The cost of passing by shared_ptr

I use std::tr1::shared_ptr extensively throughout my application. This includes passing objects in as function arguments. Consider the following: ``` class Dataset {...} void f( shared_ptr< Dataset const > pds ) {...} void g( shared_ptr< Dataset const > pds ) {...} ... ``` While passing a dataset object around via shared_ptr guarantees its existence inside f and g, the functions may be called millions of times, which causes a lot of shared_ptr objects being created and destroyed. Here's a snippet of the flat gprof profile from a recent run: ``` Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls s/call s/call name 9.74 295.39 35.12 2451177304 0.00 0.00 std::tr1::__shared_count::__shared_count(std::tr1::__shared_count const&) 8.03 324.34 28.95 2451252116 0.00 0.00 std::tr1::__shared_count::~__shared_count() ``` So, ~17% of the runtime was spent on reference counting with shared_ptr objects. Is this normal? A large portion of my application is single-threaded and I was thinking about re-writing some of the functions as ``` void f( const Dataset& ds ) {...} ``` and replacing the calls ``` shared_ptr< Dataset > pds( new Dataset(...) ); f( pds ); ``` with ``` f( *pds ); ``` in places where I know for sure the object will not get destroyed while the flow of the program is inside f(). But before I run off to change a bunch of function signatures / calls, I wanted to know what the typical performance hit of passing by shared_ptr was. Seems like shared_ptr should not be used for functions that get called very often. Any input would be appreciated. Thanks for reading. -Artem Update: After changing a handful of functions to accept `const Dataset&`, the new profile looks like this: ``` Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls s/call s/call name 0.15 241.62 0.37 24981902 0.00 0.00 std::tr1::__shared_count::~__shared_count() 0.12 241.91 0.30 28342376 0.00 0.00 std::tr1::__shared_count::__shared_count(std::tr1::__shared_count const&) ``` I'm a little puzzled by the number of destructor calls being smaller than the number of copy constructor calls, but overall I'm very pleased with the decrease in the associated run-time. Thanks to all for their advice.