How can I use an array as map value?

Question

I'm trying to create a map, where the key is an int, and the value is an array as follows:

int red[3]   = {1,0,0};
int green[3] = {0,1,0};
int blue[3]  = {0,0,1};

std::map<int, int[3]> colours;

colours.insert(std::pair<int,int[3]>(GLUT_LEFT_BUTTON,red));    // THIS IS LINE 24!
colours.insert(std::pair<int,int[3]>(GLUT_MIDDLE_BUTTON,blue));
colours.insert(std::pair<int,int[3]>(GLUT_RIGHT_BUTTON,green));

However, when I try to compile this code, I get the following error:

g++ (Ubuntu 4.4.1-4ubuntu8) 4.4.1

In file included from /usr/include/c++/4.4/bits/stl_algobase.h:66,
                 from /usr/include/c++/4.4/bits/stl_tree.h:62,
                 from /usr/include/c++/4.4/map:60,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:84: error: array used as initializer
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const std::pair<_U1, _U2>&) [with _U1 = int, _U2 = int [3], _T1 = const int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:101: error: array used as initializer
In file included from /usr/include/c++/4.4/map:61,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_map.h: In member function ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = int, _Tp = int [3], _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, int [3]> >]’:
../src/utils.cpp:30:   instantiated from here
/usr/include/c++/4.4/bits/stl_map.h:450: error: conversion from ‘int’ to non-scalar type ‘int [3]’ requested
make: *** [src/utils.o] Error 1

I really can't see where the error is. Or even if there's an error.

Solution 1

You can't copy arrays by value like that.

Here are several solutions, but I recommend #4 for your needs:

1. Use an std::vector instead of an array.

2. Use a map of pointers to arrays of 3 elements:

   int red[3]   = {1,0,0};
   int green[3] = {0,1,0};
   int blue[3]  = {0,0,1};
   std::map<int,int(*)[3]> colours;
   colours.insert(std::pair<int,int(*)[3]>(GLUT_LEFT_BUTTON,&red));
   colours.insert(std::pair<int,int(*)[3]>(GLUT_MIDDLE_BUTTON,&blue));
   colours.insert(std::pair<int,int(*)[3]>(GLUT_RIGHT_BUTTON,&green));
   // Watch out for scope here, you may need to create the arrays on the heap.
   

3. Use boost tuples instead of arrays of 3 elements.

4. Instead of using an array make a new struct that takes 3 elements. Make the map<int, newstructtype>. Or wrap your array in a struct as follows:

   struct Triple
   {
       int color[3];
   };
   
   // Later in code
   Triple red = {1, 0, 0}, green = {0, 1, 0}, blue = {0, 0, 1};
   std::map<int,Triple> colours;
   colours.insert(std::pair<int,Triple>(GLUT_LEFT_BUTTON,red));
   colours.insert(std::pair<int,Triple>(GLUT_MIDDLE_BUTTON,blue));
   colours.insert(std::pair<int,Triple>(GLUT_RIGHT_BUTTON,green));
   

Solution 2

Use std::tr1::array.

typedef std::tr1::array<int, 3> Triple;
Triple red   = {1, 0, 0};
Triple green = {0, 1, 0};
Triple blue  = {0, 0, 1};
std::map<int, Triple> colours;
colours.insert(std::make_pair(GLUT_LEFT_BUTTON,   red));
colours.insert(std::make_pair(GLUT_MIDDLE_BUTTON, blue));
colours.insert(std::make_pair(GLUT_RIGHT_BUTTON,  green));

Or std::array in C++11 and above

using  Triple = std::array<int, 3>; 
Triple red   = {1, 0, 0};
Triple green = {0, 1, 0};
Triple blue  = {0, 0, 1};
std::map<int, Triple> colours;
colours.insert(std::make_pair(GLUT_LEFT_BUTTON,   red));
colours.insert(std::make_pair(GLUT_MIDDLE_BUTTON, blue));
colours.insert(std::make_pair(GLUT_RIGHT_BUTTON,  green));

Solution 3

Arrays are not first class constructs in C++. They are not Copy Constructible nor Assignable which are requirements for values of std::map. You can use boost::array or std::vector.

Solution 4

Don't map to an int[], instead, map to an int* like this:

#include <iostream>
#include <map>
using namespace std;
int main(){
    std::map<int,int*> colors;
    int red[] = {3,7,9};
    colors[52] = red;
    cout << colors[52][1];  //prints 7
    colors[52][1] = 11;
    cout << colors[52][1];  //prints 11
    return 0;
}

Solution 5

Another alternative is to put the arrays in a wrapping struct:

    struct Wrapper { int value[3]; };

    // ...
    Wrapper red = {{1,0,0}};    
    std::map<int,Wrapper> colours;    
    colours.insert(std::pair<int,Wrapper>(1, red));

View more solutions

Comments

[Tom]

about 3) This is a small project, cant add boost, but will take a look, thanks.

[Brian R. Bondy]

@Tom: Added a 4) which may be your easiest solution.

[Eddy Pronk]

use std::make_pair. See below.

[underscore_d]

also known now as simply std::array ;-) making this really the only still-relevant answer left here. weird how it has so few votes...

[Amy Cohen]

When inserted in the 2nd way, how can they be accessed? For example, if I need to access the first element of GLUT_LEFT_BUTTON I tried doing `colours[0][0] but it's not printing the value I want.