Suppose I have a private std::map inside my class std::map<std::string, double>. How can I transform is to std::map<std::string_view, double> to return to the user? I'd like to have the following prototype here
const std::map<std::string_view, double>&
MyClass::GetInternalMap() const;
You should not return the new map by const reference. You would be returning a dangling reference to a temp map that was destroyed when GetInternalMap() exits. If you want to return a const reference, then you should return the source map as-is, eg:
const std::map<std::string, double>& MyClass::GetInternalMap() const
{
return myvalues;
}
Otherwise, return the new map by value instead:
std::map<std::string_view, double> MyClass::GetInternalMap() const;
That being said, a std::map<std::string,double> is not directly convertible to a std::map<std::string_view,double>, so you would have to manually iterate the source map one element at a time, assigning each one to the target map, eg:
std::map<std::string_view, double> MyClass::GetInternalMap() const
{
std::map<std::string_view, double> result;
for(auto &p : myvalues) {
result[p.first] = p.second;
// or: result.emplace(p.first, p.second);
}
return result;
}
Fortunately, a std::pair<std::string,double> is implicitly convertible to a std::pair<std::string_view,double>, so you could simply use the map constructor that takes an iterator range as input, and let map assign the elements for you, eg:
std::map<std::string_view, double> MyClass::GetInternalMap() const
{
return {myvalues.begin(), myvalues.end()};
}
result map directly using the two-iterator constructor. I tested GCC, Clang, and MSVC; they all accepted return { begin(myvalues), end(myvalues) }; as the entire function body.
const&is already effectively a "view". None of the underlying memory will be copied.