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Basic Usage
Advanced Topics

Basic Usage

A discriminated union container on some set of types is defined by instantiating the boost::variant class template with the desired types. These types are called bounded types and are subject to the requirements of the BoundedType concept. Any number of bounded types may be specified, up to some implementation-defined limit (see BOOST_VARIANT_LIMIT_TYPES).

For example, the following declares a discriminated union container on int and std::string:

boost::variant< int, std::string > v;

By default, a variant default-constructs its first bounded type, so v initially contains int(0). If this is not desired, or if the first bounded type is not default-constructible, a variant can be constructed directly from any value convertible to one of its bounded types. Similarly, a variant can be assigned any value convertible to one of its bounded types, as demonstrated in the following:

v = "hello";

Now v contains a std::string equal to "hello". We can demonstrate this by streamingv to standard output:

std::cout << v << std::endl;

Usually though, we would like to do more with the content of a variant than streaming. Thus, we need some way to access the contained value. There are two ways to accomplish this: apply_visitor, which is safest and very powerful, and get<T>, which is sometimes more convenient to use.

For instance, suppose we wanted to concatenate to the string contained in v. With value retrieval by get, this may be accomplished quite simply, as seen in the following:

std::string& str = boost::get<std::string>(v);
str += " world! ";

As desired, the std::string contained by v now is equal to "hello world! ". Again, we can demonstrate this by streaming v to standard output:

std::cout << v << std::endl;

While use of get is perfectly acceptable in this trivial example, get generally suffers from several significant shortcomings. For instance, if we were to write a function accepting a variant<int, std::string>, we would not know whether the passed variant contained an int or a std::string. If we insisted upon continued use of get, we would need to query the variant for its contained type. The following function, which "doubles" the content of the given variant, demonstrates this approach:

void times_two( boost::variant< int, std::string > & operand )
    if ( int* pi = boost::get<int>( &v ) )
        *pi *= 2;
    else if ( std::string* pstr = boost::get<std::string>( &v ) )
        *pstr += *pstr;

However, such code is quite brittle, and without careful attention will likely lead to the introduction of subtle logical errors detectable only at runtime. For instance, consider if we wished to extend times_two to operate on a variant with additional bounded types. Specifically, let's add std::complex<double> to the set. Clearly, we would need to at least change the function declaration:

void times_two( boost::variant< int, std::string, std::complex<double> > & operand )
    // as above...?

Of course, additional changes are required, for currently if the passed variant in fact contained a std::complex value, times_two would silently return -- without any of the desired side-effects and without any error. In this case, the fix is obvious. But in more complicated programs, it could take considerable time to identify and locate the error in the first place.

Thus, real-world use of variant typically demands an access mechanism more robust than get. For this reason, variant supports compile-time checked visitation via apply_visitor. Visitation requires that the programmer explicitly handle (or ignore) each bounded type. Failure to do so results in a compile-time error.

Visitation of a variant requires a visitor object. The following demonstrates one such implementation of a visitor implementating behavior identical to times_two:

class times_two_visitor
    : public boost::static_visitor<>

    void operator()(int & i) const
        i *= 2;

    void operator()(std::string & str) const
        str += str;


With the implementation of the above visitor, we can then apply it to v, as seen in the following:

boost::apply_visitor( times_two_visitor(), v );

As expected, the content of v is now a std::string equal to "hello world! hello world! ". (We'll skip the verification this time.)

