Re: Initializing a map...

James Kanze <>
Thu, 21 Feb 2008 01:52:10 -0800 (PST)
On Feb 21, 1:33 am, Jeff Schwab <> wrote:

Sam wrote:

barcaroller writes:

Is there a way in C++ to initialize an STL map in one
statement (the way arrays can be initialized in C)?

For example, instead of using:

    map<type1,type2> mymap;
    mymap[key1] = value1;
    mymap[key2] = value2;

I would like to use something like:

    // wrong syntax!
    map<type1,type2> mymap = { (key1, value1), (key2, value2) };

You can subclass it, and define an operator function.

template<typename keyType, typename valType> class myMap
   : public std::map<keyType, valType> {
   myMap<keyType, valType> &operator()(keyType k, valType v)
       return *this;

You can initialize these objects as follows:

   myMap<int, int> z=myMap<int, int>()(3, 4)(5, 6);

Or even:

    std::map< int, int > z = myMap< int, int >()( 3, 4 )( 5, 6 ) ;

I like it. I'm just afraid that it deviates enough from usual
practice to be a bit of obfuscation. (Maybe you should try to
get something like this into Boost, so that it can be considered
usual practice.)

Note too that as used above, there is a deep copy of the map.
Not necessarily the sort of thing you might want in a tight
loop. (But how often does one construct pre-initialized maps in
a tight loop, anyway?)

The other thing to consider is that it binds the initializer
list very tightly to the actual declaration, in a way which
makes machine generation of the initializers somewhat difficult.
And if the map is small enough that you're willing to consider
writing the initialers out by hand, it's probably small enough
for you to simply use find_if on a C style array.

.. and so on. You can use these objects anywhere std::map is

Augh! std::map is a concrete type, really not meant to be
publicly subclassed.

It's true that std::map isn't designed to be used as a base
class. But you have to weigh everything. It doesn't bother me
in the least to derive from it to provide a specialized
constructor which contains the specific initialization---the
semantics of the resulting class (with its specific
initialization) are such that no one is going to use it except
in its intended use. His class is more general, so perhaps the
risk is greater, but I still find it within the realm of
reasonableness. On the other hand...

std::map basically provides an implementation class for several
more or less distinct use cases. (See the discussions on its
operator[], which can't be used on a const object.) Perhaps the
ideal solution is to define precise interfaces for each of the
use cases, using a member std::map for the implementation.

It hasn't got a virtual destructor, for example, so the
following causes undefined behavior:

     std::map* p = new myMap;
     delete p;

I'd wonder about any code which allocated an std::map
dynamically to begin with. I don't think the risk here is very

Your idea is good, but either (1) the inheritance should be
private, or (2) myMap should contain the std::map sub-object
as a member rather than a base.

In both cases, this means a lot of brunt typing to duplicate the
interface. It might be worth it, however.

James Kanze (GABI Software)
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