map::value_type 示例代码

本文展示了如何在 C++ 中使用 `map::value_type` 插入和访问映射元素。通过实例代码解释了 `value_type` 的作用,避免了隐式类型转换,并演示了插入和打印 map 中元素的关键步骤。

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// map_value_type.cpp
// compile with: /EHsc
#include <map>
#include <iostream>

int main( )
{
   using namespace std;
   typedef pair <const int, int> cInt2Int;
   map <int, int> m1;
   map <int, int> :: key_type key1;
   map <int, int> :: mapped_type mapped1;
   map <int, int> :: value_type value1;
   map <int, int> :: iterator pIter;
   // value_type can be used to pass the correct type
   // explicitly to avoid implicit type conversion
   m1.insert ( map <int, int> :: value_type ( 1, 10 ) );

   // Compare other ways to insert objects into a map
   m1.insert ( cInt2Int ( 2, 20 ) );
   m1[ 3 ] = 30;

   // Initializing key1 and mapped1
   key1 = ( m1.begin( ) -> first );
   mapped1 = ( m1.begin( ) -> second );

   cout << "The key of first element in the map is "
        << key1 << "." << endl;

   cout << "The data value of first element in the map is "
        << mapped1 << "." << endl;

   // The following line would cause an error because
   // the value_type is not assignable
   // value1 = cInt2Int ( 4, 40 );

   cout  << "The keys of the mapped elements are:";
   for ( pIter = m1.begin( ) ; pIter != m1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are:";
   for ( pIter = m1.begin( ) ; pIter != m1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;
}

 