In addition to enhanced robustness, visitation provides another important advantage over get: the ability to write generic visitors. For instance, the following visitor will "double" the content of anyvariant (provided its bounded types each support operator+=):

class times_two_generic
    : public boost::static_visitor<>

    template <typename T>
    void operator()( T & operand ) const
        operand += operand;


Again, apply_visitor sets the wheels in motion:

boost::apply_visitor( times_two_generic(), v );

While the initial setup costs of visitation may exceed that required for get, the benefits quickly become significant. Before concluding this section, we should explore one last benefit of visitation with apply_visitor: delayed visitation. Namely, a special form of apply_visitor is available that does not immediately apply the given visitor to any variant but rather returns a function object that operates on any variant given to it. This behavior is particularly useful when operating on sequences of variant type, as the following demonstrates:

std::vector< boost::variant<int, std::string> > vec;
vec.push_back( 21 );
vec.push_back( "hello " );

times_two_generic visitor;
      vec.begin(), vec.end()
   , boost::apply_visitor(visitor)

Advanced Topics

This section discusses several features of the library often required for advanced uses of variant. Unlike in the above section, each feature presented below is largely independent of the others. Accordingly, this section is not necessarily intended to be read linearly or in its entirety.

Preprocessor macros

While the variant class template's variadic parameter list greatly simplifies use for specific instantiations of the template, it significantly complicates use for generic instantiations. For instance, while it is immediately clear how one might write a function accepting a specific variant instantiation, say variant<int, std::string>, it is less clear how one might write a function accepting any given variant.

Due to the lack of support for true variadic template parameter lists in the C++98 standard, the preprocessor is needed. While the Preprocessor library provides a general and powerful solution, the need to repeat BOOST_VARIANT_LIMIT_TYPES unnecessarily clutters otherwise simple code. Therefore, for common use-cases, this library provides its own macro BOOST_VARIANT_ENUM_PARAMS.

This macro simplifies for the user the process of declaring variant types in function templates or explicit partial specializations of class templates, as shown in the following:

// general cases
template <typename T> void some_func(const T &);
template <typename T> class some_class;

// function template overload
template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
void some_func(const boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> &);

// explicit partial specialization
template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
class some_class< boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> >;
Using a type sequence to specify bounded types

While convenient for typical uses, the variant class template's variadic template parameter list is limiting in two significant dimensions. First, due to the lack of support for true variadic template parameter lists in C++, the number of parameters must be limited to some implementation-defined maximum (namely, BOOST_VARIANT_LIMIT_TYPES). Second, the nature of parameter lists in general makes compile-time manipulation of the lists excessively difficult.

To solve these problems, make_variant_over< Sequence > exposes a variant whose bounded types are the elements of Sequence (where Sequence is any type fulfilling the requirements of MPL's Sequence concept). For instance,

typedef mpl::vector< std::string > types_initial;
typedef mpl::push_front< types_initial, int >::type types;

boost::make_variant_over< types >::type v1;

behaves equivalently to

boost::variant< int, std::string > v2;

Portability: Unfortunately, due to standard conformance issues in several compilers, make_variant_over is not universally available. On these compilers the library indicates its lack of support for the syntax via the definition of the preprocessor symbol BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT.

Recursive variant types

Recursive types facilitate the construction of complex semantics from simple syntax. For instance, nearly every programmer is familiar with the canonical definition of a linked list implementation, whose simple definition allows sequences of unlimited length:

template <typename T>
struct list_node
    T data;
    list_node * next;

The nature of variant as a generic class template unfortunately precludes the straightforward construction of recursive variant types. Consider the following attempt to construct a structure for simple mathematical expressions:

struct add;
struct sub;
template <typename OpTag> struct binary_op;

typedef boost::variant<
    , binary_op<add>
    , binary_op<sub>
    > expression;

template <typename OpTag>
struct binary_op
    expression left;  // variant instantiated here...
    expression right;

    binary_op( const expression & lhs, const expression & rhs )
        : left(lhs), right(rhs)

}; // ...but binary_op not complete until here!

While well-intentioned, the above approach will not compile because binary_op is still incomplete when the variant type expression is instantiated. Further, the approach suffers from a more significant logical flaw: even if C++ syntax were different such that the above example could be made to "work," expression would need to be of infinite size, which is clearly impossible.

To overcome these difficulties, variant includes special support for the boost::recursive_wrapper class template, which breaks the circular dependency at the heart of these problems. Further, boost::make_recursive_variant provides a more convenient syntax for declaring recursive variant types. Tutorials for use of these facilities is described in the section called “Recursive types with recursive_wrapper and the section called “Recursive types with make_recursive_variant.

