C++ 初始值设定项列表奇怪地依赖于参数的顺序?
我有以下代码片段:C++ 初始值设定项列表奇怪地依赖于参数的顺序?,c++,c++11,lambda,initializer-list,typetraits,C++,C++11,Lambda,Initializer List,Typetraits,我有以下代码片段: #include <type_traits> #include <limits> #include <initializer_list> #include <cassert> template <typename F, typename... FIn> auto min_on(F f, const FIn&... v) -> typename std::common_type<FIn...>
#include <type_traits>
#include <limits>
#include <initializer_list>
#include <cassert>
template <typename F, typename... FIn>
auto min_on(F f, const FIn&... v) -> typename std::common_type<FIn...>::type
{
using rettype = typename std::common_type<FIn...>::type;
rettype result = std::numeric_limits<rettype>::max();
(void)std::initializer_list<int>{((f(v) < result) ? (result = static_cast<rettype>(v), 0) : 0)...};
return result;
}
int main()
{
auto mod2 = [](int a)
{
return a % 2;
};
assert(min_on(mod2, 2) == 2); // PASSES as it should
assert(min_on(mod2, 3) == 3); // PASSES as it should
assert(min_on(mod2, 2, 3) == 3); // PASSES but shouldn't - should be 2
assert(min_on(mod2, 2, 3) == 2); // FAILS but shouldn't - should be 2
}
会有用的。这里可能有什么问题?如果
f(v)
,则函数将result
设置为v
。使用mod2
作为f
,f(v)
将只产生0、1或-1。这意味着,如果所有值都大于1,result
将被设置为测试的最后一个v
,因为f(v)
将始终小于result
。试着把负数放在一组正数中间,负数永远是结果,不管你把它放在哪里。
assert(min_on(mod2, 2, 3, 4, -3, 7, 6, 5) == -3);
也许你想要这个:
std::initializer_list<int>{((f(v) < f(result)) ? (result = static_cast<rettype>(v), 0) : 0)...};
这显然是错误的。因此,也许您可以要求第一个参数:
template <typename F, typename FirstT, typename... FIn>
auto min_on(F f, const FirstT& first, const FIn&... v)
-> typename std::common_type<FirstT, FIn...>::type
{
using rettype = typename std::common_type<FirstT, FIn...>::type;
rettype result = first;
(void)std::initializer_list<int>{((f(v) < f(result)) ? (result = static_cast<rettype>(v), 0) : 0)...};
return result;
}
存储最佳输入和最佳输出会更有效,避免重复调用f。完美答案!我不能要求更多了。非常感谢Benjamint这应该返回可选的,或者在空列表中失败。在编译时,您的建议在empty上失败,这很好。注意:通过跟踪答案的索引并在找到答案后切换回值,您可以不惜代价避免重复复制结果类型(可能会很昂贵!),但这是非常先进的。
[](int a) { return -a; }
template <typename F, typename FirstT, typename... FIn>
auto min_on(F f, const FirstT& first, const FIn&... v)
-> typename std::common_type<FirstT, FIn...>::type
{
using rettype = typename std::common_type<FirstT, FIn...>::type;
rettype result = first;
(void)std::initializer_list<int>{((f(v) < f(result)) ? (result = static_cast<rettype>(v), 0) : 0)...};
return result;
}
template <typename F, typename FirstT, typename... FIn>
auto min_on(F f, const FirstT& first, const FIn&... v)
-> typename std::common_type<FirstT, FIn...>::type
{
using rettype = typename std::common_type<FirstT, FIn...>::type;
rettype result = first;
auto result_trans = f(result);
auto v_trans = result_trans;
(void)std::initializer_list<int>{(
(v_trans = f(v), v_trans < result_trans)
? (result = static_cast<rettype>(v), result_trans = v_trans, 0) : 0)...};
return result;
}