(C++11) Variadic Ally 模板函数转发到另一个

Question

我正在尝试将我的尾巴从 parser 转发到 add_option，但 GCC 不允许这样做：

// Templated constructor
template <typename H = std::string, typename... T>
parser(H head, T... tail) {
    /* Add an option */
    add_option(std::forward<T>(tail)...);
}

// Base case
void add_option(std::string head) {
    /* Add an option */
}

// Recursive case
template <typename H = std::string, typename... T>
void add_option(H head, T... tail) {
    add_option(tail...);
}

相反，GCC 给了我这个错误：

parser.h: In instantiation of ‘void parser::add_option(H, T ...) [with H = const char*; T = {}]’:
parser.h:16:4:   required from ‘parser::parser(H, T ...) [with H = const char*; T = {const char*}]’
parser.cpp:7:26:   required from here
parser.h:24:4: error: no matching function for call to ‘parser::add_option()’
    add_option(tail...);
    ^
parser.h:24:4: note: candidates are:
parser.h:18:8: note: void parser::add_option(std::string)
   void add_option(std::string head) {
        ^
parser.h:18:8: note:   c1andidate expects 1 argument, 0 provided
parser.h:22:8: note: template<class H, class ... T> void parser::add_option(H, T ...)
   void add_option(H head, T... tail) {
        ^
parser.h:22:8: note:   template argument deduction/substitution failed:
parser.h:24:4: note:   candidate expects 2 arguments, 0 provided
    add_option(tail...);
    ^
Makefile:2: recipe for target 'parser' failed
make: *** [parser] Error 1

谁能告诉我我在这里做错了什么？我该如何解决这个问题？

Answer 1

struct parser {
  template <typename... Ts>
  parser(std::string head, Ts... ts) {
    add_options(std::move(head), std::forward<Ts>(ts)...);
  }

  // Base case
  void add_options(std::string head) {
    // I assume we store head somewhere.  `std::move` from it
    // if we don't directly store it, replace all std::string with std::string const&
    std::cout << "option: " << head.c_str() << "\n";
  }

  // Recursive case.  Make 2+ arguments explicit (probably not needed, but I like it)
  template <typename T0, typename... Ts>
  void add_options(std::string head, T0&& t0, Ts&&... ts) {
    add_options(std::move(head));
    add_options(std::forward<T0>(t0), std::forward<Ts>(ts)...);
  }
};

int main() {
  parser parse("hello", "world");
}

live example