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Suppose you have a type A which has some cost associated with copying it that we'd like to avoid.

I want to have a collection where A forms part of the key, say...

std::map< std::pair<A, int>, std::string> myMap;
Now I want to do a look up like this...

void SomeMethod(const A& a, int b)
{
   iterator it = myMap.find(std::make_pair(a, b));
}

...make_pair(...) copies the A because the map's key type is
std::pair<A, int>
not
std::pair<const A&, int>

Can anyone suggest a way to avoid the copy of A?
Posted 17-Dec-12 19:56pm
_Josh_53.4K
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Solution 1

// not std::pair<const A&, int>

Why not ?

It could be also std::pair<const A*, int>
or std::pair<std::auto_ptr<A>&, int> :)
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v2
Comments
_Josh_ 18-Dec-12 2:27am
   
But then I'd have to manage memory for the pointers and ill get worse cache performance because things will be all over the place in memory.
Eugen Podsypalnikov 18-Dec-12 2:56am
   
// worse cache performance, because things will be all over the place in memory
First - please remember that the all objects are in the memory (once or twice ;-))

I think, it (worse cache performance) is wrong :) ,
because a map will work with pointer- (or reference-) keys in its hash-table _more_faster_
as with full full-dumped-copies keys.

Above all, your code will be more canny (in the memory usage context).
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Solution 2

Typically you don't want to use large object for a key.

You might also be able to keep a std::pair<a,> somewhere and pass it by reference to SomeMethod to avoid extra construction particulary in a loop.

Are you using the whole A object for ordering ? If not, then maybe a better design would be to have a simpler key and a more complete value.

Generally your key should be simple and a small object and the value would have all the information. Something similar to this:

struct PairAndString 
{
    A first;
    int second;
    std::string third;
};
 
// Only fields necessary for sorting and comparison in key. Everything else moved to the value part.
std::map<a_key,> myMap;  
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