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問題描述

在我使用

 um[key] = MapNode(key, value);

把新的 MapNode 加到 unordered_map 時，出現

main.cc:17:10: note: candidate constructor not viable: requires 2 arguments, but 0 were provided

問題分析

第一時間以為 MapNode 的 constructor 沒有定義好。
但細看錯誤訊息時發現， compilation error 是在我把新的 MapNode 加到 unordered_map 時發生。

 um[key] = MapNode(key, value);

以下是完整的 code:

#include "unordered_map"
#include "iostream"
#include "string"

using namespace std;

class MapNode {
private:
  string key_;
  string value_;

public:
  explicit MapNode(string key, string value);
  string Show();
};

MapNode::MapNode(string key, string value): key_(key), value_(value){} 

string MapNode::Show(){
  size_t size = key_.size() + value_.size() + 1;
  char buffer[size];
  snprintf(buffer,size,  "%s%s", key_.c_str(), value_.c_str());
  return buffer;
}

int main(int argc, char *argv[]) {
  unordered_map<string, MapNode> um;
  string key = "hello";
  string value = "world";
  um[key] = MapNode(key, value);
  
  for (auto &n: um) {
    cout << n.second.Show()<< endl;
  }

  return 0;
}

錯誤訊息：

$ g++ -std=c++11 -o main main.cc && ./main
In file included from main.cc:1:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/unordered_map:437:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__hash_table:16:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/algorithm:653:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/functional:495:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/bind_front.h:14:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/perfect_forward.h:14:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/tuple:1522:7: error: no matching constructor for initialization of 'MapNode'
      second(_VSTD::forward<_Args2>(_VSTD::get<_I2>(__second_args))...)
      ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__utility/pair.h:252:11: note: in instantiation of function template specialization 'std::pair<const s
td::string, MapNode>::pair<const std::string &, 0UL>' requested here
        : pair(__pc, __first_args, __second_args,
          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__memory/allocator.h:154:28: note: in instantiation of function template specialization 'std::pair<con
st std::string, MapNode>::pair<const std::string &>' requested here
        ::new ((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__memory/allocator_traits.h:290:13: note: in instantiation of function template specialization 'std::a
llocator<std::__hash_node<std::__hash_value_type<std::string, MapNode>, void *>>::construct<std::pair<const std::string, MapNode>, const std::piecewise_construct_t &, std::t
uple<const std::string &>, std::tuple<>>' requested here
        __a.construct(__p, _VSTD::forward<_Args>(__args)...);
            ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__hash_table:2473:20: note: in instantiation of function template specialization 'std::allocator_trait
s<std::allocator<std::__hash_node<std::__hash_value_type<std::string, MapNode>, void *>>>::construct<std::pair<const std::string, MapNode>, const std::piecewise_construct_t 
&, std::tuple<const std::string &>, std::tuple<>, void>' requested here
    __node_traits::construct(__na, _NodeTypes::__get_ptr(__h->__value_),
                   ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__hash_table:2093:29: note: in instantiation of function template specialization 'std::__hash_table<st
d::__hash_value_type<std::string, MapNode>, std::__unordered_map_hasher<std::string, std::__hash_value_type<std::string, MapNode>, std::hash<std::string>, std::equal_to<std:
:string>, true>, std::__unordered_map_equal<std::string, std::__hash_value_type<std::string, MapNode>, std::equal_to<std::string>, std::hash<std::string>, true>, std::alloca
tor<std::__hash_value_type<std::string, MapNode>>>::__construct_node_hash<const std::piecewise_construct_t &, std::tuple<const std::string &>, std::tuple<>>' requested here
        __node_holder __h = __construct_node_hash(__hash, _VSTD::forward<_Args>(__args)...);
                            ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/unordered_map:1743:21: note: in instantiation of function template specialization 'std::__hash_table<s
td::__hash_value_type<std::string, MapNode>, std::__unordered_map_hasher<std::string, std::__hash_value_type<std::string, MapNode>, std::hash<std::string>, std::equal_to<std
::string>, true>, std::__unordered_map_equal<std::string, std::__hash_value_type<std::string, MapNode>, std::equal_to<std::string>, std::hash<std::string>, true>, std::alloc
ator<std::__hash_value_type<std::string, MapNode>>>::__emplace_unique_key_args<std::string, const std::piecewise_construct_t &, std::tuple<const std::string &>, std::tuple<>
>' requested here
    return __table_.__emplace_unique_key_args(__k,
                    ^
main.cc:30:5: note: in instantiation of member function 'std::unordered_map<std::string, MapNode>::operator[]' requested here
  um[key] = MapNode(key, value);
    ^
main.cc:7:7: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were provided
class MapNode {
      ^
main.cc:7:7: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 0 were provided
main.cc:17:10: note: candidate constructor not viable: requires 2 arguments, but 0 were provided
MapNode::MapNode(string key, string value): key_(key), value_(value){} 
         ^

待解問題：

• :question: What is Copy and Move in Cpp ?
  • Ref: 一些筆記 — C++: Move Semantics
    
• :question: What is piecewise constructor ?
  • What’s up with std::piecewise_construct and std::forward_as_tuple?