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)> >;

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 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<
      int
    , 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”.

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

class calculator : public boost::static_visitor<int>
{
public:

    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 );
    }

};

void f()
{
    // result = ((7-3)+8) = 12
    expression result(
        binary_op<add>(
            binary_op<sub>(7,3)
          , 8
          )
      );

    assert( boost::apply_visitor(calculator(),result) == 12 );
}

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

std::vector< int_tree_t > subresult;
subresult.push_back(3);
subresult.push_back(5);

std::vector< int_tree_t > result;
result.push_back(1);
result.push_back(subresult);
result.push_back(7);

int_tree_t var(result);

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>
{
public:

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

    template <typename T>
    bool operator()( const T & lhs, const T & rhs ) const
    {
        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 ");
seq1.push_back(3.14);

std::list< my_variant > seq2;
seq2.push_back("pi is close to ");
seq2.push_back(3.14);

are_strict_equals visitor;
assert( std::equal(
      v1.begin(), v1.end(), v2.begin()
    , boost::apply_visitor( visitor )
    ) );