Description

Defines a 3-parameter function if_else() that acts as an analogue to the ternary operator (?:), since the ternary operator is not user-overloadable. The return type of if_else() is an expression instantiated from the expr_template expression template.

At least one parameter to if_else() must be an expression.

For each parameter E passed to if_else(), if E is an rvalue, E is moved into the result, and otherwise E is captured by reference into the result.

Example:

template <boost::yap::expr_kind Kind, typename Tuple>
struct user_expr
{
    static const boost::yap::expr_kind kind = Kind;

    Tuple elements;
};

// Defines an overload of if_else() that returns expressions instantiated from
// user_expr.
BOOST_YAP_USER_EXPR_IF_ELSE(::user_expr)