Initializing a static std::map<int, int> in C++

Using C++11:

#include <map>
using namespace std;

map<int, char> m = {{1, 'a'}, {3, 'b'}, {5, 'c'}, {7, 'd'}};

Using Boost.Assign:

#include <map>
#include "boost/assign.hpp"
using namespace std;
using namespace boost::assign;

map<int, char> m = map_list_of (1, 'a') (3, 'b') (5, 'c') (7, 'd');

Best way is to use a function:

#include <map>

using namespace std;

map<int,int> create_map()
{
  map<int,int> m;
  m[1] = 2;
  m[3] = 4;
  m[5] = 6;
  return m;
}

map<int,int> m = create_map();

Here is another way that uses the 2-element data constructor. No functions are needed to initialize it. There is no 3rd party code (Boost), no static functions or objects, no tricks, just simple C++:

#include <map>
#include <string>

typedef std::map<std::string, int> MyMap;

const MyMap::value_type rawData[] = {
   MyMap::value_type("hello", 42),
   MyMap::value_type("world", 88),
};
const int numElems = sizeof rawData / sizeof rawData[0];
MyMap myMap(rawData, rawData + numElems);

Since I wrote this answer C++11 is out. You can now directly initialize STL containers using the new initializer list feature:

const MyMap myMap = { {"hello", 42}, {"world", 88} };

I would wrap the map inside a static object, and put the map initialisation code in the constructor of this object, this way you are sure the map is created before the initialisation code is executed.

In addition to the good top answer of using

const std::map<int, int> m = {{1,1},{4,2},{9,3},{16,4},{32,9}}

there's an additional possibility by directly calling a lambda that can be useful in a few cases:

const std::map<int, int> m = []()->auto {
  std::map<int, int> m;
  m[1]=1;
  m[4]=2;
  m[9]=3;
  m[16]=4;
  m[32]=9;
  return m;
}();

Clearly a simple initializer list is better when writing this from scratch with literal values, but it does open up additional possibilities:

const std::map<int, int> m = []()->auto {
  std::map<int, int> m;
  for(int i=1;i<5;++i) m[i*i]=i;
  m[32]=9;
  return m;
}();

(Obviously it should be a normal function if you want to re-use it; and this does require recent C++.)