One-liner for RAII on non pointer?

Question

Related topic

std::unique_ptr, deleters and the Win32 API

To use a Win32 Handle as a RAII, I can use the following line

std:         


        

Answer 1

Finally, I want with another Kerrek SB answer. It's the proposal for STD Unique Resource.

#ifndef UNIQUE_RESOURCE_H_
#define UNIQUE_RESOURCE_H_

#include <type_traits>

// From standard proposal http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3949.pdf
// Slightly modified to compile on VS2012.
namespace std
{
    namespace experimental
    {
        enum class invoke_it { once, again };
        template<typename R,typename D>
        class unique_resource_t 
        {
            R resource;
            D deleter;
            bool execute_on_destruction; // exposition only
            unique_resource_t& operator=(unique_resource_t const &);
            unique_resource_t(unique_resource_t const &); // no copies!
        public:
            // construction
            explicit unique_resource_t(R && resource, D && deleter, bool shouldrun=true)
                : resource(std::move(resource))
                , deleter(std::move(deleter))
                , execute_on_destruction(shouldrun)
            {

            }
            // move
            unique_resource_t(unique_resource_t &&other) /*noexcept*/
                :resource(std::move(other.resource))
                ,deleter(std::move(other.deleter))
                ,execute_on_destruction(other.execute_on_destruction)
            {
                    other.release();
            }
            unique_resource_t& operator=(unique_resource_t &&other) 
            {
                this->invoke(invoke_it::once);
                deleter=std::move(other.deleter);
                resource=std::move(other.resource);
                execute_on_destruction=other.execute_on_destruction;
                other.release();
                return *this;
            }
            // resource release
            ~unique_resource_t() 
            {
                this->invoke(invoke_it::once);
            }
            void invoke(invoke_it const strategy = invoke_it::once) 
            {
                if (execute_on_destruction) {
                    try {
                        this->get_deleter()(resource);
                    } catch(...){}
                }
                execute_on_destruction = strategy==invoke_it::again;
            }
            R const & release() /*noexcept*/{
                execute_on_destruction = false;
                return this->get();
            }
            void reset(R && newresource) /*noexcept*/ {
                invoke(invoke_it::again);
                resource = std::move(newresource);
            }
            // resource access
            R const & get() const /*noexcept*/ {
                return resource;
            }
            operator R const &() const /*noexcept*/ 
            {
                return resource;
            }
            R operator->() const /*noexcept*/ 
            {
                return resource;
            }

//             Couldn't make this function compile on VS2012
//             std::add_lvalue_reference<std::remove_pointer<R>::type>::type operator*() const 
//             {
//                     return * resource;
//             }

            // deleter access
            const D & get_deleter() const /*noexcept*/ 
            {
                return deleter;
            }
        };

        //factories
        template<typename R,typename D>
        unique_resource_t<R,D> unique_resource( R && r,D t) /*noexcept*/ 
        {
                return unique_resource_t<R,D>(std::move(r), std::move(t),true);
        }
            template<typename R,typename D>
        unique_resource_t<R,D>
            unique_resource_checked(R r, R invalid, D t ) /*noexcept*/ {
                bool shouldrun = (r != invalid);
                return unique_resource_t<R,D>(std::move(r), std::move(t), shouldrun);
        }
    }
}
#endif /* UNIQUE RESOURCE H */

Usage

auto listenSocket = std::experimental::unique_resource_checked(socket(AF_INET,SOCK_STREAM,IPPROTO_TCP), INVALID_SOCKET, closesocket);

Hope this makes std soon enough!

Answer 2

It is well known the example to RAII a FILE* using std::unique_ptr:

struct FILEDeleter
{
    typedef FILE *pointer;
    void operator()(FILE *fp) { fclose(fp); }
};

typedef std::unique_ptr<FILE, FILEDeleter> FilePtr;

FilePtr f(fopen("file.txt", "r"));

Alas, a similar approach to POSIX close() to RAII a file descriptor is not possible:

struct FDDeleter
{
    typedef int pointer;
    void operator()(int fd) { close(fp); }
};

typedef std::unique_ptr<int, FDDeleter> FD;

Although some compilers will work just fine, it is not valid because the fd==0 is a valid file descriptor! The null one should be -1. But anyway, even if it were 0 it is still not valid, because FDDeleter::pointer shall satisfy the requirements of NullablePointer (summing up):

1. It shall be comparable to nullptr.
2. It shall be value-initialized to a value that compares equal to nullptr.

Thus, UniqueHandle is born!

