greenplumn RelationWrapper 源码
greenplumn RelationWrapper 代码
文件路径：/src/include/gpopt/utils/RelationWrapper.h
//---------------------------------------------------------------------------
// Greenplum Database
// Copyright (C) 2020 VMware, Inc.
//---------------------------------------------------------------------------
#ifndef GPDB_RelationWrapper_H
#define GPDB_RelationWrapper_H
#include <cstddef>
using Relation = struct RelationData *;
namespace gpdb
{
/// \class
/// A transparent RAII wrapper for a pointer to a Postgres RelationData.
/// "Transparent" means that an object of this type can be used in most contexts
/// that expect a Relation. The main advantage of using this wrapper is that it
/// automatically closes the wrapper relation when exiting scope. So you no
/// longer have to write code like this:
/// \code
/// void RetrieveRel(Oid reloid) {
/// Relation rel = GetRelation(reloid);
/// if (IsPartialDist(rel)) {
/// CloseRelation(rel);
/// GPOS_RAISE(...);
/// }
/// try {
/// do_stuff();
/// CloseRelation(rel);
/// catch (...) {
/// CloseRelation(rel);
/// GPOS_RETHROW(...);
/// }
/// }
/// \endcode
/// and instead you can write this:
/// \code
/// void RetrieveRel(Oid reloid) {
/// gpdb::RelationWrapper rel = GetRelation(reloid);
/// if (IsPartialDist(rel.get())) {
/// GPOS_RAISE(...);
/// }
/// do_stuff();
/// }
/// \endcode
class RelationWrapper
{
public:
RelationWrapper(RelationWrapper const &) = delete;
RelationWrapper(RelationWrapper &&r) : m_relation(r.m_relation)
{
r.m_relation = nullptr;
};
explicit RelationWrapper(Relation relation) : m_relation(relation)
{
}
/// allows use in typical conditionals of the form
///
/// \code if (rel) { do_stuff(rel); } \endcode or
/// \code if (!rel) return; \endcode
explicit operator bool() const
{
return m_relation != nullptr;
}
// behave like a raw pointer on arrow
Relation
operator->() const
{
return m_relation;
}
// get the raw pointer, behaves like std::unique_ptr::get()
Relation
get() const
{
return m_relation;
}
/// Explicitly close the underlying relation early. This is not usually
/// necessary unless there is significant amount of time between the point
/// of close and the end-of-scope
void Close();
~RelationWrapper() noexcept(false);
private:
Relation m_relation = nullptr;
};
} // namespace gpdb
#endif // GPDB_RelationWrapper_H
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