Initializing a static std::map in C++

Question

What is the right way of initializing a static map? Do we need a static function that will initialize it?

Answer 1

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.

Answer 2

Just wanted to share a pure C++ 98 work around:

#include <map>

std::map<std::string, std::string> aka;

struct akaInit
{
    akaInit()
    {
        aka[ "George" ] = "John";
        aka[ "Joe" ] = "Al";
        aka[ "Phil" ] = "Sue";
        aka[ "Smitty" ] = "Yando";
    }
} AkaInit;

Answer 3

If you are stuck with C++98 and don't want to use boost, here there is the solution I use when I need to initialize a static map:

typedef std::pair< int, char > elemPair_t;
elemPair_t elemPairs[] = 
{
    elemPair_t( 1, 'a'), 
    elemPair_t( 3, 'b' ), 
    elemPair_t( 5, 'c' ), 
    elemPair_t( 7, 'd' )
};

const std::map< int, char > myMap( &elemPairs[ 0 ], &elemPairs[ sizeof( elemPairs ) / sizeof( elemPairs[ 0 ] ) ] );

Answer 4

This is similar to PierreBdR, without copying the map.

#include <map>

using namespace std;

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

static map<int,int> m;
static bool _dummy = create_map (m);

Answer 5

It's not a complicated issue to make something similar to boost. Here's a class with just three functions, including the constructor, to replicate what boost did (almost).

template <typename T, typename U>
class create_map
{
private:
    std::map<T, U> m_map;
public:
    create_map(const T& key, const U& val)
    {
        m_map[key] = val;
    }

    create_map<T, U>& operator()(const T& key, const U& val)
    {
        m_map[key] = val;
        return *this;
    }

    operator std::map<T, U>()
    {
        return m_map;
    }
};

Usage:

std::map mymap = create_map<int, int >(1,2)(3,4)(5,6);

The above code works best for initialization of global variables or static members of a class which needs to be initialized and you have no idea when it gets used first but you want to assure that the values are available in it.

If say, you've got to insert elements into an existing std::map... here's another class for you.

template <typename MapType>
class map_add_values {
private:
    MapType mMap;
public:
    typedef typename MapType::key_type KeyType;
    typedef typename MapType::mapped_type MappedType;

    map_add_values(const KeyType& key, const MappedType& val)
    {
        mMap[key] = val;
    }

    map_add_values& operator()(const KeyType& key, const MappedType& val) {
        mMap[key] = val;
        return *this;
    }

    void to (MapType& map) {
        map.insert(mMap.begin(), mMap.end());
    }
};

Usage:

typedef std::map<int, int> Int2IntMap;
Int2IntMap testMap;
map_add_values<Int2IntMap>(1,2)(3,4)(5,6).to(testMap);

See it in action with GCC 4.7.2 here: http://ideone.com/3uYJiH

############### EVERYTHING BELOW THIS IS OBSOLETE #################

EDIT: The map_add_values class below, which was the original solution I had suggested, would fail when it comes to GCC 4.5+. Please look at the code above for how to add values to existing map.

template<typename T, typename U>
class map_add_values
{
private:
    std::map<T,U>& m_map;
public:
    map_add_values(std::map<T, U>& _map):m_map(_map){}
    map_add_values& operator()(const T& _key, const U& _val)
    {
        m_map[key] = val;
        return *this;
    }
};

Usage:

std::map<int, int> my_map;
// Later somewhere along the code
map_add_values<int,int>(my_map)(1,2)(3,4)(5,6);

NOTE: Previously I used a operator [] for adding the actual values. This is not possible as commented by dalle.

##################### END OF OBSOLETE SECTION #####################