不能使用 map<string ，Custom_Class>除非我定义了没有参数的默认构造函数Custom_Class

Question

#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");
}

除非我做以下两件事中的任何一件，否则上面的代码都不起作用：-

(1( 删除地图的用法。

(2( 定义一个不要求参数的空构造函数。

是否正在使用空的构造函数？每次调用cout<<"usedn";时，我都尝试执行它，但输出为空。

错误：-

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

我知道地图实现中的某些部分失败了，但是什么以及为什么？

Answer 1

std::map::operator[]默认会在映射中不存在值的情况下构造该值，这就是它不起作用的原因。

使用std::map::try_emplace或std::map::insert_or_assign：

void generateHouse(string cName)
{
Cat p(cName);
this->Cats.push_back(p);
this->CatName_Cat.insert_or_assign(p.name, p);
}

Answer 2

std::map的方括号运算符default在元素不存在的情况下构造元素，如下所述。因此，如果调用该运算符，则映射的类型必须是默认可构造的。