C++ Smart Pointer performance

Question

How much do using smart pointers, particularly boost::shared_ptr cost more compared to bare pointers in terms of time and memory? Is using bare pointers better for performance i

Answer 1

When I last tested, with VC6, the compiler wasn't able to optimize the code with a smart pointer as well as it could with a raw pointer. Things might have changed since then.

Answer 2

Reference-counted smart pointers (the most common type) only cost more when you copy, create and delete them. This extra cost can be substantial if you are copying a lot, because most of them are thread-safe.

If you just want an "auto-deleting" pointer, there is the much maligned auto_ptr, or the new and shiny (but not much supported yet) unique_ptr from C++0x.

Answer 3

The only way to deal with performance problems is to profile your code. The largest part of performance problems is imagined anyway; only profiling will point out to you where your bottlenecks lie.

If it turns out that using smart pointers produces a bottleneck where raw pointers don't, use raw pointers! Until then, I wouldn't worry too much about it; most operations on smart pointers are reasonably fast. You're likely comparing strings too often (or something like that) for them to matter.

Answer 4

Dereferencing smart pointers is typically trivial, certainly for boost in release mode. All boost checks are at compile-time. (Smart pointers could in theory do smart stuff across threads). This still leaves a lot of other operations. Nicola mentioned construction, copying and destruction. This is not the complete set, though. Other important operations are swapping, assignment and resetting to NULL. Basically, any operation that requires smartness.

Note that some of these operations are excluded by some smart pointers. E.g. boost::scoped_ptr cannot even be copied, let alone be assigned. As this leaves less operations, the implementation can be optimized for these fewer methods.

In fact, with TR1 coming up, it's quite likely that compilers can do better with smart pointers than raw pointers. E.g. it's possible that a compiler can prove that a smart non-copyable pointer is not aliased in some situations, merely because it's non-copyable. Think about it: aliasing occurs when two pointers are created pointing to the same object. If the first pointer cannot be copied, how would a second pointer end up pointing to the same object? (There are ways around that, too - operator* has to return an lvalue)

Answer 5

There's an often overlooked halfway-house between a "manually" managed std::vector<T*> (ie raw pointers) and a std::vector<boost::shared_ptr<T> >, in the form of the boost::ptr_container classes.

These combine the performance of a raw pointer container with the convenience of a container of smart pointers (ie they provide the functionality people would like STL containers of std::auto_ptr to provide, if that worked).

Answer 6

Boost provide different smart pointers. Generally both the memory occupation, which varies in accordance to the kind of smart pointer, and the performance should not be an issue. For a performance comparison you can check this http://www.boost.org/doc/libs/1_37_0/libs/smart_ptr/smarttests.htm.

As you can see only construction, copy and destruction are taken into account for the performance comparison, which means that dereferencing a smart pointer has supposedly the same cost as that of a raw pointer.

The following snippet demonstrates that there's no performance loss by using a shared_ptr<> in place of a raw pointer:

#include <iostream>
#include <tr1/memory>

int main()
{
#ifdef USE_SHARED_PTR
    std::tr1::shared_ptr<volatile int> i(new int(1));
#else
    volatile int * i = new int(1);
#endif

    long long int h = 0;

    for(long long int j=0;j < 10000000000LL; j++)
    {
        h += *i;
    }

    std::cout << h << std::endl;
    return 0;
}