C : use string& and unique_ptr<SomeClass>& for map<string, unique

I have following string& and unique_ptr&

const FilePath& path; // given as parameter
auto key = path.BaseName().value();

auto value = make_unique<SomeClass>();

I would like to use these in the following map

map<string, unique_ptr<SomeClass>> myMap;

When I do the following,

myMap.emplace(key, value);

I get this error:

note: in instantiation of function template specialization 'std::map<std::string, std::unique_ptr<SomeClass>>::emplace<const std::string &, std::unique_ptr<SomeClass> &>' requested here

I think this means that I am trying to put string& and unique_ptr& where string and unique_ptr is needed.

Would there be a way to use &-variable to a non-&-parameter?

Full code:

map<string, unique_ptr<SomeClass>> myMap;

void function (const FilePath& path) {
  auto key = path.BaseName().value();
  auto value = make_unique<SomeClass>();
  myMap.emplace(key, value);
}

CodePudding user response：

std::unique_ptr can't be copied, but only moved. You can use std::move to convert value to rvalue:

myMap.emplace(key, std::move(value));

CodePudding user response：

In this case you don't even need to use std::unique_ptr! Your map will own the instances of MyClass anyway. So why not make a map of SomeClass directly?

Don't dynamically allocate memory yourself unless you really have to.

#include <iostream>
#include <filesystem> // <== STL has its own filesystem 
#include <map>
#include <string>

// do not use : using namespace std; !

class SomeClass
{
public:
    explicit SomeClass(const int, const int ) {}
};

std::map<std::string_view, SomeClass> myMap;

void function(const std::filesystem::path& path) 
{
    auto key = path.filename(); 
    myMap.emplace( key, 1, 2 ); // 1,2 are the parameters for constructor of SomeClass
}