Function polymorphic_get

boost::polymorphic_get — Retrieves a value of a specified type from a given variant.

Synopsis

// In header: <boost/variant/polymorphic_get.hpp>


template<typename U, typename T1, typename T2, ..., typename TN> 
  U * polymorphic_get(variant<T1, T2, ..., TN> * operand);
template<typename U, typename T1, typename T2, ..., typename TN> 
  const U * polymorphic_get(const variant<T1, T2, ..., TN> * operand);
template<typename U, typename T1, typename T2, ..., typename TN> 
  U & polymorphic_get(variant<T1, T2, ..., TN> & operand);
template<typename U, typename T1, typename T2, ..., typename TN> 
  const U & polymorphic_get(const variant<T1, T2, ..., TN> & operand);

Description

The polymorphic_get function allows run-time checked, type-safe retrieval of the content of the given variant. The function succeeds only if the content is of the specified type U or of type derived from type U, with failure indicated as described below.

Warning: After either operand or its content is destroyed (e.g., when the given variant is assigned a value of different type), the returned reference is invalidated. Thus, significant care and caution must be extended when handling the returned reference.

Notes:	As part of its guarantee of type-safety, `polymorphic_get` enforces `const`-correctness. Thus, the specified type `U` must be `const`-qualified whenever `operand` or its content is likewise `const`-qualified. The converse, however, is not required: that is, the specified type `U` may be `const`-qualified even when `operand` and its content are not.
Returns:	If passed a pointer, `polymorphic_get` returns a pointer to the value content if it is of the specified type `U` or of type derived from type `U`; otherwise, a null pointer is returned. If passed a reference, `polymorphic_get` returns a reference to the value content if it is of the specified type `U` or of type derived from type `U`; otherwise, an exception is thrown (see below).
Throws:	Overloads taking a `variant` pointer will not throw; the overloads taking a `variant` reference throw `bad_polymorphic_get` if the content is not of the specified type `U`or of type derived from type `U`.
Rationale:	While visitation via `apply_visitor` is generally preferred due to its greater safety, `polymorphic_get` may may be more convenient in some cases due to its straightforward usage.

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