Effective C++ Item 14 Think carefully about copying behavior in resource-managing classe

In C++, the only code that guaranteed to be executed after an exception is thrown are the destructors of objects residing in stack and that's why we need RAII. We don't always deal with head based objects, so instead with using auto_ptrs and shared_ptrs, sometimes we need to create our own resource managment class. And here are some rules you should follow:

For coping constructor and coping assignment operator

 

1. prohibit copying. If copying an RAII object makes no sense, you should explicitly prohibit it. You can either declare copy constructor private or inherit from Uncopyable class (see Item 6)

 

2. Reference-count the underlying resources. Sometimes it's desirable to hold on the resource until the last object using it is destroyed. When that's the case, copying an RAII object should increse the count of the number of referring to that object. That is how shared_ptr implement copying behavior. So RAII can implement reference-counting copying by containing a shared_ptr data member. In addition, if you don't want to delete the resource when the reference count goes to 0 which is the default behavior of shared_ptr, you can provide a customized "deleter" to shared_ptr which will be called when reference count goes to 0.

class Lock{
public:
	explicit Lock(Mutex *pm)
	: mutexPtr(pm, unlock) {
		lock(mutexPtr.get());
	}
	
private:
	std::tr1::shared_ptr<Mutex> mutexPtr;
};

　　

3. copy the underlying resources

In this case, the only reason to use recource-managing class is to make sure that resource would be released after you done with it. You need to make a deep copy of underlying resource

 

4. Transfer the ownership of underlying resources

Sometimes, you want to keep only one RAII object refers to raw resource, you need to transfer the ownership to new instance while copying. This kind of behavior is implemented by auto_ptr