C++ 不能使用map，除非我为Custom\u Class定义了没有参数的默认构造函数

除非我执行以下两项操作之一，否则上述代码不起作用：-

1删除map的用法

2定义一个不要求参数的空构造函数

是否使用了空构造函数？我试着做一个coutdefault，如果值在映射中不存在，则构造该值，这就是它不起作用的原因

使用或替代：

无效生成房屋字符串cName { Cat-pcName； 这个->Cats.push_backp； 此->CatName\u Cat.insert\u或\u assignp.name，p； }

---

如前所述，如果元素不存在，map的方括号操作符default将构造该元素。因此，如果调用该运算符，则映射类型必须是默认可构造的

这是因为map对象尝试默认初始化Cat对象。因此，在这里定义默认构造函数是一种方法。只需执行Cat=default，我知道它正在尝试这样做，但它在哪里以及为什么要尝试这样做？为什么在我没有插入或要求它这样做的情况下，它会默认初始化Cat对象。是否创建默认对象来填充某些位置？如果是，那么为什么呢？因为无论何时创建对象，构造函数都会在幕后被调用。因此，当调用map[]时，它尝试初始化Cat对象，但失败了，因为它需要一个默认的构造函数。我们的问题是从运行时的角度来看的。跟踪代码不显示正在使用的默认构造函数。但是错误消息是在编译时发出的。[]运算符需要一个默认构造函数，该函数将作为映射模板的一部分生成和编译，即使您的代码从未调用它。

#include<bits/stdc++.h>
using namespace std;

class Cat
{
    public:
        string name;
        // Cat(){}
        Cat(string name_given)
        {
            this->name=name_given;
        }
};

class House
{
    public: 
        vector<Cat> Cats;
        map<string,Cat> CatName_Cat;
        House(string CatName)
        {
            generateHouse(CatName);
        }

    private:
        void generateHouse(string cName)
        {
            Cat p(cName);
            this->Cats.push_back(p);
            this->CatName_Cat[p.name]=p;
        }
};
int main()
{
    Cat p("McCat");
}

In file included from /usr/include/c++/7/functional:54:0,
                 from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:71,
                 from dumber.cpp:1:
/usr/include/c++/7/tuple: In instantiation of ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Cat]’:
/usr/include/c++/7/tuple:1641:63:   required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; _Args2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Cat]’
/usr/include/c++/7/ext/new_allocator.h:136:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Cat>; _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Tp = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Cat> >]’
/usr/include/c++/7/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Cat>; _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Tp = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Cat> > >]’
/usr/include/c++/7/bits/stl_tree.h:626:32:   required from ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_construct_node(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, Cat>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, Cat> >; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Cat> >*]’
/usr/include/c++/7/bits/stl_tree.h:643:21:   required from ‘std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, Cat>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, Cat> >; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, Cat> >*]’
/usr/include/c++/7/bits/stl_tree.h:2398:33:   required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_emplace_hint_unique(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, Cat>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, Cat> >; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::__cxx11::basic_string<char>, Cat> >]’
/usr/include/c++/7/bits/stl_map.h:493:8:   required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = Cat; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Cat> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = Cat; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]’
dumber.cpp:30:28:   required from here
/usr/include/c++/7/tuple:1652:70: error: no matching function for call to ‘Cat::Cat()’
         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
                                                                      ^
dumber.cpp:9:3: note: candidate: Cat::Cat(std::__cxx11::string)
   Cat(string name_given)
   ^~~
dumber.cpp:9:3: note:   candidate expects 1 argument, 0 provided
dumber.cpp:4:7: note: candidate: Cat::Cat(const Cat&)
 class Cat
       ^~~
dumber.cpp:4:7: note:   candidate expects 1 argument, 0 provided
dumber.cpp:4:7: note: candidate: Cat::Cat(Cat&&)
dumber.cpp:4:7: note:   candidate expects 1 argument, 0 provided