Moving keys from unordered_map

Question

I've searched but I only found questions about move constructor with the mapped value, but I want to try something different.

Is it possible to use std::move the key from a std::unordered_map? The reason is quite simple: I'd like to construct an example where I create a vector from the map, wasting as little as possible of memory. I know it would mess up with the representation of the map, but hey, after all I will never use the map again, so it would make sense to move values out.

My guess is this: no, I cannot do that. However, I'd like some confirmation.

Here's a simple code. I expected to see the move constructor called, but I have the copy constructor called.

Cheers & Thanks!

#include <iostream>
#include <unordered_map>
#include <vector>
#include <string>
#include <utility>

class prop
{
public:
    prop(const std::string &s, int i) : s_(s), i_(i) { std::cout << "COPIED" << std::endl; };

    prop(std::string &&s, int i) : s_(std::move(s)), i_(i) { std::cout << "MOVED" << std::endl; };

    std::string s_;
    int         i_;
};

std::string gen_random(const int len) {
    static const char alphanum[] =
    "ABC";

    std::string s;
    s.resize(len);

    for (int i = 0; i < len; ++i) {
        s[i] = alphanum[rand() % (sizeof(alphanum) - 1)];
    }

    return s;
}

int main()
{
    const long n = 3, len = 4, max = 20;

    std::unordered_map<std::string, int> map;

    std::cout << ">>GENERATING" << std::endl;
    for (int i = 0; i < n; i++) map[gen_random(len)]++;

    if (map.size() < max)
    {
        std::cout << ">>MAP" << std::endl;
        for (auto &p : map) std::cout << p.first << " : " << p.second << std::endl;
    }

    std::cout << ">>POPULATING VEC" << std::endl;
    std::vector<prop> vec;
    vec.reserve(map.size());
    for (auto &p : map) vec.push_back(prop(p.first, p.second));

    if (map.size() < max)
    {
        std::cout << ">>VEC" << std::endl;
        for (auto &p : vec) std::cout << p.s_ << " : " << p.i_ << std::endl;
        std::cout << ">>MAP" << std::endl;
        for (auto &p : map) std::cout << p.first << " : " << p.second << std::endl;
    }

    std::cout << ">>POPULATING MOV" << std::endl;
    std::vector<prop> mov;
    mov.reserve(map.size());
    for (auto &p : map) mov.push_back(prop(std::move(p.first), p.second));

    if (map.size() < max)
    {
        std::cout << ">>MOV" << std::endl;
        for (auto &p : mov) std::cout << p.s_ << " : " << p.i_ << std::endl;
        std::cout << ">>MAP" << std::endl;
        for (auto &p : map) std::cout << p.first << " : " << p.second << std::endl;
    }

    return 0;
}

output

>>GENERATING
>>MAP
CBAC : 1
BCAC : 1
BBCC : 1
>>POPULATING VEC
COPIED
COPIED
COPIED
>>VEC
CBAC : 1
BCAC : 1
BBCC : 1
>>MAP
CBAC : 1
BCAC : 1
BBCC : 1
>>POPULATING MOV
COPIED
COPIED
COPIED
>>MOV
CBAC : 1
BCAC : 1
BBCC : 1
>>MAP
CBAC : 1
BCAC : 1
BBCC : 1
Program ended with exit code: 0

Answer 1

You cannot move, keys will be copied, since

value_type  std::pair<const Key, T>

http://en.cppreference.com/w/cpp/container/unordered_map so, here

for (auto &p : map)

p will be deduced to std::pair<const std::string, int>.