#
# Copyright (C) 2013 Canonical Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License version 3 as
# published by the Free Software Foundation.
#
# 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 Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# Authored by: Michi Henning <michi.henning@canonical.com>
#

This is an explanation of the layout of the source tree, and the
conventions used by this library. Please read this before making changes
to the source!

For instructions on how to build the code, see the INSTALL file.


Build targets
-------------

The build produces the Lomiri API library (liblomiri-api).

TODO: Flesh this out


Source tree layout
------------------

At the top level, we have src, include, and test, which contain what
they suggest.

Underneath src and include, we have subdirectories that correspond to
namespaces, that is, if we have src/internal, that automatically means
that there is a namespace lomiri::scopes::internal. This one-to-one correspondence
makes it easier to work out what is defined where.


Namespaces
----------

The library maintains a compilation firewall, strictly separating
external API and internal implementation.  This reduces the chance of
breaking the API, and it makes it clear what parts of the API are for
public consumption.

Anything that is not public is implemented in a namespace called "internal".

TODO: More explanation of the namespaces used and for what.


Header file conventions
-----------------------

All header files are underneath include/lomiri. Source code always includes
headers using the full pathname, for example:

  #include <lomiri/internal/ExceptionImpl.h>

All source code uses angle brackets (<...>) for #include
directives. Double quotes ("...") are never used because the lookup
semantics can be surprising if there are any headers with the same name
in the tree. (Not that this should happen, but it's better to be safe. If
there are no duplicate names, inclusion with angle brackets behaves the
same way as inclusion with double quotes.)

All headers that are for public consumption appear in include/lomiri/*
(provided the path does not include "internal").

Public header directories contain a private header directory called
"internal". This directory contains all the headers that are private
and specific to the implementation.

No public header file is allowed to include any header from one of the
internal directories. (Doing so would break the compilation firewall
and also prevent API clients from compiling because the internal headers
are not installed when lomiri is deployed. (This is enforced by the tests.)

All header files, whether public or internal, compile stand-alone, that
is, no header relies on another header being included first in order to
compile. (This is enforced by the tests.)


Compilation firewall
--------------------

Public classes use the pimpl (pointer-to-implemtation) idiom, also
known as "envelope-letter idiom", "Cheshire Cat", or "Compiler firewall
idiom". This makes it less likely that an update to the code will break
the ABI.

Many classes that are part of the public API contain only one private data
member, namely the pimpl. No other data members (public, protected,
or private) are permitted.

For public classes with shallow-copy semantics (classes that are
immutable after instantiation), the code uses std::shared_ptr as the
pimpl type because std::shared_ptr provides the correct semantics. (See
lomiri::Exception and its derived classes for an example.)

For classes that are non-copyable, std::unique_ptr is used as the
pimpl type.

If classes form derivation hierarchies, by convention, the pimpl is a
private data member of the root base class. Derived classes can access the
pimpl by calling the protected pimpl() method of the root base class. This
avoids redundantly storing a separated pimpl to the derived type in each
derived class. Instead, polymorphic dispatch to virtual methods in the
derived classes is achieved by using a dynamic_cast to the derived
type to forward an invocation to the corresponding virtual method in
the derived implementation class.


Error handling
--------------

No error codes, period. All errors are reported via exceptions. All
exceptions thrown by the library are derived from lomiri::Exception (which
is abstract). lomiri::Exception provides a to_string() method. This method
returns the exception history and prints all exceptions that were raised
along a particular throw path, even if a low-level exception was caught
and translated into a higher level exception. This works for exceptions
derived from std::nested_exception. (The exception chaining ends when it
encounters an exception that does not derive from std::nested_exception.)

If API clients intercept lomiri exceptions and rethrow their own exceptions,
it is recommended that API clients derive their exceptions from
lomiri::Exception or, alternatively, derive them from std::nested_exception
and implement a similar to_string() operation that, if it encounters a
lomiri::Exception while following a chain, calls the to_string() method on
lomiri::Exception. That way, the entire exception history will be returned
from to_string().

Functions that throw exceptions should, if at all possible, provide
the strong exception guarantee. Otherwise, they must provide the basic
(weak) exception guarantee. If it is impossible to maintain even the basic
guarantee, the code must call abort() instead of throwing an exception.


Resource management
-------------------

The code uses the RAII (resource acquisition is initialization)
idiom extensively. If you find yourself writing free, delete, Xfree,
XCloseResource, or any other kind of clean-up function, your code has a
problem. Instead of explictly cleaning up in destructors, *immediately*
assign the resource to a unique_ptr or shared_ptr with a custom deleter.

This guarantees that the resource will be released without having to
remember anything.  In particular, it guarantees that the resource
will be released even if allocated in a constructor near the beginning
and something called by the constructor throws before the constructor
completes.

For resources that cannot be managed by unique_ptr or shared_ptr
(because the allocator does not return a pointer or returns a
pointer to an opaque type), use the RAII helper class in ResourcePtr.h.
It does much the same thing as unique_ptr, but is more permissive
in the types it can manage.

Note the naming convention established by util/DefinesPtrs.h. All
classes that return or accept smart pointers derive from DefinesPtrs,
which is a code injection template that creates typedefs for Ptr and
UPtr (shared_ptr and unique_ptr, respectively) as well as CPtr and UCPtr
(shared_ptr and unique_ptr to constant instances).

Ideally, classes are fully initialized by their constructor so it is
impossible for a class to exist but not being in a usable state.

For some classes, it is unavoidable to provide a default constructor (for
example, if we want to put instances into an array). It is also sometimes
impossible to fully construct an instance immediately, for example if
the instance is member variable and the necessary initialization data
is not available until some time afterwards.

In this case, the default constructor must initialize the class to
a fail-safe state, that is, it must behave sanely in the face of
methods being invoked on it. This means that calling a method on a
default-constructed instance should throw a logic exception to
indicate to the caller that the instance is not fully initialized yet.

Note that turning method calls on not-yet-initialized instances into
no-ops is usually a bad idea: the caller thinks that everything worked
fine when, in fact, it did nothing. If such no-op methods do something
sensible (that is, they can do their job even on an incompletely
initialized instance), this begs the question of why the instance wasn't
default-constructible in the first place...

To sum it up: try to enforce complete initialization from the
constructor wherever possible. If it is impossible to do that, follow
the principle of least surprise for the caller if a method is called on
a not-yet-initialized instance.


Loose ends
----------

Things that need fixing or checking are marked with a TODO comment in
the code.

Things that are work-arounds for bugs in the compiler or libraries are
marked with a BUGFIX comment.

Things that are dubious in some way with no better solution available
are marked with a HACK comment.

Style
-----

Consider running astyle over the code before checking it in.
See astyle-config for more details.


Versioning
----------

When adding API, remember to both bump:
- the debian/changelog version and
- the VERSION field in the relevant CMakeLists.txt file


Test suite
----------

The test suite lives underneath the test directory.

test/headers contains tests relating to header file integrity.

test/gtest contains the C++ tests (which use Google test).

The Google gtest authors are adamant that it is a bad idea to just
link with a pre-installed version of libgtest.a. Therefore, libgtest.a
(and libgtest_main.a) are created as part of the build and can be found
in test/gtest/libgtest.

See the INSTALL file for how to run the tests, particularly the caveat
about "make test" not rebuilding the tests!


Building and installing the code
--------------------------------

See the INSTALL file.
