Click here to Skip to main content
Click here to Skip to main content

Tiny Template Library: variant

, 23 Feb 2004
Rate this:
Please Sign up or sign in to vote.
An article on how to implement and use variant. Variant is useful for creating heterogeneous containers and much more.
<!------------------------------- STEP 3 ---------------------------><!-- Add the article text. Please use simple formatting (

,

etc) -->

Introduction

My standard warning: Don't try to compile this project with MSVC v6.0/v7.0. This project requires a compliant compiler. MSVC v7.1 or GCC v3.2.3 will work just fine. In the article about typelist, I briefly mentioned variant. Here, I'd like to discuss it in more detail. The code in this article is to demonstrate the basic ideas only. The actual implementation can be found in TTL. In C++, it is not legal to have non-POD data types in union. For example the following code won't compile.

struct my_type
{
   int x;
   my_type() : x(0) {}
   virtual ~my_type();
};

union my_union
{
   my_type a;
   double b;
};

The variant template solves this problem and adds a lot of other cool features. One application of variant is heterogeneous containers.

typedef variant< my_type, double > mv;

main()
{
  my_type a;
  
  std::vector< mv > v;
  v.push_back(2.3); //add double 
  v.push_back(a); //add my_type
}

The variant semantic was inspired by boost::variant. A very good discussion about variant can be found in "Modern C++ Design" by A. Alexandrescu.

Implementation

variant has a variable number of template parameters.

variant<int>

variant<int, double>

...
To support variable numbers of template parameters, we use the technique that was suggested in the typelist article.

The main variant implementation ideas are:

  • Compile-time: convert variant template parameters to typelist.
  • Compile-time: using the typelist, find the largest element and reserve a buffer of this element size.
  • Run-time: use the reserved buffer to construct controlled objects in place.
  • Run-time: keep the index of the current instance type.
  • Run-time: if not initialized, variant is in a singular state.

    The variant pseudo-code looks like this:

    template < typename T1, typename T2, ... > 
    struct variant
    {
        //list of user types
        typedef meta::typelist< T1, T2,...> types;
        
        //initialization 
        variant() : p_(0) {}
        
        template< typename T >
        variant( const T& d ) : p_(0) 
        {
            //find the index of this type in the variant typelist
            which_ = find_type<T>::value;
            //in place construction
            p_ = new(buffer_) data_holder<T>(d);
        }
        
        virtual ~variant() { destroy(); }
        
        inline int which() const { return which_; }
        
        inline bool is_valid() const { return p_ != 0; }
        
        void destroy() 
        { 
            if(!is_valid()) return;
            p_->~data_holder_base(); 
            p_ = 0;
        }
        
    private:
        
        //data_holder is a wrapper for the user types
        
        struct data_holder_base 
        { 
            virtual ~data_holder_base() {}
        };
        
        temlate< typename T >
        struct data_holder : data_holder_base
        {
            T d_;
            data_holder( const T& d ) : d_(d) {}
        };
        
        //list of data holder types
        typedef meta::typelist<data_holder<T1>, data_holder<T2>,...> holder_types;
        
        //reserve enough space to hold the largest type
        char buffer_[find_largest_type<holder_types>::value];
    
    
        
        //pointer to the controlled object
        data_holder_base *p_;
        
        //object type index
        int which_;
    };
    
    Please note: the above code is only a pseudo-code. The actual implementation is more complex. It might take a small book to describe all the details. I'd rather talk about how to use variant in practice.

    Using variant

    Let's consider a simple example:
    typedef variant<int, double> my_variant;
    
    This typedef defines a data type that can contain a double or int variable. Suppose that we need to write a function that does something with my_variant depending on the variable data type. We can use a simple switch/case statement.
    void f( my_variant& v )
    {
        int n;
        double x;
        switch( v.which() )
        {
        case 0:  //int variable
            n = get<int>(v);
            //do something with int;
            break;
            
        case 1:  //double variable
            x = get<double>(v);
            //do something with the double;
            break;
        }
    };
    

    As you can see, the get<> function can be used to retrieve the typed data from variant<>. Obviously this switch statement is ugly and not very flexible. The function f() has to know the type indexes in my_variant. One way to solve these problems is to utilize the Gof visitor pattern ideas.

  • Define a variant visitor functor that has a separate operator() for all types in variant.
  • When applied to the variant, an appropriate visitor's operator() is called.

    TTL's variant has the apply_visitor<> function that takes care of calling the appropriate visitor operator(). Using this technique, the above example can be implemented as follows.
    typedef variant<int, double> my_variant;
    
    struct visitor
    {
        void operator()(int n)
        {
        //do something with the int;
        ...
        }
        void operator()(double x)
        {
        //do something with the double;
        ...
        }
        
        //ignore any other types
        template< typename T >
        void operator()( T d )
        {
        }
    };
    
    my_variant var;
    visitor vis;
    apply_visitor(var, vis);
    

    I think that it looks much nicer and we don't have to worry about type indexes or any other type identifiers for the same matter. The apply_visitor() function is implemented in TTL. apply_visitor does the following:

  • finds what type is identified by the which_ member;
  • casts the object's pointer to this type;
  • passes the casted pointer to the user supplied visitor.
  • the compiler automatically selects the appropriate operator().