Recursive types with recursive_wrapper

The following example demonstrates how recursive_wrapper could be used to solve the problem presented in the section called “Recursive variant types”:

typedef boost::variant<
    , boost::recursive_wrapper< binary_op<add> >
    , boost::recursive_wrapper< binary_op<sub> >
    > expression;

Because variant provides special support for recursive_wrapper, clients may treat the resultant variant as though the wrapper were not present. This is seen in the implementation of the following visitor, which calculates the value of an expression without any reference to recursive_wrapper:

class calculator : public boost::static_visitor<int>

    int operator()(int value) const
        return value;

    int operator()(const binary_op<add> & binary) const
        return boost::apply_visitor( calculator(), binary.left )
             + boost::apply_visitor( calculator(), binary.right );

    int operator()(const binary_op<sub> & binary) const
        return boost::apply_visitor( calculator(), binary.left )
             - boost::apply_visitor( calculator(), binary.right );


Finally, we can demonstrate expression in action:

void f()
    // result = ((7-3)+8) = 12
    expression result(
          , 8

    assert( boost::apply_visitor(calculator(),result) == 12 );
Recursive types with make_recursive_variant

For some applications of recursive variant types, a user may be able to sacrifice the full flexibility of using recursive_wrapper with variant for the following convenient syntax:

typedef boost::make_recursive_variant<
    , std::vector< boost::recursive_variant_ >
    >::type int_tree_t;

Use of the resultant variant type is as expected:

std::vector< int_tree_t > subresult;

std::vector< int_tree_t > result;

int_tree_t var(result);

To be clear, one might represent the resultant content of var as ( 1 ( 3 5 ) 7 ).

Finally, note that a type sequence can be used to specify the bounded types of a recursive variant via the use of boost::make_recursive_variant_over, whose semantics are the same as make_variant_over (which is described in the section called “Using a type sequence to specify bounded types”).

Portability: Unfortunately, due to standard conformance issues in several compilers, make_recursive_variant is not universally supported. On these compilers the library indicates its lack of support via the definition of the preprocessor symbol BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT. Thus, unless working with highly-conformant compilers, maximum portability will be achieved by instead using recursive_wrapper, as described in the section called “Recursive types with recursive_wrapper.

Binary visitation

As the tutorial above demonstrates, visitation is a powerful mechanism for manipulating variant content. Binary visitation further extends the power and flexibility of visitation by allowing simultaneous visitation of the content of two different variant objects.

Notably this feature requires that binary visitors are incompatible with the visitor objects discussed in the tutorial above, as they must operate on two arguments. The following demonstrates the implementation of a binary visitor:

class are_strict_equals
    : public boost::static_visitor<bool>

    template <typename T, typename U>
    bool operator()( const T &, const U & )
        return false; // cannot compare different types

    template <typename T>
    bool operator()( const T & lhs, const T & rhs )
        return lhs == rhs;


As expected, the visitor is applied to two variant arguments by means of apply_visitor:

boost::variant< int, std::string > v1( "hello" );

boost::variant< double, std::string > v2( "hello" );
assert( boost::apply_visitor(are_strict_equals(), v1, v2) );

boost::variant< int, const char * > v3( "hello" );
assert( !boost::apply_visitor(are_strict_equals(), v1, v3) );

Finally, we must note that the function object returned from the "delayed" form of apply_visitor also supports binary visitation, as the following demonstrates:

typedef boost::variant<double, std::string> my_variant;

std::vector< my_variant > seq1;
seq1.push_back("pi is close to ");

std::list< my_variant > seq2;
seq2.push_back("pi is close to ");

are_strict_equals visitor;
assert( std::equal(
      v1.begin(), v1.end(), v2.begin()
    , boost::apply_visitor( visitor )
    ) );
Copyright 2002, 2003 Eric Friedman, Itay Maman