英文:
Will copy constructor be called when calling insert of std::unordered_map
问题
以下是您要翻译的内容：
"I have a std::unordered_map and I found that the object I insert into it is different from that I got from it by using for range traversing.
I doubt that there might be some object copying happening here but after I added some dumping to the copy constructor, it is not called at all.
Could any one tell me what is happening in the background when inserting and traversing a std::unordered_map.
I tried the following code and it dumped:
[Result]
mypair constuctormypair constuctorstring1 address:0x7ffccb813ba0string2 address:0x7ffccb813bd0++++++++++++++++auto &x address:0x55fb40529378string2: 0.5auto &x address:0x55fb405292c8string1: 0.3++++++++++++++++auto x address:0x7ffccb813c00string2: 0.5auto x address:0x7ffccb813c00string1: 0.3++++++++++++++++
[Source]
#include <iostream>#include <string>#include <unordered_map>class mypair : public std::pair<std::string,double> {public:mypair(std::string str, double num):std::pair<std::string,double>(str, num) {std::cout<<"mypair constuctor"<<std::endl;}mypair( const mypair& ) {std::cout<<"mypair copy constuctor"<<std::endl;}mypair& operator=(const mypair&) {std::cout<<"mypair copy assignment"<<std::endl;return *this;}};int main (){std::unordered_map<std::string,double> myMap;mypair string1 ("string1", 0.3);mypair string2 ("string2", 0.5);std::cout << "string1 address:" << &string1 << std::endl;std::cout << "string2 address:" << &string2 << std::endl;std::cout <<"++++++++++++++++"<< std::endl;myMap.insert (string1);myMap.insert (string2);for (auto& x: myMap) {std::cout << "auto &x address:"<< &x << std::endl<< x.first << ": " << x.second << std::endl;}std::cout <<"++++++++++++++++"<< std::endl;for (auto x: myMap) {std::cout << "auto x address:"<< &x << std::endl<< x.first << ": " << x.second << std::endl;}std::cout <<"++++++++++++++++"<< std::endl;return 0;}
英文:
I have a std::unordered_map and I found that the object I insert into it is different from that I got from it by using for range traversing.
I doubt that there might be some object copying happening here but after I added some dumping to the copy constructor, it is not called at all.
Could any one tell me what is happening in the background when inserting and traversing a std::unordered_map.
I tried the following code and it dumped:
[Result]
mypair constuctormypair constuctorstring1 address:0x7ffccb813ba0string2 address:0x7ffccb813bd0++++++++++++++++auto &x address:0x55fb40529378string2: 0.5auto &x address:0x55fb405292c8string1: 0.3++++++++++++++++auto x address:0x7ffccb813c00string2: 0.5auto x address:0x7ffccb813c00string1: 0.3++++++++++++++++
[Source]
#include <iostream>#include <string>#include <unordered_map>class mypair : public std::pair<std::string,double> {public:mypair(std::string str, double num):std::pair<std::string,double>(str, num) {std::cout<<"mypair constuctor"<<std::endl;}mypair( const mypair& ) {std::cout<<"mypair copy constuctor"<<std::endl;}mypair& operator=(const mypair&) {std::cout<<"mypair copy assignment"<<std::endl;return *this;}};int main (){std::unordered_map<std::string,double> myMap;mypair string1 ("string1", 0.3);mypair string2 ("string2", 0.5);std::cout << "string1 address:" << &string1 << std::endl;std::cout << "string2 address:" << &string2 << std::endl;std::cout <<"++++++++++++++++"<< std::endl;myMap.insert (string1);myMap.insert (string2);for (auto& x: myMap) {std::cout << "auto &x address:"<< &x << std::endl<< x.first << ": " << x.second << std::endl;}std::cout <<"++++++++++++++++"<< std::endl;for (auto x: myMap) {std::cout << "auto x address:"<< &x << std::endl<< x.first << ": " << x.second << std::endl;}std::cout <<"++++++++++++++++"<< std::endl;return 0;}
答案1
得分: 5
以下是翻译好的内容：
你没有看到对mypair的复制构造函数的任何调用，因为std::unordered_map内部不使用该对类型。
using value_type = std::pair<const Key, Value>;
...实际上是内部使用的对类型。你的mypair可以隐式转换为value_type，因此应该调用以下成员函数：
template<class P>std::pair<iterator, bool> insert(P&& value);
这既不复制也不复制分配mypair，它将从传递的对的内容构造一个std::pair。
如果你想尽量减少复制和开销，你应该使用emplace或try_emplace。
英文:
You're not seeing any calls to the copy constructor of mypair, because that is not the pair type that the std::unordered_map uses internally.
using value_type = std::pair<const Key, Value>;
... is the pair type that is actually used inside. Your mypair is implicitly convertible to the value_type, so the following member function should be called:
template< class P >std::pair<iterator, bool> insert( P&& value );
This neither copies nor copy-assigns a mypair, it will construct a std::pair from the contents of the pair you pass.
If you want to minimize copying and overhead as much as possible, you should instead use emplace or try_emplace.
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