懒惰评估 [英] Lazy evaluation
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问题描述
我如何才能懒惰地评估std :: conditional中的第二个arg?
How can I lazy evaluate second arg in std::conditional?
#include "stdafx.h"
#include <type_traits>
struct Null{};
struct _1{enum {one = true,two = false};};
struct _2{enum {two = true, one = false};};
template<class T>
struct is_nulltype
{
enum {value = false};
};
template<>
struct is_nulltype<Null>
{
enum {value = true};
};
template<class T>
struct X : std::conditional<is_nulltype<T>::value,Null,typename std::conditional<T::one,_1,_2>::type>::type
{
};
int _tmain(int argc, _TCHAR* argv[])
{
X<Null> x;//won't compile no Null::one but I don't need that member in Null at all
return 0;
}
推荐答案
通常的方法是让std::conditional
在两个元函数之间进行选择:
The usual technique for this is to have the std::conditional
choose between two metafunctions:
template <typename T>
struct false_case {
typedef typename std::conditional<T::one,_1,_2>::type type;
};
struct always_null {typedef Null type;};
template<class T>
struct X :
std::conditional<is_nulltype<T>::value,
always_null,
false_case<T>
>::type::type { ... };
现在注意std::conditional
之后的两个::type
.
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