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 b         


        

Answer 1

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));

Answer 2

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;
}

Answer 3

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));
   

Answer 4

Approach using structure in C++

int MAX_DATA_PER_INSTR = 8;
//struct to hold the values. remember to write the constructor
struct InstrChar
{
  InstrChar(int in[MAX_DATA_PER_INSTR]) { 
    //c alternative is memcopy
    std::copy(in, in+MAX_DATA_PER_INSTR, data);
  }
  int data[MAX_DATA_PER_INSTR];
};

// create a key value pair 
std::map <int, InstrChar> address_instructions;
std::map <int, InstrChar>::iterator it;

// sample array, 8 elements
int xp[MAX_DATA_PER_INSTR ] = {31,4,3,4,4,3,1,2};
address_instructions.insert(std::pair<int, InstrChar>(PC, xp));
it = address_instructions.find(PC);
InstrChar buf1 = it->second;
//integer pointer to the array, can be dereferenced as *p, *(p+1), ....    //*(p+7)
int *p = buf1.data;

//in case you need to print these values out. They can also be referred to as buf1.data[0], buf1.data[1], buf1.data[2]
printf("%d\n", (*p));
printf("%d\n", *(p+1));
printf("%d\n", *(p+2));
printf("%d\n", *(p+3));
printf("%d\n", *(p+4));
printf("%d\n", *(p+5));
printf("%d\n", *(p+6));
printf("%d\n", *(p+7));

Answer 5

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.

Answer 6

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));