using character array as a key in map

Question

I am trying to use char[] as a key for a map:

#include<iostream>
#include<map>
#include<string>
#include<utility>
#include<list>

using namespace std;

int main(void)
{
    map<char[10],int> m;

    char c[10]={'1','2','3','4','5','6','7','8','9','0'};
    m[c]=78;
    return 0;
}

But is throwing an error:

error: array used as initializer

second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)

even this doesn't work: m["abcdefghi"]=4;

How to use char [] as a key? I have couple of questions on SO but they didn't help much.

NOTE: I have used string but I want to try char[] just for curiosity

Vlad from Moscow · Accepted Answer · 2020-02-09 11:35:52Z

3

Arrays have neither the copy constructor nor the copy assignment operator. And there is no default operator < for arrays.

Instead of array use the standard container std::array.

For example

#include<iostream>
#include <array>
#include<map>

int main()
{
    std::map< std::array<char, 10>, int> m;

    std::array<char, 10> c = {'1','2','3','4','5','6','7','8','9','0'};
    m[c]=78;

    return 0;
}

answered Feb 9, 2020 at 11:35

Vlad from Moscow

313k27 gold badges204 silver badges358 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Agrudge Amicus Over a year ago

I think you are absolutely right, strict weak ordering (<) is required condition for the key which is not possible in char[].

Ted Lyngmo Over a year ago

@AgrudgeAmicus That part isn't a big problem. You could provide a comparator solving that:

template<typename T> struct less {     using type = typename std::remove_extent_t<T>;     static constexpr size_t extent = std::extent_v<T>;      constexpr bool operator()(const type (&a)[extent],                               const type (&b)[extent]) const {         return std::lexicographical_compare(std::begin(a), std::end(a),                                             std::begin(b), std::end(b));     } };

Aykhan Hagverdili · Accepted Answer · 2020-02-09 11:36:50Z

0

Use std::array:

#include <array>
#include <iostream>
#include <map>

int
main()
{
  using key_type = std::array<char, 10>;
  std::map<key_type, int> m;

  key_type c{ '1', '2', '3', '4', '5', '6', '7', '8', '9', '0' };
  m[c] = 78;
}

If you want variable size, use std::string_view:

#include <iostream>
#include <map>
#include <string_view>

int
main()
{
  std::map<std::string_view, int> m;

  char c[10] = { '1', '2', '3', '4', '5', '6', '7', '8', '9', '0' };
  m[{c, sizeof(c)}] = 78;
}

edited Feb 9, 2020 at 11:36

answered Feb 9, 2020 at 11:30

Aykhan Hagverdili

30.5k6 gold badges53 silver badges109 bronze badges

Collectives™ on Stack Overflow

using character array as a key in map

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related