smart pointer to manage socket file descriptor

Question

A smart pointer clears the memory if the pointer gets out of scope. I wanted to adapt this to a file descriptor, like a socket. There you need a user defined deleter, because close() is the function to free the file descriptor (fd) resources.

I found this useful page, unfortunately, most approaches did not work for me. Below is a working solution I found up to now, which is a little nasty. Because uniqu_ptr expects a pointer I created int *fd to store the fd value, therefore, I had to close(*fd) and delete fd in my custom deleter.

(1) Is there a better way?

Options A and B, which are based on the hints provided by the mentioned web page, are much nicer but causing odd compiler errors.

(2) Does anyone know how to correctly use these alternatives?

I'm using Qt Creator 3.0.1 with CONFIG += c++11 option and gcc version 4.8.2

#include "ccommhandler.h"

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <memory>

#include <qdebug.h>

//for Option A and B
struct CloseHandleDeleter {
    typedef int pointer;
    void operator()(int handle) const
    {
    }
};

//custom deleter, working
class MyComplexDeleter
{

public:
    MyComplexDeleter() {}

    void operator()(int* ptr) const
    {
        qDebug() << "Deleting ";
        close(*ptr);
        delete ptr;
    }
};

CCommHandler::CCommHandler()
{
    //Option A doesn't work
    //std::unique_ptr<int, CloseHandleDeleter> file( socket(AF_INET, SOCK_STREAM, 0) );
    //Option B doesn't work
    //std::unique_ptr<int, int()(int)> filePtr(  socket(AF_INET, SOCK_STREAM, 0) , close);

    MyComplexDeleter deleter;
    int *fd = new int;
    *fd = socket(AF_INET, SOCK_STREAM, 0);
    std::unique_ptr<int, MyComplexDeleter> p( fd , deleter);

}

Edit:

The posted answer by Nevin is right, it solves my initial problem.

The comment of learnvst caused to rethink my problem, and I have to say I may made it much more complex than needed, because the following simple class should also solve my problem of auto-free the memory of a resource or as in my case, to close the file descriptor:

class SocketHandler
{
   int _fd;
 public:
   SocketHandler(int FD):_fd(FD){}
   ~SocketHandler() { if(_fd!=-1) close(_fd); }

   operator int() const { return _fd; }
};

Why not just wrap the socket stuff into a class and close the socket in the dtor? Then you do not need to use a deleter as everything will close out nicely when the smart pointer frees the resource. — learnvst, Apr 13 '15 at 21:31
http://stackoverflow.com/questions/27440953/stdunique-ptr-for-c-functions-that-need-free/27452707#27452707 — T.C., Apr 13 '15 at 21:49
A `unique_ptr` must hold a **pointer**. `socket()` does not return a **pointer** on non-Windows platforms. That is why options A and B do not work, but option C does. — Remy Lebeau, Apr 14 '15 at 02:01

score 3 · Accepted Answer · answered Apr 13 '15 at 21:33

Because fd isn't a pointer, I wouldn't try to pigeonhole it into unique_ptr. Instead, create a custom class whose interface is based on unique_ptr, as in (caution: totally untested):

class unique_fd
{
public:
    constexpr unique_fd() noexcept = default;
    explicit unique_fd(int fd) noexcept : fd_(fd) {}
    unique_fd(unique_fd&& u) noexcept : fd_(u.fd_) { u.fd_ = -1; }

    ~unique_fd() { if (-1 != fd_) ::close(fd_); }

    unique_fd& operator=(unique_fd&& u) noexcept { reset(u.release()); return *this; }

    int get() const noexcept { return fd_; }
    operator int() const noexcept { return fd_; }

    int release() noexcept { int fd = fd_; fd_ = -1; return fd; }
    void reset(int fd = -1) noexcept { unique_fd(fd).swap(*this); }
    void swap(unique_fd& u) noexcept { std::swap(fd_, u.fd_); }

    unique_fd(const unique_fd&) = delete;
    unique_fd& operator=(const unique_fd&) = delete;

    // in the global namespace to override ::close(int)
    friend int close(unique_fd& u) noexcept { int closed = ::close(u.fd_); u.fd_ = -1; return closed; }

private:
    int fd_ = -1;
};

smart pointer to manage socket file descriptor

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