Re: [Maria-developers] e2017fc3a5f: MDEV-22441 implement a generic way to change a value of a variable in scope

Hi, Eugene! On Jun 03, Eugene Kosov wrote:

revision-id: e2017fc3a5f (mariadb-10.2.31-219-ge2017fc3a5f)

This should go into 10.5, not into old-old GA 10.2. Besides, 10.2 cannot use C++11 features, 10.5 can.

parent(s): 1b3adaab25c author: Eugene Kosov committer: Eugene Kosov timestamp: 2020-05-27 12:45:41 +0300 message:

MDEV-22441 implement a generic way to change a value of a variable in scope

Scope_value<T> class changes a variable value to some temporary value while it exits and restores an old value in it's destructor.

make_scope_value() is a function targeted for C++11 which allows to omit a template parameter which is required by Scope_value<T>

I think I'd name it backup_and_change_scoped() or something like that. But it's just a suggestion, feel free to keep make_scope_value() name if you like it better.

Sql_mode_save removed and the only usage is replaced with Scope_value<T>

Sql_mode_save removal should not be merged in newer branches!

--- include/scope.h | 46 ++++++++++++++++++++++++++++++++++++++++++++++ sql/sql_acl.cc | 7 ++++--- sql/sql_class.h | 16 ---------------- 3 files changed, 50 insertions(+), 19 deletions(-)

diff --git a/include/scope.h b/include/scope.h new file mode 100644 index 00000000000..81061ceed37 --- /dev/null +++ b/include/scope.h @@ -0,0 +1,46 @@ +/* + Copyright (c) 2020, MariaDB Corporation. + + This program is free software; you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; version 2 of the License. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program; if not, write to the Free Software + Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */ + +#pragma once

use include guards, please, not a pragma

+ +namespace maria

and, as you remember, we didn't quite decide on a global server-wide namespace, so it's not the MDEV that should introduce it.

+{ + +template <typename T> class Scope_value +{ +public: + Scope_value(T &variable, const T &scope_value) + : variable_(variable), saved_value_(variable) + { + variable= scope_value; + } + + ~Scope_value() { variable_= saved_value_; } + +private: + T &variable_; + T saved_value_; +}; + +// Starting from C++11 this allows to omit template argument like this: +// auto _ = make_scope_value(var, tmp_value); +template <typename T> +Scope_value<T> make_scope_value(T &variable, const T &scope_value) +{ + return Scope_value<T>(variable, scope_value); +}

as you've seen from Sql_mode_save usage, sometimes the value is assigned later, not when it's saved. Please, add another helper that doesn't require to provide a value. Like template <typename T> Scope_value<T> backup_scoped(T &variable) { return Scope_value<T>(variable, variable); }

+ +} // namespace maria

Regards, Sergei VP of MariaDB Server Engineering and security@mariadb.org