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Posted 23 Feb 2004

Tiny Template Library: variant

, 23 Feb 2004
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An article on how to implement and use variant<>. Variant is useful for creating heterogeneous containers and much more.
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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;

  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.


variant has a variable number of template parameters.


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;
        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_ = 0;
        //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;
        case 1:  //double variable
            x = get<double>(v);
            //do something with the double;

    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);
        event_source src(my_callback);
    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


    About the Author

    Software Developer (Senior)
    United States United States
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    Comments and Discussions

    GeneralPerformance and Details Pin
    tpolzin25-Aug-04 4:27
    membertpolzin25-Aug-04 4:27 
    I looked at different possibilities for event dispatching, similiar to the one described in your example. I did some basic perfomance tests and found the following:

    union, enum, switch-statement 0.05
    union, functionptr 0.13
    base class/virtual method 0.52
    base class/virtual method using boost::object_pool lazy-delete
    base class/virtual method using boost::pool
    boost::bind 0.44
    boost::variant 0.28
    ttl::varaint 0.25

    (microseconds on my PC, gcc 3.3.3 -O3 )

    I won't go into details how the different methods were implemented and the results are nothing to rely on. Overall it seems, that using variant has the best tradeoff between performance and ease of coding.
    Still, I was surprised that boost::pool is faster than variant and that the gap to using a hand coded union, _which enum and a switch statement or a functionpoiter is noticable.
    Do you have an explaination for this?

    A disadvantage of using variant is that without optimization, it is the slowest. (>1 microsecond).
    A sidenote: Using gcc 3.4.1 each of the two variant's get slower, and ttl::variant is then slower than boost::variant.

    Concerning your article, I have some probably silly questions:
    1. What is the reasoning for making ~variant "virtual"?
    2. What does "p_->~data_holder_base();" do, as there is no destructor defined in data_holder_base or any data_holder?
    3. In the description of "apply_visitor": "the compiler automatically selects the appropriate operator()".
    Did I get it right that in the case the compiler cannot detect the current type at runtime (e.g. there is a vector of events, created at runtime) the following happens: Using typelists and inlining the compiler generates automatically an if-then-else chain: "if (which_ == 1 ) visitor( get<type1>(v)); else if (which_ == 2 ) ..."

    GeneralRe: Performance and Details Pin
    kig25-Aug-04 19:02
    memberkig25-Aug-04 19:02 
    GeneralRe: Performance and Details Pin
    tpolzin25-Aug-04 23:26
    membertpolzin25-Aug-04 23:26 
    GeneralRe: Performance and Details Pin
    kig26-Aug-04 18:16
    memberkig26-Aug-04 18:16 
    GeneralRe: Performance and Details Pin
    tpolzin26-Aug-04 22:58
    membertpolzin26-Aug-04 22:58 
    GeneralRe: Performance and Details Pin
    kig1-Sep-04 20:50
    memberkig1-Sep-04 20:50 
    GeneralRe: Performance and Details Pin
    tpolzin2-Sep-04 1:15
    membertpolzin2-Sep-04 1:15 
    GeneralRe: Performance and Details Pin
    kig6-Sep-04 15:45
    memberkig6-Sep-04 15:45 
    QuestionWhat's the difference to Boost.Variant? Pin
    Hartmut Kaiser25-Feb-04 21:47
    memberHartmut Kaiser25-Feb-04 21:47 
    AnswerRe: What's the difference to Boost.Variant? Pin
    kig26-Feb-04 7:01
    memberkig26-Feb-04 7:01 
    GeneralRe: What's the difference to Boost.Variant? Pin
    Hartmut Kaiser26-Feb-04 9:08
    memberHartmut Kaiser26-Feb-04 9:08 
    GeneralRe: What's the difference to Boost.Variant? Pin
    kig26-Feb-04 11:28
    memberkig26-Feb-04 11:28 
    AnswerRe: What's the difference to Boost.Variant? Pin
    kig26-Feb-04 11:49
    memberkig26-Feb-04 11:49 
    GeneralRe: What's the difference to Boost.Variant? Pin
    Hartmut Kaiser26-Feb-04 19:40
    memberHartmut Kaiser26-Feb-04 19:40 

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