如何定义可变参数 class 模板的成员模板 function

Question

我正在尝试使用成员模板函数实现可变参数 class 模板，其模板 arguments 独立于 class 模板参数，但我在定义成员模板时遇到了麻烦。

我将我的问题简化为尝试编译它（抱歉无法弄清楚如何进一步简化）：

#include <iostream>
#include <string>
#include <typeindex>
#include <typeinfo>
#include <unordered_map>
#include <utility>
#include <vector>

template <class... Types>
class Foo {
public:
  Foo();

  template <class T>
  T& at(const std::string& key);

  template <class T>
  void insert(const std::string& key, const T& value);

private:
  std::tuple<std::unordered_map<std::string, Types>...> sets_;
  std::unordered_map<std::type_index, size_t> type_to_pos_;
};

template<class... Types>
Foo<Types...>::Foo() {
  std::vector<std::type_index> type_indices{std::type_index(typeid(Types))...};
  for (size_t i = 0; i < type_indices.size(); i++) {
    this->type_to_pos_.insert({type_indices[i], i});
  }
}

template<class T, class... Types>
T& Foo<Types...>::at(const std::string& key) {
  std::type_index type_idx{std::type_index(typeid(T))};
  size_t pos;

  pos = this->type_to_pos_.at(type_idx);
  return std::get<pos>(this->sets_).at(key);
}

template <class T, class... Types>
void Foo<Types...>::insert(const std::string& key, const T& value) {
  std::type_index type_idx{std::type_index(typeid(T))};
  size_t pos;

  pos = this->type_to_pos_.at(type_idx);
  std::get<pos>(this->sets_).insert({key, value});
}

int main(int argc, char** argv) {
  Foo<int, float, double> foo{};
  foo.insert("key", 1.0f);
  std::cout << foo.at<float>("key") << std::endl;
  return 0;
}

尝试编译 (C++11) 时，出现以下错误：

$ make
Scanning dependencies of target test
[ 50%] Building CXX object CMakeFiles/test.dir/main.cpp.o
/Users/Jasper/cpp_projects/playground/main.cpp:33:19: error: nested name specifier 'Foo<Types...>::'
      for declaration does not refer into a class, class template or class template partial
      specialization
T& Foo<Types...>::at(const std::string& key) {
   ~~~~~~~~~~~~~~~^
/Users/Jasper/cpp_projects/playground/main.cpp:37:9: error: invalid use of 'this' outside of a
      non-static member function
  pos = this->type_to_pos_.at(type_idx);
        ^
/Users/Jasper/cpp_projects/playground/main.cpp:38:24: error: invalid use of 'this' outside of a
      non-static member function
  return std::get<pos>(this->sets_).at(key);
                       ^
/Users/Jasper/cpp_projects/playground/main.cpp:38:40: error: use of undeclared identifier 'key'
  return std::get<pos>(this->sets_).at(key);
                                       ^
/Users/Jasper/cpp_projects/playground/main.cpp:42:21: error: nested name specifier 'Foo<Types...>::'
      for declaration does not refer into a class, class template or class template partial
      specialization
void Foo<Types...>::insert(const std::string& key, const T& value) {
     ~~~~~~~~~~~~~~~^
/Users/Jasper/cpp_projects/playground/main.cpp:43:19: error: redefinition of 'type_idx'
  std::type_index type_idx{std::type_index(typeid(T))};
                  ^
/Users/Jasper/cpp_projects/playground/main.cpp:34:19: note: previous definition is here
  std::type_index type_idx{std::type_index(typeid(T))};
                  ^
/Users/Jasper/cpp_projects/playground/main.cpp:44:10: error: redefinition of 'pos'
  size_t pos;
         ^
/Users/Jasper/cpp_projects/playground/main.cpp:35:10: note: previous definition is here
  size_t pos;
         ^
/Users/Jasper/cpp_projects/playground/main.cpp:46:9: error: invalid use of 'this' outside of a
      non-static member function
  pos = this->type_to_pos_.at(type_idx);
        ^
8 errors generated.
make[2]: *** [CMakeFiles/test.dir/main.cpp.o] Error 1
make[1]: *** [CMakeFiles/test.dir/all] Error 2
make: *** [all] Error 2

我很确定它归结为第一个和第五个错误，但无法弄清楚我做错了什么。 为什么Foo<Types...>不引用 class 模板？ 我怎样才能解决这个问题？

编辑：添加了实用程序库并修复了insert的返回值。

PS 为了简单起见，我删除了所有异常检查。

—————————- @songyuanyao 给出的答案解决了问题，但正如@songyuanyao 指出的那样，在编译时get不知道pos所以它不会编译。 这个解决方案有助于解决这个问题。

Answer 1

您应该分开两组模板参数：一组用于封闭的 class 模板，另一组用于成员 function 模板本身。 例如

template<class... Types> // for the enclosing class template
template<class T>        // for the member template
T& Foo<Types...>::at(const std::string& key) {
  ...
}

template<class... Types> // for the enclosing class template
template<class T>        // for the member template
void Foo<Types...>::insert(const std::string& key, const T& value) {
  ...
}