Making a noncopyable class

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 
}

Getting ready

Only very basic knowledge of C++ is required for this recipe.

How to do it...

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); 
}

How it works...

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_fixed private
• 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)?"

See also

• 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.Core library'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