  Output

The key of first element in the map is 1.
The data value of first element in the map is 10.
The keys of the mapped elements are: 1 2 3.
The values of the mapped elements are: 10 20 30.
In file included from D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/hashtable_policy.h:34, from D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/hashtable.h:35, from D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/unordered_map.h:33, from D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/unordered_map:41, from D:/obj/i wanna own world to me/i wanna own world to me.cpp:9: D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/tuple: In instantiation of 'constexpr std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; long long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long long unsigned int ..._Indexes2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = SDL_GL_Image]': D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/tuple:2877:63: required from 'constexpr std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; _Args2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = SDL_GL_Image]' 2877 | typename _Build_index_tuple<sizeof...(_Args2)>::__type()) | ^ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/stl_construct.h:97:14: required from 'constexpr decltype (::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...) [with _Tp = pair<const __cxx11::basic_string<char>, SDL_GL_Image>; _Args = {const piecewise_construct_t&, tuple<__cxx11::basic_string<char, char_traits<char>, allocator<char> >&&>, tuple<>}; decltype (::new(void*(0)) _Tp) = pair<const __cxx11::basic_string<char>, SDL_GL_Image>*]' 97 | { return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); } | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/alloc_traits.h:536:21: required from 'static constexpr void std::allocator_traits<std::allocator<_Up> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>; _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Tp = std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>, true>; allocator_type = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>, true> >]' 536 | std::construct_at(__p, std::forward<_Args>(__args)...); | ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/hashtable_policy.h:2024:36: required from 'std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>, true> >; __node_ptr = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>, true> >::value_type*]' 2024 | __node_alloc_traits::construct(__alloc, __n->_M_valptr(), | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~ 2025 | std::forward<_Args>(__args)...); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/hashtable.h:312:35: required from 'std::_Hashtable<_Key, _Value, _Alloc, _ExtractKey, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits>::_Scoped_node::_Scoped_node(std::_Hashtable<_Key, _Value, _Alloc, _ExtractKey, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits>::__hashtable_alloc*, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Key = std::__cxx11::basic_string<char>; _Value = std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image> >; _ExtractKey = std::__detail::_Select1st; _Equal = std::equal_to<std::__cxx11::basic_string<char> >; _Hash = std::hash<std::__cxx11::basic_string<char> >; _RangeHash = std::__detail::_Mod_range_hashing; _Unused = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<true, false, true>; std::_Hashtable<_Key, _Value, _Alloc, _ExtractKey, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits>::__hashtable_alloc = std::_Hashtable<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image>, std::allocator<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image> >, std::__detail::_Select1st, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true> >::__hashtable_alloc]' 312 | _M_node(__h->_M_allocate_node(std::forward<_Args>(__args)...)) | ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/hashtable_policy.h:870:42: required from 'std::__detail::_Map_base<_Key, std::pair<const _Key, _Val>, _Alloc, std::__detail::_Select1st, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true>::mapped_type& std::__detail::_Map_base<_Key, std::pair<const _Key, _Val>, _Alloc, std::__detail::_Select1st, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true>::operator[](key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Val = SDL_GL_Image; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image> >; _Equal = std::equal_to<std::__cxx11::basic_string<char> >; _Hash = std::hash<std::__cxx11::basic_string<char> >; _RangeHash = std::__detail::_Mod_range_hashing; _Unused = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<true, false, true>; mapped_type = SDL_GL_Image; key_type = std::__cxx11::basic_string<char>]' 870 | typename __hashtable::_Scoped_node __node { | ^~~~~~ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/bits/unordered_map.h:992:20: required from 'std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type& std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::operator[](key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = SDL_GL_Image; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, SDL_GL_Image> >; mapped_type = SDL_GL_Image; key_type = std::__cxx11::basic_string<char>]' 992 | { return _M_h[std::move(__k)]; } | ~~~~^ D:/obj/i wanna own world to me/i wanna own world to me.cpp:164:57: required from here 164 | glBindTexture(GL_TEXTURE_2D, images["sprBrownBlock"s].textureID); | ^ D:/SelfUsing/Language/CLion 2025.1.3/mingw/lib/gcc/x86_64-w64-mingw32/14.2.0/include/c++/tuple:2888:9: error: no matching function for call to 'SDL_GL_Image::SDL_GL_Image()' 2888 | second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...) | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:/obj/i wanna own world to me/i wanna own world to me.cpp:43:14: note: candidate: 'SDL_GL_Image::SDL_GL_Image(const char*)' 43 | explicit SDL_GL_Image(const char* path) { | ^~~~~~~~~~~~ D:/obj/i wanna own world to me/i wanna own world to me.cpp:43:14: note: candidate expects 1 argument, 0 provided D:/obj/i wanna own world to me/i wanna own world to me.cpp:39:7: note: candidate: 'constexpr SDL_GL_Image::SDL_GL_Image(const SDL_GL_Image&)' 39 | union SDL_GL_Image { | ^~~~~~~~~~~~ D:/obj/i wanna own world to me/i wanna own world to me.cpp:39:7: note: candidate expects 1 argument, 0 provided ninja: build stopped: subcommand failed.
