我是否总是必须将可变参数模板参数放在模板参数的末尾?
template <size_t begin = 0U, typename... Tp>
void foo(tuple<Tp...> t);
例如,我遇到了各种错误:
#include <functional>
#include <iostream>
#include <string>
#include <tuple>
using namespace std;
template <typename... Tp, size_t begin = 0U>
enable_if_t<begin == sizeof...(Tp), void> foo(tuple<Tp...>& t){
cout << endl;
}
template <typename... Tp, size_t begin = 0U>
enable_if_t<begin < sizeof...(Tp), void> foo(tuple<Tp...>& t) {
cout << get<begin>(t) << ' ';
foo<Tp..., begin + 1>(t);
}
int main() {
tuple<int, string, float> t = make_tuple(42, "Jonathan Mee", 13.13);
foo(t);
}
When 在 gcc 5.1 上运行给我:
prog.cpp:在实例化中std::enable_if_t<(begin < sizeof... (Tp)), void> foo(std::tuple<_Elements ...>&) [with Tp = {int, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, float}; unsigned int begin = 0u; std::enable_if_t<(begin < sizeof... (Tp)), void> = void]
:
prog.cpp:21:7:从这里需要
prog.cpp:15:23: 错误:没有匹配的函数可调用foo(std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, float>&)
foo<Tp..., begin + 1>(t);
prog.cpp:8:43:注意:候选人:template<class ... Tp, unsigned int begin> std::enable_if_t<(begin == sizeof... (Tp)), void> foo(std::tuple<_Elements ...>&)
enable_if_t<begin == sizeof...(Tp), void> foo(tuple<Tp...>& t){
prog.cpp:8:43:注意:模板参数推导/替换失败:
prog.cpp:13:42:注意:候选人:template<class ... Tp, unsigned int begin> std::enable_if_t<(begin < sizeof... (Tp)), void> foo(std::tuple<_Elements ...>&)
enable_if_t<begin < sizeof...(Tp), void> foo(tuple<Tp...>& t) {
prog.cpp:13:42:注意:模板参数推导/替换失败:
当参数交换为:
template <size_t begin = 0U, typename... Tp>
void foo(tuple<Tp...> t);
程序正确运行:http://ideone.com/SozUbb
如果这确实要求可变参数模板参数放在最后,有人可以给我这个信息的来源吗?