如何在 std::unordered_map 中插入一个键但没有值
How to insert a key but no value into a std::unordered_map
我有一个
std::unordered_map< std::string, std::unordered_map<std::string, std::string> >
我只想插入密钥并稍后填写值。
例如:
#include <string>
#include <unordered_map>
int main() {
std::unordered_map< std::string, std::unordered_map<std::string, std::string> > mymap;
mymap.insert("TestKey");
}
给出错误:
source.cpp(6): error C2664: 'void std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>::insert(std::initializer_list<std::pair<const _Kty,_Ty>>)' : cannot convert argument 1 from 'const char [8]' to 'std::pair<const _Kty,_Ty> &&'
1> with
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> , _Hasher=std::hash<std::string>
1> , _Keyeq=std::equal_to<std::string>
1> , _Alloc=std::allocator<std::pair<const std::string,std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>>>
1> ]
1> and
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> ]
1> Reason: cannot convert from 'const char [8]' to 'std::pair<const _Kty,_Ty>'
1> with
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
如何添加没有值的键?
如果具有给定键的元素尚不存在,operator[]
将插入一个值初始化值。所以你可以简单地这样做:
mymap["TestKey"];
如果你想获取值的引用,那么
auto& val = mymap["TestKey"];
我有一个
std::unordered_map< std::string, std::unordered_map<std::string, std::string> >
我只想插入密钥并稍后填写值。
例如:
#include <string>
#include <unordered_map>
int main() {
std::unordered_map< std::string, std::unordered_map<std::string, std::string> > mymap;
mymap.insert("TestKey");
}
给出错误:
source.cpp(6): error C2664: 'void std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>::insert(std::initializer_list<std::pair<const _Kty,_Ty>>)' : cannot convert argument 1 from 'const char [8]' to 'std::pair<const _Kty,_Ty> &&'
1> with
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> , _Hasher=std::hash<std::string>
1> , _Keyeq=std::equal_to<std::string>
1> , _Alloc=std::allocator<std::pair<const std::string,std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>>>
1> ]
1> and
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> ]
1> Reason: cannot convert from 'const char [8]' to 'std::pair<const _Kty,_Ty>'
1> with
1> [
1> _Kty=std::string
1> , _Ty=std::unordered_map<std::string,std::string,std::hash<std::string>,std::equal_to<std::string>,std::allocator<std::pair<const std::string,std::string>>>
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
如何添加没有值的键?
operator[]
将插入一个值初始化值。所以你可以简单地这样做:
mymap["TestKey"];
如果你想获取值的引用,那么
auto& val = mymap["TestKey"];