#include <memory>

template <typename T, T TNul = T()>
class UniqueHandle
{
public:
    UniqueHandle(std::nullptr_t = nullptr)
        :m_id(TNul)
    { }
    UniqueHandle(T x)
        :m_id(x)
    { }
    explicit operator bool() const { return m_id != TNul; }

    operator T&() { return m_id; }
    operator T() const { return m_id; }

    T *operator&() { return &m_id; }
    const T *operator&() const { return &m_id; }

    friend bool operator == (UniqueHandle a, UniqueHandle b) { return a.m_id == b.m_id; }
    friend bool operator != (UniqueHandle a, UniqueHandle b) { return a.m_id != b.m_id; }
    friend bool operator == (UniqueHandle a, std::nullptr_t) { return a.m_id == TNul; }
    friend bool operator != (UniqueHandle a, std::nullptr_t) { return a.m_id != TNul; }
    friend bool operator == (std::nullptr_t, UniqueHandle b) { return TNul == b.m_id; }
    friend bool operator != (std::nullptr_t, UniqueHandle b) { return TNul != b.m_id; }

private:
    T m_id;
};

Its use is pretty easy, best seen with an example:

struct FDDeleter
{
    typedef UniqueHandle<int, -1> pointer;
    void operator()(pointer p)
    {
        close(p);
    }
};
typedef std::unique_ptr<int, FDDeleter> FD;

FD fd(open("test.txt", O_RDONLY));

If you truly want a one-liner you could go with this generalization:

template <typename T, T TNul = T(), typename RD, RD (*D)(T)>
struct OLDeleter
{
    typedef UniqueHandle<T, TNul> pointer;
    void operator()(pointer p)
    {
        D(p);
    }
};

And then just one line:

std::unique_ptr<int, OLDeleter<int, -1, int, close> > FD fd(open("test.txt", O_RDONLY));

The problem is that you must add the return of close() as a template argument and assume that there isn't anything funny about this function that prevents its conversion to a int(*)(int) (weird calling conventions, extra parameters, macros...) and that is quite inconvenient.

You could add a function wrapper:

void my_close(int fd) { close(fd); }

But if you are into it, you could as well write the whole struct FDDeleter.

Answer 3

A slightly different approach (within the premises of the RAII idiom though) is to use boost's scope exit.

Example :

#include <boost/scope_exit.hpp>
#include <cstdlib>
#include <cstdio>
#include <cassert>

int main() 
{
    std::FILE* f = std::fopen("example_file.txt", "w");
    assert(f);

    BOOST_SCOPE_EXIT(f) {
    // Whatever happened in scope, this code will be
    // executed  and  file  will be correctly closed.
        std::fclose(f);
    } BOOST_SCOPE_EXIT_END

    // Some code that may throw or return.
    // ...
}

Using this functionality, you'd be practically specifying freestanding "RAII destructor actions". Use where it makes your code clearer and cleaner and avoid when all functionality would be more easily incorporated (or already is) inside a class' destructor.

---

It seems that soon additional RAII functionality will be added to the language. When available you'll be able to use something like scoped_resource which looks like this (I'd refer to that link for a full fledged implementation of what you ask)

Answer 4

Kerrek SB answered in the comments and it was exactly what I was looking for!

template <typename T, typename D, D Deleter> 
struct stateless_deleter 
{
    typedef T pointer; 

    void operator()(T x) 
    { 
        Deleter(x); 
    } 
};
std::unique_ptr<SOCKET, stateless_deleter<SOCKET, int(*)(SOCKET), &::closesocket>> listenSocket(socket(AF_UNSPEC, SOCK_STREAM, IPPROTO_UDP));

Answer 5

I often use this in C++11:

 #include <utility>

 namespace{
    template<typename F>
    struct RAII_Helper{
        template<typename InitFunction>
        RAII_Helper(InitFunction &&init, F &&exit) : f_(std::forward<F>(exit)), canceled(false){
            init();
        }
        RAII_Helper(F &&f) : f_(f), canceled(false){
        }
        ~RAII_Helper(){
            if (!canceled)
                f_();
        }
        void cancel(){
            canceled = true;
        }
    private:
        F f_;
        bool canceled;
    };
 }
 template<class F>
 RAII_Helper<F> RAII_do(F &&f){
    return RAII_Helper<F>(std::forward<F>(f));
 }

 template<class Init, class Exit>
 RAII_Helper<Exit> RAII_do(Init &&init, Exit &&exit){
    return RAII_Helper<Exit>(std::forward<Init>(init), std::forward<Exit>(exit));
 }

usage:

FILE *f = fopen("file", "r");
if (!f)
    return "error";
auto filecloser = RAII_do([=]{fclose(f);});

//also makes for good init / exit objects
static auto initExit = RAII_do([]{initializeLibrary();}, []{exitLibrary();});

I like it because it works for arbitrary code, not just pointers or handles. Also the cancel function could be omitted if never used.

Answer 6

Here's one possible solution using as an example the NetCDF C API, which has plain ints as handles:

retval = nc_open(..., &id);
...  // validate
std::unique_ptr<int, void(*)(int*)> always_be_closing(&id, [](int* p){nc_close(*p);});

Of course you can check the value in the lambda, if necessary.

... [](int* p){ if(p) nc_close(*p); }

And a typedef makes it a little nicer:

typedef std::unique_ptr<int, void(*)(int*)> nc_closer;
...
nc_closer abc(&id, [](int* p){nc_close(*p);});

And you probably want a function to reduce duplication:

static void nc_close_p(int* p) { nc_close(*p); }
...
nc_closer abc(&id, &nc_close_p);

or:

auto abc = auto_nc_close(&id, &nc_close_p);

Since unique_ptr implements operator bool, you can also use this as a block scope, like using in C#:

if (auto abc = auto_nc_close(&id, &nc_close_p))
{
    ...
}