Re: Initializing a map...

From:
Jeff Schwab <jeff@schwabcenter.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 20 Feb 2008 16:33:21 -0800
Message-ID:
<sMudnUmIvdgxWyHanZ2dnUVZ_tGonZ2d@comcast.com>
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> {

public:
   myMap<keyType, valType> &operator()(keyType k, valType v)
   {
    (*this)[k]=v;
       return *this;
   };
};

You can initialize these objects as follows:

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

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


Augh! std::map is a concrete type, really not meant to be publicly
subclassed. It hasn't got a virtual destructor, for example, so the
following causes undefined behavior:

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

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.

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