C++ STL 内 std::{bind/tuple/function} 简单实现

基本逻辑思考

首先是实现 function，这个比较简单，重载 operator() 就好，这里只实现对函数指针的包装

其次是实现 tuple，这个会比较绕，通过模板取第一个参数，然后用剩下的参数继续生成  tuple并继承，是一种递归的思想

有了 tuple 就要有 get()，这个就更比较绕了，首先是需要类似的方式实现获得 tuple 的值类型与元组类型，然后通过强制类型转换，获取对应的层级的 value

接下来是 bind，首先要解决的就是如何保存创建时的参数列表，这里就用到 tuple 来保存了

奇技淫巧还是运行函数时取相应的元组的对应位置的值，还是类似的方式，通过特化模板，公式是  <n, indexs...> => <n - 1, n - 1, indexs...>，比如 3 最后会生成 0 0 1 2 那么抛弃第一个，并用来展开元组，传递给函数指针

最重要的来了，就是如何实现 placeholders，简单来说就是在上一步的 operator() 增加传入参数，并制造成元组 r_args，然后带进一个 _unwrap_tuple 类，这里会重载 operator[] 根据传入数据结构，如果是 _placeholders<index> 那么取 r_args 相应的 index 位置，否则会直接  return

代码

不多说，还是直接放代码，仅作为参考，有写的不好的地方轻喷

/*
 * Author: SpringHack - springhack@live.cn
 * Last modified: 2020-02-19 10:16:17
 * Filename: main.cpp
 * Description: Created by SpringHack using vim automatically.
 */
#include <iostream>

namespace dosk { // begin namespace dosk

// function
template <typename... T>
class function;

template <typename Result, typename... Args>
class function<Result(Args...)> {
 private:
  Result (*function_)(Args...);
 public:
  typedef Result return_type;
  function() = default;
  function(Result (*fn)(Args...)) : function_(fn) {};
  Result operator()(Args... a) {
   return function_(a...);
  }
  function& operator=(Result (*fn)(Args...)) {
   function_ = fn;
   return *this;
  }
};

// tuple
template <typename... T>
class tuple;

template <typename HEAD, typename... LIST>
class tuple<HEAD, LIST...> : public tuple<LIST...> {
 public:
  HEAD value;
  tuple(HEAD head, LIST... list) : tuple<LIST...>(list...), value(head) {};
};

template <>
class tuple<> {};

// tuple get
template <int index, typename... T>
class _tuple_type;

template <int index, typename HEAD, typename... LIST>
class _tuple_type<index, tuple<HEAD, LIST...>> {
 public:
  typedef typename _tuple_type<index - 1, tuple<LIST...>>::value_type value_type;
  typedef typename _tuple_type<index - 1, tuple<LIST...>>::tuple_type tuple_type;
};

template <typename HEAD, typename... LIST>
class _tuple_type<0, tuple<HEAD, LIST...>> {
 public:
  typedef HEAD value_type;
  typedef tuple<HEAD, LIST...> tuple_type;
};

template <int index, typename HEAD, typename... LIST>
typename _tuple_type<index, tuple<HEAD, LIST...>>::value_type get(tuple<HEAD, LIST...> t) {
 typedef typename _tuple_type<index, tuple<HEAD, LIST...>>::value_type value_type;
 typedef typename _tuple_type<index, tuple<HEAD, LIST...>>::tuple_type tuple_type;
 value_type rv = ((tuple_type)t).value;
 return rv;
}

// bind
template <size_t...>
class _tuple_index {};

template <size_t n, size_t... indexs>
class _make_indexs : public _make_indexs<n - 1, n - 1, indexs...> {};

template<size_t... indexs>
class _make_indexs<0, indexs...> {
 public:
  typedef _tuple_index<indexs...> index_type;
};

namespace placeholders {

template <size_t index>
class _placeholders {};

_placeholders<0> _1;
_placeholders<1> _2;
_placeholders<2> _3;
_placeholders<3> _4;
_placeholders<4> _5;
_placeholders<5> _6;
_placeholders<6> _7;
_placeholders<7> _8;
_placeholders<8> _9;
_placeholders<9> _10;

template <typename... RArgs>
class _unwrap_tuple {
 public:
  tuple<RArgs...> r_args; 
  _unwrap_tuple(tuple<RArgs...> r_args) : r_args(r_args) {};
  template <typename R>
  R operator[](R r) {
   return r;
  }
  template <size_t index>
  auto operator[](placeholders::_placeholders<index>) {
   return get<index>(r_args);
  }
};

};

template <typename Func, typename... Args>
class bind_t {
 public:
  typedef typename _make_indexs<sizeof...(Args)>::index_type _indexs;
  typedef typename Func::return_type return_type;
  Func func;
  tuple<Args...> args;
  bind_t(Func func, Args... args): func(func), args(args...) {}
  template <typename... RArgs>
  return_type operator()(RArgs&&... _r_args) {
   tuple<RArgs...> r_args = tuple<RArgs...>(_r_args...);
   return run(_indexs(), r_args);
  }
  template <size_t... Idx, typename... RArgs>
  return_type run(_tuple_index<Idx...>, tuple<RArgs...> r_args) {
   return func(unwrap_args<Idx>(r_args)...);
  }
  template <size_t index, typename... RArgs>
  auto unwrap_args(tuple<RArgs...> r_args) {
   placeholders::_unwrap_tuple<RArgs...> _u_a(r_args);
   auto _m_a = get<index>(args);
   return _u_a[_m_a];
  }
};

template <typename Func, typename... Args>
bind_t<Func, Args...> bind(Func& func, Args&&... args) {
 return bind_t<Func, Args...>(func, args...);
}

}; // end namespace dosk



// Test code
std::string test_func(int a, const char * b) {
 return std::to_string(a) + std::string(b);
}

std::string test_bind_args(int a, int b, int c, int d, int e) {
 return std::to_string(a) + std::to_string(b) + std::to_string(c) + std::to_string(d) + std::to_string(e);
}

int main() {
 // Test tuple
 dosk::tuple<int, const char *> t(123, "456");
 std::cout << dosk::get<0>(t) << dosk::get<1>(t) << std::endl;
 // Test function
 dosk::function<std::string(int, const char *)> closure_1 = test_func;
 std::cout << closure_1(123, "456") << std::endl;
 // Test bind
 dosk::function<std::string(int, int, int, int, int)> closure_2 = test_bind_args;
 auto binder = dosk::bind(closure_2, 1, dosk::placeholders::_2, 3, dosk::placeholders::_1, 5);
 std::cout << binder(4, 2, 0) << std::endl;
 return 0;
}