Optimizing initialization on maps: forwarding the key - c++

Question

I've got a question that should be interesting. I'd like to "forward initialize" an item in a std::unordered_map upon construction.
These are the details. I've got a hash map from std::string to a custom class prop, which in my dreams, would initialize a member variable calculating the hash of the string passed to std::unordered_map::operator[].
This is a handy code I've written, but I don't know where to start.
Why this trouble? Because I'd like to avoid something like "if the string is NOT in the container calculate the hash; do stuff with prop". Avoiding this if could be something that might affect my performances. So the constructor, as well as the hashing, will be executed only once, when the map adds a new item in the container. It would be great.
Any hints?
Thanks & Cheers!
#include <iostream>
#include <string>
#include <unordered_map>
class prop
{
public:
prop(std::string s = "") : s_(s), hash_(std::hash<std::string>()(s))
{
// Automagically forwarding the string in the unordered_map...
};
std::string s_;
std::size_t hash_;
int x;
};
int main(int argc, const char * argv[])
{
// Forward the std::string to the prop constructor... but how?
std::unordered_map<std::string, prop> map;
map["ABC"].x = 1;
map["DEF"].x = 2;
map["GHI"].x = 3;
map["GHI"].x = 9; // This should not call the constructor: the hash is there already
std::cout << map["ABC"].x << " : " << map["ABC"].s_ << " : " << map["ABC"].hash_ << std::endl;
std::cout << map["DEF"].x << " : " << map["DEF"].s_ << " : " << map["DEF"].hash_ << std::endl;
std::cout << map["GHI"].x << " : " << map["GHI"].s_ << " : " << map["GHI"].hash_ << std::endl;
std::cout << map["XXX"].x << " : " << map["XXX"].s_ << " : " << map["XXX"].hash_ << std::endl;
return 0;
}

Answer 1

Just use your prop class as a key, instead of string:
#include <iostream>
#include <string>
#include <unordered_map>
class prop
{
public:
prop(std::string s = "") : s_(s), hash_(std::hash<std::string>()(s))
{
// Automagically forwarding the string in the unordered_map...
};
std::string s_;
std::size_t hash_;
};
int main(int argc, const char * argv[])
{
// Forward the std::string to the prop constructor... but how?
std::unordered_map<prop, int, ...> map( ... );
prop pABC( "ABC" ), pDEF( "DEF" ), pGHI( "GHI" );
map[pABC] = 1;
map[pDEF] = 2;
map[pGHI] = 3;
map[pGHI] = 9;
std::cout << map[pABC] << " : " << pABC.s_ << " : " << pABC.hash_ << std::endl;
std::cout << map[pDEF] << " : " << pDEF.s_ << " : " << pDEF.hash_ << std::endl;
std::cout << map[pGHI] << " : " << pGHI.s_ << " : " << pGHI.hash_ << std::endl;
prop pXXX( "XXX" );
std::cout << map[pXXX] << " : " << pXXX.s_ << " : " << pXXX.hash_ << std::endl;
return 0;
}
I omitted custom hash and compare function, the idea should be clear without it.