Re: Initializing member references to dummy member variables

From:

Luca Risolia <luca.risolia@studio.unibo.it>

Newsgroups:

comp.lang.c++

Date:

Mon, 12 Aug 2013 07:29:23 +0200

Message-ID:

<ku9rri$gjm$1@speranza.aioe.org>

K. Frank wrote:

   struct Handle {
     Handle (int iHandle) :
       iHandle_(iHandle),
       sHandle_(dummy),
       useIHandle_(true)
     {}
     Handle (const std::string& sHandle) :
       iHandle_(0),
       sHandle_(sHandle),
       useIHandle_(false)
     {}
     const int iHandle_;
     const std::string& sHandle_;
     bool useIHandle_;
     std::string dummy_;
   }

The point is that the member reference variable sHandle_
is supposed to be initialized (in the constructors'
initialization lists), whether or not it's actually
going to be used. The constructor that takes an int
argument doesn't have any std::strings floating
around with which to initialize sHandle_, hence the
introduction of the member variable dummy_.

Is this a reasonable approach? Is there an established
idiom for doing this kind of thing?

Use boost::optional or wait for std::optional :

#include <iostream>
#include <string>
#include <boost/optional.hpp>

using boost::optional; // there will be std::optional in C++1y

struct Handle {
    Handle(int iHandle) : iHandle_(iHandle) { }
    Handle(const std::string& sHandle) : iHandle_(0), sHandle_(sHandle) { }
    const int iHandle_;
    optional<const std::string&> sHandle_;
};

int main() {
    std::string s = "Hello World!";
    Handle h1(2);
    Handle h2(s);
    // ...
    if (h2.sHandle_)
        std::cout << h2.sHandle_.get() << '\n';
}