我是否总是必须将可变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);
}
当 run on gcc 5.1给我:
prog.cpp: In instantiation of
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: required from here
prog.cpp:15:23: error: no matching function for call tofoo(std::tuple<int, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, float>&)
foo<Tp..., begin + 1>(t);
prog.cpp:8:43: note: candidate: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: note: template argument deduction/substitution failed:
prog.cpp:13:42: note: candidate: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: note: template argument deduction/substitution failed:
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
如果这真的是可变参数模板参数最后的要求,有人可以给我这个信息的来源吗?
最佳答案
问题不在于模板声明。这很好:
template <typename... Tp, size_t begin = 0U>
void foo(tuple<Tp...> t);
问题是这个调用:
foo<Tp..., begin + 1>(t);
虽然您可以在参数包之后提供默认的模板参数,但您无法在以后实际设置它。编译器无法知道包在哪里结束以及包开始后的参数。
你应该翻转顺序将 begin
作为第一个参数,默认:
template <size_t begin = 0U, typename... Tp>
void foo(tuple<Tp...> t);
这样你的递归调用就可以:
foo<begin + 1>(t);
关于c++ - Variadic 模板参数总是必须放在最后吗?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35701888/