#include <iostream>
#include <any>
#include <vector>
#include <map>
#include <unordered_map>
#include <string>
using namespace std;
// enable_if_t = MapType implements .find(KeyType key), .begin(), .end(), and .begin()
// and MapType::iterator it has it->first and it->second
template<typename MapType>
decltype(declval<MapType>().begin()->second) MapGetOrDefault(
MapType mymap,
decltype(declval<MapType>().begin()->first) key,
decltype(declval<MapType>().begin()->second) defaultValue = decltype(declval<MapType>().begin()->second){}
)
{
auto it = mymap.find(key);
if (it!=mymap.end()) return it->second;
else return defaultValue;
}
int main()
{
map<string, int> a;
unordered_map<int, string> b;
a["1"] = 2;
cout << MapGetOrDefault(a, "1") << " " << MapGetOrDefault(a, "2") << " " << MapGetOrDefault(a, "2", -1) << "\n";
b[1] = "hello";
cout << MapGetOrDefault(b, 1) << " " << MapGetOrDefault(b, 2) << " " << MapGetOrDefault(b, 3, "world") << "\n";
return 0;
}
I'm trying to make a generic get_or_default()
function that can be used with all types of map. There are 4 conditions that a class must meet to be counted as map, like shown in the code comment.
How can I do this using C type traits? Also, how to change decltype(declval<MapType>().begin()->first)
into something more clean?
Edit: is enable_if_t
even needed in this case? My goal is to just prevent compilation
CodePudding user response:
Maps have member aliases mapped_type
and key_type
(and value_type
) that you can use :
#include <iostream>
#include <any>
#include <vector>
#include <map>
#include <unordered_map>
#include <string>
using namespace std;
template<typename MapType>
typename MapType::mapped_type MapGetOrDefault(
MapType mymap,
typename MapType::key_type key,
typename MapType::mapped_type defaultValue = typename MapType::mapped_type{}
)
{
auto it = mymap.find(key);
if (it!=mymap.end()) return it->second;
else return defaultValue;
}
int main()
{
map<string, int> a;
unordered_map<int, string> b;
a["1"] = 2;
cout << MapGetOrDefault(a, "1") << " " << MapGetOrDefault(a, "2") << " " << MapGetOrDefault(a, "2", -1) << "\n";
b[1] = "hello";
cout << MapGetOrDefault(b, 1) << " " << MapGetOrDefault(b, 2) << " " << MapGetOrDefault(b, 3, "world") << "\n";
return 0;
}
Template aliases can help for more terse syntax:
template <typename T> using mapped_type = typename T::mapped_type;
template <typename T> using key_type = typename T::key_type;
template<typename MapType>
mapped_type<MapType> MapGetOrDefault(
MapType mymap,
key_type<MapType> key,
mapped_type<MapType> defaultValue = mapped_type<MapType>{}
)
{
auto it = mymap.find(key);
if (it!=mymap.end()) return it->second;
else return defaultValue;
}
PS: I didnt change it because it wasn't part of the question, but you should not make a copy of the map when passing it to the function, rather use const MapType& mymap
for the argument.
PPS: As mentioned by Caleth in a comment, with a transparent comparator, ie one that has a Comparator::is_transparent
member type, you might want to support the overload (4) listed here: https://en.cppreference.com/w/cpp/container/map/find by introducing another template argument KeyType
that can be deduced from the key
paramter.