我已经用模板元编程快速编写了一个reduce函数。我知道它并不完美,我应该检查要兼容的类型,返回类型......
#include <iostream>
#include <functional>
template <typename Functor, typename T>
T reduce(Functor f, T v) {
return v;
}
template<typename Functor, typename T1, typename... Ts>
T1 reduce(Functor f, T1 t1, Ts... ts) {
return f(t1, reduce(f, ts...));
}
int main() {
std::cout << reduce(std::plus<int>(), 1, 2, 3, 4, 5, 6, 7) << std::endl;
return 0;
}
关键是我想写像
这样的东西std::cout << reduce(std::plus<int>(), {1, 2, 3, 4, 5, 6, 7}) << std::endl;
或如果可能的话
const int input[3] = {1, 2, 3};
std::cout << reduce(std::plus<int>(), input) << std::endl;
答案 0 :(得分:1)
您不需要可变参数模板:
#include <algorithm>
#include <iostream>
#include <iterator>
namespace Detail {
template <typename Functor, typename List>
typename Functor::result_type reduce(Functor f, const List& list) {
using std::begin;
using std::end;
typename Functor::result_type result{};
auto pos = begin(list);
if(pos != end(list)) {
result = *pos;
while(++pos != end(list))
result = f(result, *pos);
}
return result;
}
} // namespace Detail
template <typename Functor, typename List>
typename Functor::result_type reduce(Functor f, const List& list) {
return Detail::reduce(f, list);
}
template <typename Functor, typename T>
typename Functor::result_type reduce(Functor f, std::initializer_list<T> list) {
return Detail::reduce(f, list);
}
int main () {
std::cout << reduce(std::plus<int>(), {1, 2, 3, 4, 5, 6, 7}) << std::endl;
const int input[3] = {1, 2, 3};
std::cout << reduce(std::plus<int>(), input) << std::endl;
}
使用'constexpr'的C ++ 11(g ++ 4.8.4)变体:
#include <algorithm>
#include <iostream>
#include <iterator>
// constexpr plus is C++14
template<typename T>
struct plus : public std::binary_function<T, T, T>
{
constexpr T operator()(const T& x, const T& y) const { return x + y; }
};
// constexpr begin is C++14
template<class T, std::size_t N>
inline constexpr T* begin(T (&array)[N]) { return array; }
// constexpr end is C++14
template<class T, std::size_t N>
inline constexpr T* end(T (&array)[N]) { return array + N; }
template <typename Functor, typename Iterator>
inline constexpr typename Functor::result_type
reduce(
const Functor& f,
const typename Functor::result_type& value,
Iterator first,
Iterator last)
{
return (first != last)
? reduce(f, f(value, *first), first + 1, last)
: value;
}
template <typename Functor, typename T, std::size_t N>
constexpr typename Functor::result_type reduce(const Functor& f, T (&array)[N]) {
// constexpr begin/end is C++14
// using std::begin;
// using std::end;
return reduce(f, typename Functor::result_type{}, begin(array), end(array));
}
template <typename Functor, typename T>
inline constexpr typename Functor::result_type reduce(
const Functor& f,
std::initializer_list<T> list)
{
return reduce(f, typename Functor::result_type{}, list.begin(), list.end());
}
int main () {
static constexpr int input[3] = {1, 2, 3};
static_assert(28 == reduce(plus<int>(), {1, 2, 3, 4, 5, 6, 7}), "Failure");
static_assert(6 == reduce(plus<int>(), input), "Failure");
}
答案 1 :(得分:1)
#include <iostream>
#include <functional>
#include <initializer_list>
template<typename Functor, typename T, size_t N>
T constexpr reduce(Functor f, T(&arr)[N]) {
typename std::remove_const<T>::type val = arr[0];
for(auto i = std::begin(arr)+1; i != std::end(arr); ++i) val = f(val, *i);
return val;
}
template<typename Functor, typename T>
T constexpr reduce(Functor f, std::initializer_list<T> il) {
T val = *il.begin();
for(auto i = il.begin()+1; i != il.end(); ++i) val = f(val, *i);
return val;
}
template<typename Functor, typename T, typename... Ts>
T constexpr reduce(Functor f, T t1, Ts... ts) {
return f(t1, reduce(f, std::initializer_list<T>({ts...})));
}
template<int value>
void print_constexpr() { std::cout << value << std::endl; }
int main() {
// run-time or compile-time
std::cout << reduce(std::plus<int>(), 1, 2, 3, 4, 5, 6, 7) << std::endl; // 28
std::cout << reduce(std::plus<int>(), {1, 2, 3, 4, 5, 6, 7}) << std::endl;// 28
const int input[3] = {1, 2, 3}; // 6
std::cout << reduce(std::plus<int>(), input) << std::endl;
// compile-time
print_constexpr< reduce(std::plus<int>(), 1, 2, 3, 4, 5, 6, 7) >(); // 28
print_constexpr< reduce(std::plus<int>(), {1, 2, 3, 4, 5, 6, 7}) >(); // 28
constexpr int input_constexpr[3] = {1, 2, 3}; // 6
print_constexpr< reduce(std::plus<int>(), input_constexpr) >(); // 6
return 0;
}
输出:
28
28
6
28
28
6
您可以在编译时使用以下任何一个:
reduce(std::plus<int>(), 1, 2, 3, 4, 5, 6, 7)
reduce(std::plus<int>(), {1, 2, 3, 4, 5, 6, 7})
reduce(std::plus<int>(), input)
使用任何constexpr
- 仿函数。