如何在可变参数模板中有多个参数包? [英] How can I have multiple parameter packs in a variadic template?
本文介绍了如何在可变参数模板中有多个参数包?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
函数 one()接受一个参数包。函数 two()接受两个。每个包被约束为以类型 A 和 B 包装。为什么不能实例化 two()
Function one() accepts one parameter pack. Function two() accepts two. Each pack is constrained to be wrapped in types A and B. Why is it impossible to instantiate two()?
template <typename T>
struct A {};
template <typename T>
struct B {};
template <typename... Ts>
void one(A<Ts> ...as) {
}
template <typename... Ts, typename... Us>
void two(A<Ts> ...as, B<Us> ...bs) {
}
int main() {
auto a = A<int>();
auto b = B<int>();
// Just fine
one();
one(a);
one(a, a);
// All errors
two();
two(a);
two(a, b);
}
尝试使用gcc和clang。
Tried with gcc and clang.
sam@wish:~/x/cpp$ gcc -std=c++0x variadic_templates.cpp
variadic_templates.cpp: In function ‘int main()’:
variadic_templates.cpp:23:7: error: no matching function for call to ‘two()’
variadic_templates.cpp:23:7: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
variadic_templates.cpp:24:8: error: no matching function for call to ‘two(A<int>&)’
variadic_templates.cpp:24:8: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
variadic_templates.cpp:25:11: error: no matching function for call to ‘two(A<int>&, B<int>&)’
variadic_templates.cpp:25:11: note: candidate is:
variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...)
sam@wish:~/x/cpp$ clang -std=c++0x variadic_templates.cpp
variadic_templates.cpp:23:3: error: no matching function for call to 'two'
two();
^~~
variadic_templates.cpp:11:6: note: candidate function template not viable: requires at least 1 argument, but 0 were provided
void two(A<Ts> ...as, B<Us> ...bs) {}
^
variadic_templates.cpp:24:3: error: no matching function for call to 'two'
two(a);
^~~
variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 1 was provided
void two(A<Ts> ...as, B<Us> ...bs) {}
^
variadic_templates.cpp:25:3: error: no matching function for call to 'two'
two(a, b);
^~~
variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 2 were provided
void two(A<Ts> ...as, B<Us> ...bs) {}
^
3 errors generated.
推荐答案
这里是另一种使用模板模板参数:
Here is another way to have several parameters packs using template template parameters:
#include <iostream>
template <typename... Types>
struct foo {};
template < typename... Types1, template <typename...> class T
, typename... Types2, template <typename...> class V
, typename U >
void
bar(const T<Types1...>&, const V<Types2...>&, const U& u)
{
std::cout << sizeof...(Types1) << std::endl;
std::cout << sizeof...(Types2) << std::endl;
std::cout << u << std::endl;
}
int
main()
{
foo<char, int, float> f1;
foo<char, int> f2;
bar(f1, f2, 9);
return 0;
}
这篇关于如何在可变参数模板中有多个参数包?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文