boost::shared_ptr boost::mutex and copy constructor

Question

I need to protect the access to a data structure in my class. As I can't have mutex (because I can't copy it) I am considering to have shared_ptr and keep the mutex there. Here is a sample code of my idea:

class Sample {
    typedef boost::lock_guard<boost::mutex> AcquireLock;
    boost::shared_ptr<boost::mutex> mutt;

public:
    Sample() : mutt(new boost::mutex) {}

    void Method()
    {
        AcquireLock lock(*mutt);

        //do some work here
    }
};

I've got the following questions:

Is it a bad practice to use the mutex that way (as member of the class, via shared_ptr)?
Should I have copy constructor for this class, as it has memory allocated on heap via shared_ptr?

EDIT: Maybe I need to give a bit more details: I'll create this object only once and save it in std::vector. I don't need to make copies of it and if the vector needs to make copies, I don't want to have different mutex for each copy. That's why I think the copy constructor will work for me.

Igor R. Igor R. · Accepted Answer · 2012-02-28T18:54:06

This approach is pretty valid and legitimate, but note that as your class evolves, you might want to apply the same technique to some more class members. That's why I'd recommend you to consider taking an advantage of pImpl idiom:

// in hpp:
class Sample
{
  Impl();
private:
  struct Impl;
  // compiler generated copy-constructor will copy only this shared_ptr
  shared_ptr<void> pImpl_;
};

// in cpp:
struct Sample::Impl
{
  mutex mut_;
  // put here whatever members you need, extend Impl without affecting the Sample interface
};

Impl::Impl() : pImpl_(new Impl)
{}

boost::shared_ptr boost::mutex and copy constructor

2 Answers