Alternative to boost::shared_ptr in an embedded environment

Question

I'm using C++ in an embedded linux environment which has GCC version 2.95.

I just can't extract boost::shared_ptr files with bcp, it is just too heavy.

What I'd like would be a simple smart pointer implementation of boost::shared_ptr but without all boost overheads (if it is possible...).

I could come up with my own version reading boost source but I fear missing one or more points, it seems easy to make a faulty smart pointer and I can't afford to have a buggy implementation.

So, does a "simple" implementation or implementation example of boost::shared_ptr (or any reference counting equivalent smart pointer) exists that I could use or that I could take as an inspiration?

Answer 1

if you don't need mixing shared and weak ptr,and don't need coustom deletors, you can just use the quick and dirty my_shared_ptr:

template<class T>
class my_shared_ptr
{
    template<class U>
    friend class my_shared_ptr;
public:
    my_shared_ptr() :p(), c() {}
    explicit my_shared_ptr(T* s) :p(s), c(new unsigned(1)) {}

    my_shared_ptr(const my_shared_ptr& s) :p(s.p), c(s.c) { if(c) ++*c; }

    my_shared_ptr& operator=(const my_shared_ptr& s) 
    { if(this!=&s) { clear(); p=s.p; c=s.c; if(c) ++*c; } return *this; }

    template<class U>
    my_shared_ptr(const my_shared_ptr<U>& s) :p(s.p), c(s.c) { if(c) ++*c; }

    ~my_shared_ptr() { clear(); }

    void clear() 
    { 
        if(c)
        {
            if(*c==1) delete p; 
            if(!--*c) delete c; 
        } 
        c=0; p=0; 
    }

    T* get() const { return (c)? p: 0; }
    T* operator->() const { return get(); }
    T& operator*() const { return *get(); }

private:
    T* p;
    unsigned* c;
}

---

For anyone interested in make_my_shared<X>, it can be trivially implemented as

template<class T, class... U>
auto make_my_shared(U&&... u)
{ 
    return my_shared_ptr<T>(new T{std::forward<U>(u)...});
}

to be called as

auto pt = make_my_shared<T>( ... );

Answer 2

There is also std::tr1::shared_ptr which is just C++11 standard's lifting from boost. You could pick that if it is allowed, or write your own with reference counting.

Answer 3

Which "boost overheads" are you concerned about, and in what way is shared_ptr "too heavy" for your application? There are no overheads simply from using Boost (at least if you only use header-only libraries, such as the smart pointer library); the only overheads I can think of concerning shared_ptr are:

• Thread-safe reference counting; since you're using an ancient compiler, I assume you also have an ancient runtime library and kernel, so this may be very inefficient. If your application is single threaded, then you can disable this by #defining BOOST_DISABLE_THREADS.
• Allocation of the reference-counting structure alongside the managed object. You can eliminate the extra allocation by creating the object using make_shared() or allocate_shared().

In many cases, you can eliminate the overhead from speed-critical code by not creating objects or copying shared pointers - pass pointers by reference and only copy them when you actually need to.

If you need thread safety for some pointers, but not others, and (after profiling, and removing all unnecessary allocations and pointer copying) you find that the use of shared pointers is still causing significant overhead, then you could consider using intrusive_ptr, and manage your own reference counting within the object.

There may also be benefit to updating to a modern GNU/Linux version, if that's feasible. Thread synchronisation is much more efficient since the introduction of futexes in Linux 2.6. You may find this helps in other ways too; there have been many improvements over the last decade. A more modern compiler would also provide the standard (TR1 or C++11) shared pointer, so you wouldn't need Boost for that.

Answer 4

I'd suggest you can use shared_ptr in isolation. However, if you are looking for simple implementation

• with some unit tests
• not thread safe
• basic support for polymorphic assignment <-- let me know if you're intrested
• custom deletors (i.e. array deletors, or custom functors for special resources)

Have a look here: Creating a non-thread safe shared_ptr

Answer 5

You can use shared_ptr without all the Boost overhead: it's a header-only implementation. If you don't use anything other classes, only shared_ptr will be compiled in.

The implementation of shared_ptr is already pretty lean, but if you want to avoid the intermediate reference count block and a (potential) virtual call to deleter function, you can use boost::intrusive_ptr instead, which is more suited for embedded environments: it operates on a reference counter embedded in the object itself, you just have to provide a couple of functions to increment/decrement it. The downside is that you won't be able to use weak_ptr.

I can't comment how well gcc 2.95 would inline/fold template instantiations (it's a very old compiler), more recent versions of gcc handle it rather well, so you are on your own here.