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.

Answer 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));