Initializing an int array as a type argument

Question

This code fails to compile:

  unordered_map<char, int[4]> inv = {
    { 'a', {{0,0,1,0}} } 
  }

What's the proper way to initialize this int array when passed as a type argument?

I've tried: int[], array<int,4> but they all give the error:

no instance of constructor "std::unordered_map<_Kty, _Ty, _Hasher,
_Keyeq, _Alloc>::unordered_map [with _Kty=char, _Ty=std::pair<Cell *, int []>, _Hasher=std::hash<char>, _Keyeq=std::equal_to<char>,
 _Alloc=std::allocator<std::pair<const char, std::pair<Cell *, int []>>>]" matches the argument list

This code should complain that unordered_map is undefined. Post actual code that fails to compile. — Öö Tiib
– Öö Tiib, Commented Mar 13, 2019 at 9:38
@ÖöTiib: ??? Should user provide the needed header files? Is this now common in SO? — Klaus
– Klaus, Commented Mar 13, 2019 at 9:39
@Klaus The user should provide a minimal reproducible example. — super
– super, Commented Mar 13, 2019 at 9:40
@Klaus he is hiding a using std; so what unordered_map is he using? He may have implemented his own :) — Lars Kakavandi-Nielsen
– Lars Kakavandi-Nielsen, Commented Mar 13, 2019 at 9:41
@Klaus considering OP's comments to accepted answer it seems that defect was in a #include being missing. — Öö Tiib
– Öö Tiib, Commented Mar 13, 2019 at 9:46

lubgr · Accepted Answer · 2019-03-13 09:37:03Z

5

This should work:

#include <array>

std::unordered_map<char, std::array<int, 4>> inv = {{ 'a', {{0, 0, 1, 0}} }};

edited Mar 13, 2019 at 9:37

answered Mar 13, 2019 at 9:33

lubgr

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4 Comments

lubgr Over a year ago

Which compiler are you using? Works for me with clang 7 and gcc 8.

dukevin Over a year ago

Actually include <array> fixed it, threw me off because the error was the same as when I used int[]

Lars Kakavandi-Nielsen Over a year ago

@lubgr can you explain why int[4] doesn't work but std::array<int, 4> does? Just curious :)

lubgr Over a year ago

@LarsNielsen std::unordered_map<char, int[4]> myMap; will compile, but any insertion will fail to compile, because you can't assign to an existing int[4] array, e.g. myMap['a'] = {1, 2, 3, 4}; gives you something like "int[4] is not assignable". Same as int test[4] = {1, 2, 3, 4}; (which works) and then test = {5, 6, 7, 8}.

Constantinos Glynos · Accepted Answer · 2019-03-13 09:39:18Z

2

You can initialize the array with one set of angle brackets.

int main()
{
    std::unordered_map<char,std::array<int,4>> inv = {{ 'a', {1,2,3,4} }};

    for (auto &&i : inv)
        std::cout<< i.first << "->" <<i.second[0] <<std::endl;
}

Example: https://rextester.com/FOVMLC70132

answered Mar 13, 2019 at 9:39

Constantinos Glynos

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