Re: STL map, compare function

James Kanze <>
Tue, 24 Jun 2008 01:36:19 -0700 (PDT)
On Jun 23, 3:27 pm, S S <> wrote:

I have a requirement where I am declaring a map within a class.

class abc {
map <void*, void*> mMap; // I do not pass compare struct here.

Here I am not passing compare function, but I want to do it
into the constructor of class abc.

There are two issues: the type of the comparator, and its
"value". The type becomes part of the type of the map, and must
be specified when you declare the map. (The map itself holds an
instance of this type, so the compiler must know its size.) The
value can be specified when you initialize the object, in this
case, in the initializer list of abc::abc. (It cannot be
changed later, except with swap, for obvious reasons.)

By default, the *type* of the comparator is std::less<Key>.
Since all instances of this class have the same behavior,
there's never any point of setting it to a particular value in
the constructor, but this is not necessarily true for other
comparators. If you declare:

    map< void*, void*, bool (*)( void*, void* ) > mMap ;

for example, you almost certainly want to pass an actual
instance of the comparator to the constructor. (The default
value here is a null pointer, which isn't going to work too

Is it possible? Above void* could well be char* and in that
case user will initialize it with some char compare function ,
but I don't know how to accomplish above task. Can I do
something like

abc::abc() : mMap(compare_string) {}

But above dosen't work !!

It does if you declare mMap so that it takes a comparator of
type compare_string. (Or if compare_string has the type
std::less< key_type >, but of course, there's no point in it in
that case.)

So I want to try something like function pointers, see the
code below

bool comp_default(const void *a, const void *b) { return a < b; }
bool CharStringCompare(const void* a, const void* b) {return
strcmp((char*)a, (char*)b) < 0;}
struct compare_key_ {
   bool operator()(const void* a, const void* b)
       return (abc::*mComp)(a, b); // I know this is wrong as I am not
using object here

You don't need the structure.

class abc {
friend struct compare_key_;
bool (abc::*mComp) (const void*, const void*) ;
map<void*, void*, compare_key_> mMap;

Why not
    std::map< void*, void*, bool (*)( void const*, void const* ) >
                        mMap ;
initialized with:

        : mMap( &CharStringCompare )



But above code is not the correct thing I am doing, I just
wanted to let you know the direction I am thinking? Can
someone please help me out.

Well, I'm very suspicious of your use of void* to begin with,
and I think you probably have to think about const correctness a
bit more as well. But there's no problem of having a comparator
which does something different depending on its state; a pointer
to function, for example (which will do something different
depending on which function it points to).

James Kanze (GABI Software)
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