Make a class with no default constructor a private property of another class

Question

I am trying to make a class that manipulates with Boost sockets to make the conections simple to use.
My SocketClient class has a few properties with boost::asio::ip::tcp::socket being one of them. But I get C2512 error in my constructor, because boost::asio::ip::tcp::socket cannot exist unitialised, as it has no constructor.
Here, see the code of the class:

class SocketClient {
private:
    int port;    //Port, currently unused
    boost::asio::io_service io_service;  
    boost::asio::ip::tcp::resolver::iterator endpoint_iterator;  
    boost::asio::ip::tcp::socket sock;  //This causes the error
    //It wants to be like this (impossible too):
  //boost::asio::ip::tcp::socket sock(io_service);
public:
    void init(const char*, const char* );
    SocketClient();  //Default constructor
    bool connect();
    bool read(int bytes, char *text);
    bool send(int length, char *text);
    bool send(std::string text);
    unsigned int timeout;
};

And here is the constructor:

SocketClient::SocketClient() {  //ERROR: (23): error C2512: 'boost::asio::basic_stream_socket<Protocol>' : no appropriate default constructor available
    sock=boost::asio::ip::tcp::socket(io_service);  //Adding this didn't help
}

So what to do? Do I have to keep sock as void*?

possible duplicate of [Avoid default constructor for member variable](http://stackoverflow.com/questions/15420547/avoid-default-constructor-for-member-variable) — David G, Mar 14 '13 at 21:36
I find [this question](http://stackoverflow.com/questions/15420547/avoid-default-constructor-for-member-variable), posed just six minutes prior to this one, strikingly similar. — chris, Mar 14 '13 at 21:36
You are right, but note that six minutes it takes to write the question, and google indexes a bit slower. — Tomáš Zato, Mar 14 '13 at 21:43

Andy Prowl · Accepted Answer · 2013-03-14T22:06:01.173

5

Use initialization lists:

SocketClient::SocketClient() 
    :
    sock(io_service)
{
    // Other initialization code here...
}

Be careful though: this is well-defined because member variables are constructed in the order they appear in the class definition, and io_service appears before sock. If that wasn't the case, you would pass an uninitialized object to the constructor of socket, most likely resulting in Undefined Behavior.

edited Mar 14 '13 at 22:06

answered Mar 14 '13 at 21:35

Andy Prowl

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Could you explain it more? – Tomáš Zato Mar 14 '13 at 21:35
@TomášZato, http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor – chris Mar 14 '13 at 21:37
I just noticed that using your trick, I can not define default constructor any more... – Tomáš Zato Mar 14 '13 at 21:51
@TomášZato: What do you mean? Initialization lists don't prevent you from defining a default constructor. In fact, that's what I defined. – Andy Prowl Mar 14 '13 at 21:51
When I try to define `SocketClient::SocketClient()` I get **redefinition error** - and I am warned that I am redefining the constructor defined by your code. Isn't there anything I should add to it? – Tomáš Zato Mar 14 '13 at 21:59
@TomášZato: My definition above *is* the definition of the default constructor. Why do you want to define two default constructors? Of course you can't do that – Andy Prowl Mar 14 '13 at 22:00
Maybe I want another things to happen, when class is defined, not only initialise `socket`. – Tomáš Zato Mar 14 '13 at 22:04
@TomášZato: Then add those things in the body of the constructor, as you would normally do. The constructor body in my example is empty because the constructor body in *your* example did nothing else than initializing `sock`, but you are free to change it – Andy Prowl Mar 14 '13 at 22:05

score 0 · Answer 2 · answered Sep 29 '14 at 15:54

If you are using C++ 11, you can do it like this:

boost::asio::io_service io_service;  
boost::asio::ip::tcp::resolver::iterator endpoint_iterator;  
boost::asio::ip::tcp::socket sock { io_service };

But (as Andy Prowl said) io_service must be must be placed before sock in the member list.

This would compile, but may lead to unpredictable errors:

boost::asio::ip::tcp::socket sock { io_service };
boost::asio::io_service io_service;

stefan · Answer 3 · 2014-10-16T12:27:12.177

generally - as already stated - whenever possible use initialization lists. also use a naming scheme that identifies members.

also already stated - initialization depends on member order. if you have dependencies between members this is most often a sign for a design error. it smells. at least document this well in the code - there is always a next maintainer. of course you can alway hold elements via typed (smart) pointers.

for the concrete problem I'd suggest to pass the io_service via reference. this would give you more control over the io_service (eg. use async mode and run multiple sockets within one io_service)

#include <boost/asio.hpp>

class SocketClient {
private:
    boost::asio::io_service& m_io_service;
    boost::asio::ip::tcp::socket m_socket;

[...]

public:
    SocketClient(boost::asio::io_service& io_service);

[...]

};


SocketClient::SocketClient(boost::asio::io_service& io_service)
    : m_io_service(io_service)
    , m_socket(io_service)
    [...]
{
}

score 0 · Answer 4 · answered Mar 20 '15 at 08:35

0

header .h

boost::asio::ip::tcp::socket *sock;

body .cpp

sock = new boost::asio::ip::tcp::socket(service);

answered Mar 20 '15 at 08:35

sitev_ru

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Make a class with no default constructor a private property of another class

4 Answers4