我一直在尝试一切我能想到的方法来获得 _CallWithRightmostArgsInner
函数正确失败,以便 SFINAE 可以正常工作,通过这次尝试,VS2013 给了我错误:
error C2039: 'type' : is not a member of 'std::enable_if<false,void>'
有什么想法吗?有没有更好的替代方案?这里的想法是,我想对 Function 进行函数调用,前提是 Function 采用 NumArgs 表示的数字或参数。最后两个可变参数应转发给函数并返回结果。
template <typename Function, int NumArgs>
class SplitParameters {
public:
typedef typename function_traits<Function>::result_type result_type;
template <typename ... RightArgs>
static result_type CallWithRightmostArgs(const Function& call, RightArgs && ... rightArgs) {
static_assert(sizeof...(RightArgs) >= NumArgs, "Unable to make function call with fewer than minimum arguments.");
return _CallWithRightmostArgs(call, std::forward<RightArgs>(rightArgs)...);
}
private:
template <typename ... RightArgs>
static result_type _CallWithRightmostArgs(const Function& call, RightArgs && ... rightArgs) {
return _CallWithRightmostArgsInner(call, std::forward<RightArgs>(rightArgs)...);
}
// note the '==' vs '!=' in these two functions. I would assume that only one could exist
template <typename LeftArg, typename ... RightArgs, typename std::enable_if<sizeof...(RightArgs) != NumArgs>::type* = 0>
static result_type _CallWithRightmostArgsInner(const Function& call, LeftArg, RightArgs && ... rightArgs) {
return _CallWithRightmostArgs(call, std::forward<RightArgs>(rightArgs)...);
}
template <typename LeftArg, typename ... RightArgs, typename std::enable_if<sizeof...(RightArgs) == NumArgs>::type* = 0>
static result_type _CallWithRightmostArgsInner(const Function& call, LeftArg, RightArgs && ... rightArgs) {
return call(std::forward<RightArgs>(rightArgs)...);
}
};
最佳答案
通过将代码更改为
,我可以在 g++-4.8 上使用此功能 #include <iostream>
template <class T>
struct function_traits
{
typedef void result_type;
};
template <typename Function, int NumArgs>
class SplitParameters {
public:
typedef typename function_traits<Function>::result_type result_type;
template <typename ... RightArgs>
static result_type CallWithRightmostArgs(const Function& call, RightArgs && ... rightArgs) {
static_assert(sizeof...(RightArgs) >= NumArgs,
"Unable to make function call with fewer than minimum arguments.");
return _CallWithRightmostArgs(call, std::forward<RightArgs>(rightArgs)...);
}
private:
template <typename ... RightArgs>
static result_type _CallWithRightmostArgs(const Function& call, RightArgs && ... rightArgs) {
return _CallWithRightmostArgsInner(call, std::forward<RightArgs>(rightArgs)...);
}
// note the '==' vs '!=' in these two functions. I would assume that only one could exist
template <typename LeftArg, typename ... RightArgs, class = typename std::enable_if<sizeof...(RightArgs) != NumArgs -1 >::type>
static result_type _CallWithRightmostArgsInner(const Function& call, LeftArg, RightArgs && ... rightArgs) {
return _CallWithRightmostArgsInner(call, std::forward<RightArgs>(rightArgs)...);
}
template <typename ... RightArgs, class = typename std::enable_if<sizeof...(RightArgs) == NumArgs>::type>
static result_type _CallWithRightmostArgsInner(const Function& call, RightArgs && ... rightArgs) {
return call(std::forward<RightArgs>(rightArgs)...);
}
};
void f(int i, int j)
{
std::cout << i << ' ' << j << std::endl;
}
int main()
{
SplitParameters<decltype(f), 2>::CallWithRightmostArgs(f, 1, 2, 3, 4);
}
编译器不喜欢你调用 _CallWithRightmostArgs
来自_CallWithRightmostArgsInner
,我假设您实际上是在尝试调用 Inner
功能。
g++ 也不喜欢转换 0
至void*
在模板参数列表中,所以我将其更改为 class = enable_if<...>::type
相反。
虽然我没有详细调查失败的原因,但希望这对您来说足够好。
编辑:
关于typename enable_if<...>::type* = 0
被拒绝了,我记得std::array
也有类似的问题:
template <class T, int size>
void f(const std::array<T,size>&){}
这个小片段本身编译得很好,但是当你这样做时:
std::array<int,4> a;
f(a);
g++ gives:
test3.cpp: In function ‘int main()’:
test3.cpp:9:8: error: no matching function for call to ‘f(std::array<int, 4ul>&)’
f(a);
^
test3.cpp:9:8: note: candidate is:
test3.cpp:4:6: note: template<class T, int size> void f(const std::array<T, size>&)
void f(const std::array<T,size>&){}
^
test3.cpp:4:6: note: template argument deduction/substitution failed:
test3.cpp:9:8: note: mismatched types ‘int’ and ‘#‘integer_cst’ not supported by dump_type#<type error>’
f(a);
^
test3.cpp:9:8: note: ‘std::array<int, 4ul>’ is not derived from ‘const std::array<T, size>’
事实证明,问题是我将模板声明为 int
对于size
参数,但编译器得到的是 std::size_t
这与 int
不同。即使您可以轻松地在它们之间进行转换。
在上面的例子中,我什至无法替换 = 0
与 = NULL
因为那只是一个0L
从字面上看,我必须这样做 = (void*)0
让编译器接受它(因为 enable_if<true>::type
的默认类型是 void
)。
关于c++ - VS2013 上出现 SFINAE 错误?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/18745814/