最新发布
08-05
<think>我们正在讨论的是在C++中使用`unordered_map`插入`SDL_GL_Image`对象时遇到的错误,错误信息是“default constructor missing”。这意味着在尝试插入元素时,`unordered_map`需要调用`SDL_GL_Image`的默认构造函数,但该构造函数不存在。 ### 问题分析 `std::unordered_map`的`insert`方法在插入元素时,有几种方式,其中一种是通过`std::pair`来插入键值对。但有时,`unordered_map`可能会尝试使用值类型的默认构造函数来创建临时对象,然后再进行复制或移动。如果值类型(这里是`SDL_GL_Image`)没有默认构造函数,就会导致编译错误。 具体来说,当我们使用`unordered_map::insert`的其中一种形式,例如: ```cpp std::unordered_map<Key, T> myMap; myMap.insert(std::make_pair(key, T())); // 这里需要T有默认构造函数 ``` 或者在某些情况下,`unordered_map`内部可能会尝试构造一个默认初始化的值对象。 ### 解决方案 1. **避免需要默认构造函数**:使用`unordered_map`的`emplace`方法或`insert`方法中不需要默认构造函数的版本。 - `emplace`方法:直接在容器内构造元素,避免临时对象的创建。这样,我们可以使用`SDL_GL_Image`的任意构造函数。 ```cpp myMap.emplace(key, arg1, arg2, ...); // 传递构造SDL_GL_Image所需的参数 ``` 或者,如果`SDL_GL_Image`支持移动构造,也可以这样: ```cpp myMap.emplace(key, SDL_GL_Image(...)); // 使用参数构造一个临时对象,然后移动 ``` 2. **使用`insert`的正确形式**:如果一定要用`insert`,可以传递一个已经构造好的键值对,这样就不会调用默认构造函数。 ```cpp myMap.insert({key, SDL_GL_Image(arg1, arg2, ...)}); ``` 3. **检查`SDL_GL_Image`的构造函数**:确保你有一个合适的构造函数来创建对象。如果`SDL_GL_Image`没有默认构造函数,那么你就不能无参构造它,必须提供参数。 ### 示例代码 假设`SDL_GL_Image`有一个构造函数,它接受一个文件路径字符串作为参数: ```cpp class SDL_GL_Image { public: SDL_GL_Image(const std::string& filepath); // 构造函数,需要文件路径 // ... 没有默认构造函数 }; ``` 那么,在`unordered_map`中插入元素可以这样: ```cpp #include <unordered_map> #include <string> int main() { std::unordered_map<std::string, SDL_GL_Image> imageMap; // 使用emplace imageMap.emplace("texture1", "path/to/image1.png"); // 或者使用insert imageMap.insert(std::make_pair("texture2", SDL_GL_Image("path/to/image2.png"))); return 0; } ``` ### 注意事项 - 如果`SDL_GL_Image`的拷贝构造函数被删除(例如,因为资源管理需要移动语义),那么你可能需要使用`emplace`来直接构造,或者使用移动语义(如`std::move`)。 - 如果`SDL_GL_Image`对象很大,考虑使用指针(如`std::unique_ptr`)存储在`unordered_map`中,以避免拷贝开销,同时也能解决默认构造问题(因为指针有默认构造)。但这样会增加一层间接访问。 ### 总结 错误“default constructor missing”是因为`unordered_map`的某些操作需要值类型具有默认构造函数。解决方法是通过`emplace`或带参数的`insert`来插入元素,避免调用默认构造函数。同时,确保提供正确的参数来调用`SDL_GL_Image`的构造函数。 如果问题仍然存在,请检查`SDL_GL_Image`的构造函数是否可用,以及是否所有必要的参数都已提供。 §§相关问题§§ 1. C++中`emplace`和`insert`有什么区别? 2. 如何为自定义类实现移动语义? 3. 在`unordered_map`中存储大型对象的最佳实践是什么? 4. 如何避免`unordered_map`插入时的拷贝开销?</think>### 解决C++ `unordered_map`插入`SDL_GL_Image`时缺少默认构造函数的错误 当您在C++中使用`std::unordered_map`插入`SDL_GL_Image`对象时遇到"default constructor missing"错误,这是因为`unordered_map`的某些操作需要值类型具有默认构造函数。以下是详细分析和解决方案: #### 问题原因分析 1. **`unordered_map`的底层机制**: - `unordered_map`在插入元素时(如`insert()`或`operator[]`),可能需要先构造一个值类型的默认初始化对象 - 当您尝试访问不存在的键时(如`map[key]`),容器会自动插入一个默认构造的值对象 - 如果值类型`SDL_GL_Image`没有默认构造函数,就会触发编译错误 2. **SDL_GL_Image的限制**: - SDL图像类通常禁用默认构造函数,因为它们需要特定资源初始化 - 错误示例代码: ```cpp std::unordered_map<std::string, SDL_GL_Image> textureMap; textureMap["player"]; // 需要默认构造SDL_GL_Image → 错误! ``` #### 解决方案 ##### 方法1:使用`emplace()`直接构造(推荐) ```cpp std::unordered_map<std::string, SDL_GL_Image> textureMap; // 使用emplace直接传递构造参数 textureMap.emplace("player", "textures/player.png", SDL_IMAGE_FORMAT_PNG); // 或者使用try_emplace(C++17+) textureMap.try_emplace("enemy", "textures/enemy.jpg", SDL_IMAGE_FORMAT_JPEG); ``` - ✅ 优点:避免临时对象构造,直接使用参数创建对象 - ✅ 无需默认构造函数 ##### 方法2:使用指针存储 ```cpp std::unordered_map<std::string, std::unique_ptr<SDL_GL_Image>> textureMap; // 插入时直接构造对象 textureMap["player"] = std::make_unique<SDL_GL_Image>("textures/player.png"); // 访问时使用指针语法 if (textureMap.find("player") != textureMap.end()) { textureMap["player"]->Render(); // 使用->操作符 } ``` - ✅ 优点:避免对象拷贝,支持多态 - ✅ 指针类型有默认构造函数(nullptr) ##### 方法3:自定义默认构造行为 如果必须使用值语义,可通过包装类实现: ```cpp struct TextureWrapper { std::optional<SDL_GL_Image> texture; TextureWrapper() = default; // 默认构造合法 // 延迟初始化 void Init(const std::string& path) { texture.emplace(path, SDL_IMAGE_FORMAT_AUTO); } }; std::unordered_map<std::string, TextureWrapper> textureMap; textureMap["player"].Init("textures/player.png"); ``` #### 关键注意事项 1. **避免使用`operator[]`进行查找**: ```cpp // 错误:可能触发默认构造 auto& tex = textureMap["missing_texture"]; // 正确:使用find() if (auto it = textureMap.find("player"); it != textureMap.end()) { it->second.Render(); } ``` 2. **插入前检查键是否存在**: ```cpp if (textureMap.count("new_texture") == 0) { textureMap.emplace("new_texture", "path/to/texture.png"); } ``` 3. **C++版本影响**: - C++17+:优先使用`try_emplace()`,性能更好且不要求可默认构造 - C++11:使用`emplace()`或指针方案 #### 完整示例 ```cpp #include <SDL_image.h> #include <unordered_map> #include <memory> int main() { std::unordered_map<std::string, std::unique_ptr<SDL_GL_Image>> textures; // 安全插入 textures.emplace("background", std::make_unique<SDL_GL_Image>("bg.png", SDL_IMAGE_FORMAT_PNG)); // 安全访问 if (auto it = textures.find("background"); it != textures.end()) { it->second->Render(); } // 添加新纹理(避免operator[]) textures.emplace("character", std::make_unique<SDL_GL_Image>("char.png", SDL_IMAGE_FORMAT_PNG)); return 0; } ``` 通过上述方法,您可以绕过SDL_GL_Image缺少默认构造函数的问题,同时保持代码的安全性和高效性。选择方案时优先考虑`emplace`/`try_emplace`或智能指针方案,它们能最好地满足资源管理需求。
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