问题
What is the right way of initializing a static map? Do we need a static function that will initialize it?
回答1:
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');
回答2:
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();
回答3:
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############### EVERYTHING BELOW THIS IS OBSOLETE #################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 #####################
回答4:
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} };
回答5:
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.
回答6:
For example:
const std::map<LoggerLevel, const char*> g_logLevelsDescriptions =
{
{ LoggerLevel::llNothing, "Logging disabled" },
{ LoggerLevel::llInfo, "Base information" },
{ LoggerLevel::llWarn, "Warnings" },
{ LoggerLevel::llError, "Errors" },
{ LoggerLevel::llDebug, "All information: debug-mode" }
};
回答7:
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;
回答8:
You can try:
std::map <int, int> mymap =
{
std::pair <int, int> (1, 1),
std::pair <int, int> (2, 2),
std::pair <int, int> (2, 2)
};
回答9:
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);
回答10:
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 ] ) ] );
回答11:
You have some very good answers here, but I'm to me, it looks like a case of "when all you know is a hammer"...
The simplest answer of to why there is no standard way to initialise a static map, is there is no good reason to ever use a static map...
A map is a structure designed for fast lookup, of an unknown set of elements. If you know the elements before hand, simply use a C-array. Enter the values in a sorted manner, or run sort on them, if you can't do this. You can then get log(n) performance by using the stl::functions to loop-up entries, lower_bound/upper_bound. When I have tested this previously they normally perform at least 4 times faster than a map.
The advantages are many fold... - faster performance (*4, I've measured on many CPU's types, it's always around 4) - simpler debugging. It's just easier to see what's going on with a linear layout. - Trivial implementations of copy operations, should that become necessary. - It allocates no memory at run time, so will never throw an exception. - It's a standard interface, and so is very easy to share across, DLL's, or languages, etc.
I could go on, but if you want more, why not look at Stroustrup's many blogs on the subject.
来源：https://stackoverflow.com/questions/138600/initializing-a-static-stdmapint-int-in-c