Making a noncopyable but movable class

Now, imagine the following situation: we have a resource that cannot be copied, which should be correctly freed in a destructor, and we want to return it from a function:

descriptor_owner construct_descriptor() 
{ 
    return descriptor_owner("Construct using this string"); 
} 

Actually, you can work around such situations using the swap method:

void construct_descriptor1(descriptor_owner& ret) 
{ 
    descriptor_owner("Construct using this string").swap(ret); 
} 

However, such a workaround does not allow us to use descriptor_owner in containers. By the way, it looks awful!

Getting ready

It is highly recommended that you are at least familiar with the basics of C++11 rvalue references. Reading the Using C++11 move emulation recipe is also recommended.

How to do it...

Those readers who use C++11, already know about the move-only classes (like std::unique_ptr or std::thread). Using such an approach, we can make a move-only descriptor_owner class:

class descriptor_owner1 {
    void* descriptor_;

public:
    descriptor_owner1()
        : descriptor_(nullptr)
    {}

    explicit descriptor_owner1(const char* param);

    descriptor_owner1(descriptor_owner1&& param)
        : descriptor_(param.descriptor_)
    {
        param.descriptor_ = nullptr;
    }

    descriptor_owner1& operator=(descriptor_owner1&& param) {
        descriptor_owner1 tmp(std::move(param));
        std::swap(descriptor_, tmp.descriptor_);
        return *this;
    }

    void clear() {
        free(descriptor_);
        descriptor_ = nullptr;
    }

    bool empty() const {
        return !descriptor_;
    }

    ~descriptor_owner1() {
        clear();
    }
};

// GCC compiles the following in C++11 and later modes.
descriptor_owner1 construct_descriptor2() {
    return descriptor_owner1("Construct using this string");
}

void foo_rv() {
    std::cout << "C++11n";
    descriptor_owner1 desc;
    desc = construct_descriptor2();
    assert(!desc.empty());
} 

This will work only on the C++11 compatible compilers. That is the right moment for Boost.Move! Let's modify our example, so it can be used on C++03 compilers.

According to the documentation, to write a movable but noncopyable type in portable syntax, we need to follow these simple steps:

1. Put the BOOST_MOVABLE_BUT_NOT_COPYABLE(classname) macro in the private section:

#include <boost/move/move.hpp>

class descriptor_owner_movable {
    void* descriptor_;

    BOOST_MOVABLE_BUT_NOT_COPYABLE(descriptor_owner_movable

2. Write a move constructor and a move assignment, taking the parameter as BOOST_RV_REF(classname):

public:
    descriptor_owner_movable()
        : descriptor_(NULL)
    {}
 
    explicit descriptor_owner_movable(const char* param)
        : descriptor_(strdup(param))
    {}
 
    descriptor_owner_movable(
        BOOST_RV_REF(descriptor_owner_movable) param
    ) BOOST_NOEXCEPT
        : descriptor_(param.descriptor_)
    {
        param.descriptor_ = NULL;
    }
 
    descriptor_owner_movable& operator=(
        BOOST_RV_REF(descriptor_owner_movable) param) BOOST_NOEXCEPT
    {
        descriptor_owner_movable tmp(boost::move(param));
        std::swap(descriptor_, tmp.descriptor_);
        return *this;
    }

    // ...
};

descriptor_owner_movable construct_descriptor3() {
    return descriptor_owner_movable("Construct using this string");
} 

How it works...

Now, we have a movable, but non copyable, class that can be used even on C++03 compilers and in Boost.Containers:

#include <boost/container/vector.hpp> 
#include <your_project/descriptor_owner_movable.h>

int main() {
    // Following code will work on C++11 and C++03 compilers 
    descriptor_owner_movable movable; 
    movable = construct_descriptor3(); 
    boost::container::vector<descriptor_owner_movable> vec; 
    vec.resize(10); 
    vec.push_back(construct_descriptor3()); 
 
    vec.back() = boost::move(vec.front()); 
}

Unfortunately, C++03 standard library containers still won't be able to use it (that is why we used a vector from Boost.Containers in the previous example).

There's more...

If you want to use Boost.Containers on C++03 compilers, but standard library containers on C++11 compilers, you can do the following simple trick. Add the header file to your project with the following content:

// your_project/vector.hpp 
// Copyright and other stuff goes here 
 
// include guards 
#ifndef YOUR_PROJECT_VECTOR_HPP 
#define YOUR_PROJECT_VECTOR_HPP 
 
// Contains BOOST_NO_CXX11_RVALUE_REFERENCES macro.
#include <boost/config.hpp>
 
#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES) 
// We do have rvalues 
#include <vector> 
 
namespace your_project_namespace { 
  using std::vector; 
} // your_project_namespace 
 
#else 
// We do NOT have rvalues 
#include <boost/container/vector.hpp> 
 
namespace your_project_namespace { 
  using boost::container::vector; 
} // your_project_namespace 
 
#endif // !defined(BOOST_NO_CXX11_RVALUE_REFERENCES) 
#endif // YOUR_PROJECT_VECTOR_HPP 

Now, you can include <your_project/vector.hpp> and use a vector from the namespace your_project_namespace:

int main() {
    your_project_namespace::vector<descriptor_owner_movable> v; 
    v.resize(10); 
    v.push_back(construct_descriptor3()); 
    v.back() = boost::move(v.front()); 
}

However, beware of compiler and standard library implementation-specific issues! For example, this code will compile on GCC 4.7 in C++11 mode only if you mark the move constructor, destructor, and move assignment operators with noexcept or BOOST_NOECEPT.

See also

• The Reducing code size and increasing performance of user-defined type in C++11 recipe in Chapter 10, Gathering Platform and Compiler Information, provides more info on noexcept and BOOST_NOEXCEPT.
• More information about Boost.Move can be found on Boost's website http://boost.org/libs/move.