std :: unordered_set :: find-仅为find（）构造一个实例

Question

A lot of times I see my key is actually inside my value .
For example:

struct Elem {
    int key;
    // ... Other variables ...
}

That makes me want to use std::unordered_set instead of std::unordered_map , because I already have the key stored inside my value - no need to waste more place for std::unordered_map 's .first ( key ).

Then I start implementing with std::unordered_set and get to the place I need to perform a find() over my std::unordered_set .
Then I realize I need to create an empty-shell Elem so I would be able to find() , beacuse std::unordered_set::find gets a Key for input

template < class Key,                    // unordered_set::key_type/value_type
       class Hash = hash<Key>,           // unordered_set::hasher
       class Pred = equal_to<Key>,       // unordered_set::key_equal
       class Alloc = allocator<Key>      // unordered_set::allocator_type
       > class unordered_set;

Sometimes building an empty-shell Elem is hard / wasteful / maybe even not possible?

For example, when my key/value is

• An iterator
• A reference
• A class with specific c'tor (not constructing the instance only with the key )

Q. Am I missing something?
Q. Is there a way to do find() that isn't wasteful? I mean that doesn't make me create an instance I didn't want to

• Something really strange to me - that I already should have the element I'm looking for in order to find it , or at least an empty-shell of it.

Answer 1

When choosing a data structure to hold your data you need to consider your use case.

If you want to look up data from a key you should use a map . If you just want to store unique values in a collection and you don't need to look them up use set .

I don't see why its so much trouble to insert a element as map.emplace_back(elem.key, elem) vs set.emplace_back(elem) if it means that down the road you can just query the elem as map.at(key) or map[key] vs having create an empty elem .

Besides, std::set does the whole key thingamajig (roughly) underwater anyway. (source: What is the difference between set vs map in C++? )