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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