Suppose I have a std::map<std::string, int>. Is there any way to use its at method with std::string_view? Here is a code snippet :
std::string_view key{ "a" };
std::map<std::string, int> tmp;
tmp["a"] = 0;
auto result = tmp.at(key);
Here is the output I have from clang 12.0
error: no matching member function for call to 'at'
auto result = tmp.at(key);
Three things are required for something like this to happen:
The map's comparator must be a transparent comparator (requires C++14, but you're already using string_view which is C++17, so this is a moot point).
The at() method must have an overload that participates in overload resolution when the container has a transparent comparator.
the parameter must be convertible to the map's key_type.
Neither of these are true in your example. The default std::less comparator is not a transparent comparator, there is no such overload for at(), and std::string does not have an implicit conversion from std::string_view.
There's nothing you can do about at(), however you can do something about the comparator namely using the (transparent std::void comparator), and then use find() instead of at(), which does have a suitable overload:
#include <map>
#include <string>
#include <string_view>
int main()
{
std::string_view key{ "a" };
std::map<std::string, int, std::less<void>> tmp;
tmp["a"] = 0;
auto iter=tmp.find(key);
}
There is no implicit conversion from std::string_view to std::string, that is why you get a "no matching member function" error. See: Why is there no implicit conversion from std::string_view to std::string?
There is a std::string constructor that will accept a std::string_view as input, however it is marked as explicit, so you will have to do this instead:
auto result = tmp.at(std::string(key));
Same if you wanted to use the map's operator[] with std::string_view:
tmp[std::string(key)] = ...;
auto result = tmp[std::string(key)];
std::string_viewwas introduced in C++17.atis expectingstd::string. That is why you are getting error. Just wrapped key with std::string.