Is it possible to defer member initialization to t

问题:

I have a class with an object as a member which doesn't have a default constructor. I'd like to initialize this member in the constructor, but it seems that in C++ I can't do that. Here is the class:

#include <boost/asio.hpp>
#include <boost/array.hpp>

using boost::asio::ip::udp;

template<class T>
class udp_sock
{
    public:
        udp_sock(std::string host, unsigned short port);
    private:
        boost::asio::io_service _io_service;
        udp::socket _sock;
        boost::array<T,256> _buf;
};

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost",
  unsigned short port = 50000)
{
    udp::resolver res(_io_service);
    udp::resolver::query query(udp::v4(), host, "spec");
    udp::endpoint ep = *res.resolve(query);
    ep.port(port);
    _sock(_io_service, ep);
}

The compiler tells me basically that it can't find a default constructor for udp::socket and by my research I understood that C++ implicitly initializes every member before calling the constructor. Is there any way to do it the way I wanted to do it, or is it too "Java-oriented" and not feasible in C++?

I worked around the problem by defining my constructor like this:

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost",
  unsigned short port = 50000) : _sock(_io_service)
{
    udp::resolver res(_io_service);
    udp::resolver::query query(udp::v4(), host, "spec");
    udp::endpoint ep = *res.resolve(query);
    ep.port(port);
    _sock.bind(ep);
}

So my question is more out of curiosity and to better understand OOP in C++

回答1:

When you define a constructor, you have 2 ways to "initialize" attributes:

• the initializer list
• the constructor body

If you do not explictly initialize one of the attributes in the initializer list, it is nonetheless initialized (by calling its default constructor) for you...

So in essence:

class Example
{
public:
  Example();
private:
  Bar mAttr;
};

// You write
Example::Example() {}

// The compiler understands
Example::Example(): mAttr() {}

And this of course fails if the underlying type does not have a Default Constructor.

There are various ways to defer this initialization. The "standard" way would be to use a pointer:

class Example { public: Example(); private: Bar* mAttr; };

However I prefer using Boost.Optional combined with suitable accessors:

class Example
{
public: Example();
private:
  Bar& accessAttr() { return *mAttr; }
  const Bar& getAttr() const { return *mAttr; }
  boost::Optional<Bar> mAttr;
};

Example::Example() { mAttr = Bar(42); }

Because Boost.Optional means that there is no overhead on the allocation and no overhead on the dereferencing (the object is created in place) and yet carries the correct semantic.

回答2:

I think that's one possible use case for boost::optional.

回答3:

In C++ it's preferable to initialize members in the initializer list, rather than the body of the constructor, so in fact you might consider putting other members in the initialization list

If you're thinking about creating a constructor that other ctors call, that's not available til c++0x (see inheriting constructors)

回答4:

I think your solution is the correct way to do things.

You can also postpone the creation of the object by making is pointer (however it changes the code and data type):

std::auto_ptr<udp::socket> _sock;

And then in body:

_sock.reset(new udp::soket(_io_service, ep));

But I think that your "workaround" is rather correct solution then workaround.

回答5:

You could turn the _sock member into a smart pointer:

#include <boost/asio.hpp>
#include <boost/array.hpp>
#include <boost/scoped_ptr.hpp>

using boost::asio::ip::udp;

template<class T>
class udp_sock
{
    public:
        udp_sock(std::string host, unsigned short port);
    private:
        boost::asio::io_service _io_service;
        boost::scoped_ptr<udp::socket> _sock_ptr;
        boost::array<T,256> _buf;
};

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost",
  unsigned short port = 50000)
{
    udp::resolver res(_io_service);
    udp::resolver::query query(udp::v4(), host, "spec");
    udp::endpoint ep = *res.resolve(query);
    ep.port(port);
    _sock_ptr.reset(new udp::socket(_io_service, ep));
}

回答6:

If it's to initialize a variable during construction in a class's constructor the right way is:

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost", unsigned short port = 50000)
    :res(_io_service)
    ,query(udp::v4(), host, "spec")
    ,ep(*res.resolve(query))
    ,_sock(_io_service, ep)
{
}

Edit: Forgot to mention that 'res', 'query' and 'ep' should be part of the class. Another crude method (without having _sock as a pointer) is as given below:

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost", unsigned short port = 50000)
    :_sock(_io_service, udp::resolver(_io_service).resolve(udp::resolver::query(udp::v4(),host,"spec"))
{
}

回答7:

Another option in this case is to work around the issue by creating a static function to build ep:

#include <boost/asio.hpp>
#include <boost/array.hpp>

using boost::asio::ip::udp;

template<class T>
class udp_sock
{
    public:
        udp_sock(std::string host, unsigned short port);
    private:
        static udp::endpoint build_ep(const std::string &host,
          unsigned short port, boost::asio::io_service &io_service);

        boost::asio::io_service _io_service;
        udp::socket _sock;
        boost::array<T,256> _buf;
};

template<class T>
udp::endpoint udp_sock<T>::build_ep(const std::string &host,
  unsigned short port, boost::asio::io_service &io_service)
{
    udp::resolver res(io_service);
    udp::resolver::query query(udp::v4(), host, "spec");
    udp::endpoint ep = *res.resolve(query);
    ep.port(port);
    return ep;
}

template<class T>
udp_sock<T>::udp_sock(std::string host = "localhost",
  unsigned short port = 50000)
    : _sock(_io_service, build_ep(host, port, _io_service))
{
}