Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit dcea408971.
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Function relaxed_get

boost::relaxed_get — Retrieves a value of a specified type from a given variant. Unlike strict_get does not assert at compile time that type U is one of the types that can be stored in variant.

Synopsis

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


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

Description

The 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, with failure indicated as described below.

Recomendation: Use get or strict_get in new code. strict_get provides more compile time checks and its behavior is closer to std::get from C++ Standard Library.

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, 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, get returns a pointer to the value content if it is of the specified type U; otherwise, a null pointer is returned. If passed a reference, get returns a reference to the value content if it is of the specified 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_get if the content is not of the specified type U.

Rationale:

While visitation via apply_visitor is generally preferred due to its greater safety, get may may be more convenient in some cases due to its straightforward usage.

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