://www.boost.org/doc/libs/1_43_0/libs/mpl/doc/index.html

或使用BOOST_PP_SEQ_FOR_EACH/BOOST_PP_REPEAT_FROM_TO產生，如果對function_arity<F>::value

Answer 3

如果你需要的是閱讀升壓的元數::功能，那麼你不需要做太多的工作：

#include <boost/function.hpp> 
#include <iostream> 
int main() { 
    std::cout << boost::function<void()>::arity << std::endl; 
    std::cout << boost::function<void(int)>::arity << std::endl; 
    std::cout << boost::function<void(int, int)>::arity << std::endl; 
}