You have almost certainly encountered certain situations, where a class owns some resources that must not be copied for technical reasons:
class descriptor_owner {
void* descriptor_;
public:
explicit descriptor_owner(const char* params);
~descriptor_owner() {
system_api_free_descriptor(descriptor_);
}
}; The C++ compiler in the preceding example generates a copy constructor and an assignment operator, so the potential user of the descriptor_owner class will be able to create the following awful things:
void i_am_bad() {
descriptor_owner d1("O_o");
descriptor_owner d2("^_^");
// Descriptor of d2 was not correctly freed
d2 = d1;
// destructor of d2 will free the descriptor
// destructor of d1 will try to free already freed descriptor
}To avoid such situations, the boost::noncopyable class was invented. If you derive your own class from it, the copy constructor and assignment operator won't be generated by the C++ compiler:
#include <boost/noncopyable.hpp>
class descriptor_owner_fixed : private boost::noncopyable {
// ... Now, the user won't be able to do bad things:
void i_am_good() {
descriptor_owner_fixed d1("O_o");
descriptor_owner_fixed d2("^_^");
// Won't compile
d2 = d1;
// Won't compile either
descriptor_owner_fixed d3(d1);
}A refined reader will note that we can achieve exactly the same result by:
- Making a copy constructor and an assignment operator of
descriptor_owning_fixedprivate - Defining them without actual implementation
- Explicitly deleting them using C++11 syntax
= delete;
Yes, you are correct. Depending on the abilities of your compiler, boost::noncopyable class chooses the best way to make the class noncopyable.
boost::noncopyable also serves as a good documentation for your class. It never raises questions such as "Is the copy constructor body defined elsewhere?" or "Does it have a nonstandard copy constructor (with a non-const referenced parameter)?"
- The Making a noncopyable, but movable class recipe will give you ideas on how to allow unique owning of a resource in C++03 by moving it
- You may find a lot of helpful functions and classes in the
Boost.Corelibrary's official documentation at http://boost.org/libs/core - The Initializing a base class by the member of derived recipe in Chapter 2, Managing Resources
- The Using C++11 move emulation recipe