    Another interesting implementation of variant is event dispatching. Suppose we have an event source that can generate multiple event types. For the simplicity sake, the event types are int and double.  We can define the event type as following:

    typedef variant< int, double > event;
    
    Now we need a way to specify a callback function that will be called by the event source to notify the client or observer. It is convenient to define callbacks using generic functors (see, TTL:implementing functors)

    typedef function< void (event&) > callback;
    
    Now we can put everything together:
    typedef variant< int, double > event;
    typedef function< void (event&) > callback;
    
    struct event_source
    {
        callback cb_
    
        event_source( callback& cb ) : cb_(cb) {}
    
        void do_something()
        {
            event ev;
    
            ...
            //generate int event
            ev = 1;
            cb_( ev );
    
            ....
            //generate double event
            ev = 2.3;
            cb_( ev );
        }
    };
    
    //define our event vistor
    struct event_visitor
    {
        //process int event
        void operator()(int n)
        { 
            cout << "got int:" << n;
        }
        
        //process double event
        void operator()(double n)
        { 
            cout << "got double:" << n;
        }
        
        //ignore any other events
        template< typename T >
        void operator()( T d )
        {
        }
    }
    
    void my_callback( event& e )
    {
        event_visitor vistor;
        apply_visitor(e, vistor);
    }
    
    main()
    {
        event_source src(my_callback);
        src.do_something();
    }
    
    You can find a working example in the samples/test folder. It is not hard to extend this example to a complete Observer pattern implementation w/o any polymorphic inheritances. As a result, a "strong" type checking is performed at compile time.
  • License

    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here

    Share

    About the Author

    kig
    Web Developer
    United States United States
    No Biography provided

    Comments and Discussions

     
    GeneralPerformance and Details Pinmembertpolzin25-Aug-04 4:27 
    GeneralRe: Performance and Details Pinmemberkig25-Aug-04 19:02 
    GeneralRe: Performance and Details Pinmembertpolzin25-Aug-04 23:26 
    GeneralRe: Performance and Details Pinmemberkig26-Aug-04 18:16 
    GeneralRe: Performance and Details Pinmembertpolzin26-Aug-04 22:58 
    GeneralRe: Performance and Details Pinmemberkig1-Sep-04 20:50 
    GeneralRe: Performance and Details Pinmembertpolzin2-Sep-04 1:15 
    GeneralRe: Performance and Details Pinmemberkig6-Sep-04 15:45 
    QuestionWhat's the difference to Boost.Variant? PinmemberHartmut Kaiser25-Feb-04 21:47 
    AnswerRe: What's the difference to Boost.Variant? Pinmemberkig26-Feb-04 7:01 
    Boost.Variant is a fine piece of code.
    However there are few important differences between boost and ttl semantics.
     
    1. ttl::variant can be in an uninitialized (singular) state. It means that the variant doesn't have a valid data. This concept is similar to 'union' that hasn't been initialized except that ttl::variant has the is_singular() method to check it. ttl::variant becomes singular if it has not been initialized or there was an exception during assignment. On the other hand boost::variant always contains a user defined type. If the assignment fails the content of boost::variant might be changed under the covers to a default-constrictable value. boost::variant visitors cannot assume that the variant content has not been changed by the variant internally. I find it disturbing. On the other hand, ttl::variant visitors simply won't be called if the variant is in singular state. So that if a visitor is called, the variant content is guaranteed to be valid (that was set by the user).
     

    2. To support the requirement of non-singularity, boost::variant has to sometimes allocate objects on the heap.
    If none of your variant types has a non-throw default constructible type, boost::variant creates (during assignements) an instance on the heap. For example the following variant type will allocate a default constrictable instance of the user type on heap each time you do an assignment.
     
    struct my_type {}
    typedef boost::variant< my_type > my_variant.
     
    ttl::variant never touches the memory heap! So in some cases ttl::variant can be a much more efficient solution.
     
    You can always prevent boost::variant from using the memory heap by adding a a non-throw default-constrictable type such as 'int'. For example the following variant won't touch the memory heap.
     
    typedef boost::variant< my_type, int > my_variant.
     
    However you'll have to add handlers for this dummy 'int' to all of your visitors. Also adding 'int' might be problemtic in generic context.
     

     
    3. ttl::variant visitors are not required to be derived from another class like boost::variant visitors need to be derived from boost::variant::static_visitor.
     
    4. ttl visitors cannot return a value. This limitation has a simple workaround with a wrapper class.
     
    int visitor( my_type& x );
     
    struct visitor_wrapper
    {
    int return_value;
     
    void operator()( my_type& x )
    {
    return_value = visitor(x);
    }
    };
     
    variant< my_type& > var;
    visitor_wrapper vw;
    apply_visitor(var, vw);
    int r = vw.return_value;
     
    I am planning to generalize such a wrapper latter.
     
    5. IMHO, ttl::variant implementation is much simpler. As a result the compilation times are shorter.
     
    Best,
    kig

    GeneralRe: What's the difference to Boost.Variant? PinmemberHartmut Kaiser26-Feb-04 9:08 
    GeneralRe: What's the difference to Boost.Variant? Pinmemberkig26-Feb-04 11:28 
    AnswerRe: What's the difference to Boost.Variant? Pinmemberkig26-Feb-04 11:49 
    GeneralRe: What's the difference to Boost.Variant? PinmemberHartmut Kaiser26-Feb-04 19:40 

    General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

    Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

    | Advertise | Privacy | Mobile
    Web02 | 2.8.140827.1 | Last Updated 24 Feb 2004
    Article Copyright 2004 by kig
    Everything else Copyright © CodeProject, 1999-2014
    Terms of Service
    Layout: fixed | fluid