.. title:: clang-tidy - cppcoreguidelines-owning-memory

cppcoreguidelines-owning-memory
===============================

This check implements the type-based semantics of ``gsl::owner<T*>``, which allows 
static analysis on code, that uses raw pointers to handle resources like 
dynamic memory, but won't introduce RAII concepts.

The relevant sections in the `C++ Core Guidelines <https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md>`_ are I.11, C.33, R.3 and GSL.Views
The definition of a ``gsl::owner<T*>`` is straight forward

.. code-block:: c++

  namespace gsl { template <typename T> owner = T; }

It is therefore simple to introduce the owner even without using an implementation of
the `Guideline Support Library <https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gsl-guideline-support-library>`_.

All checks are purely type based and not (yet) flow sensitive.

The following examples will demonstrate the correct and incorrect initializations
of owners, assignment is handled the same way. Note that both ``new`` and 
``malloc()``-like resource functions are considered to produce resources.

.. code-block:: c++

  // Creating an owner with factory functions is checked.
  gsl::owner<int*> function_that_returns_owner() { return gsl::owner<int*>(new int(42)); }

  // Dynamic memory must be assigned to an owner
  int* Something = new int(42); // BAD, will be caught
  gsl::owner<int*> Owner = new int(42); // Good
  gsl::owner<int*> Owner = new int[42]; // Good as well

  // Returned owner must be assigned to an owner
  int* Something = function_that_returns_owner(); // Bad, factory function
  gsl::owner<int*> Owner = function_that_returns_owner(); // Good, result lands in owner

  // Something not a resource or owner should not be assigned to owners
  int Stack = 42;
  gsl::owner<int*> Owned = &Stack; // Bad, not a resource assigned

In the case of dynamic memory as resource, only ``gsl::owner<T*>`` variables are allowed
to be deleted.

.. code-block:: c++

  // Example Bad, non-owner as resource handle, will be caught.
  int* NonOwner = new int(42); // First warning here, since new must land in an owner
  delete NonOwner; // Second warning here, since only owners are allowed to be deleted

  // Example Good, Ownership correclty stated
  gsl::owner<int*> Owner = new int(42); // Good
  delete Owner; // Good as well, statically enforced, that only owners get deleted
  
The check will furthermore ensure, that functions, that expect a ``gsl::owner<T*>`` as
argument get called with either a ``gsl::owner<T*>`` or a newly created resource.

.. code-block:: c++

  void expects_owner(gsl::owner<int*> o) { delete o; }

  // Bad Code
  int NonOwner = 42;
  expects_owner(&NonOwner); // Bad, will get caught

  // Good Code
  gsl::owner<int*> Owner = new int(42);
  expects_owner(Owner); // Good
  expects_owner(new int(42)); // Good as well, recognized created resource

  // Port legacy code for better resource-safety
  gsl::owner<FILE*> File = fopen("my_file.txt", "rw+");
  FILE* BadFile = fopen("another_file.txt", "w"); // Bad, warned

  // ... use the file

  fclose(File); // Ok, File is annotated as 'owner<>'
  fclose(BadFile); // BadFile is not an 'owner<>', will be warned


Options
-------

.. option:: LegacyResourceProducers

   Semicolon-separated list of fully qualified names of legacy functions that create
   resources but cannot introduce ``gsl::owner<>``.
   Defaults to ``::malloc;::aligned_alloc;::realloc;::calloc;::fopen;::freopen;::tmpfile``.


.. option:: LegacyResourceConsumers

   Semicolon-separated list of fully qualified names of legacy functions expecting
   resource owners as pointer arguments but cannot introduce ``gsl::owner<>``.
   Defaults to ``::free;::realloc;::freopen;::fclose``.


Limitations
-----------

Using ``gsl::owner<T*>`` in a typedef or alias is not handled correctly. 

.. code-block:: c++

  using heap_int = gsl::owner<int*>;
  heap_int allocated = new int(42); // False positive!

The ``gsl::owner<T*>`` is declared as a templated type alias.
In template functions and classes, like in the example below, the information
of the type aliases gets lost. Therefore using ``gsl::owner<T*>`` in a heavy templated
code base might lead to false positives. 

Known code constructs that do not get diagnosed correctly are:

- ``std::exchange``
- ``std::vector<gsl::owner<T*>>``

.. code-block:: c++

  // This template function works as expected. Type information doesn't get lost.
  template <typename T>
  void delete_owner(gsl::owner<T*> owned_object) {
    delete owned_object; // Everything alright
  }

  gsl::owner<int*> function_that_returns_owner() { return gsl::owner<int*>(new int(42)); }

  // Type deduction does not work for auto variables. 
  // This is caught by the check and will be noted accordingly.
  auto OwnedObject = function_that_returns_owner(); // Type of OwnedObject will be int*

  // Problematic function template that looses the typeinformation on owner
  template <typename T>
  void bad_template_function(T some_object) {
    // This line will trigger the warning, that a non-owner is assigned to an owner
    gsl::owner<T*> new_owner = some_object;
  }

  // Calling the function with an owner still yields a false positive.
  bad_template_function(gsl::owner<int*>(new int(42)));


  // The same issue occurs with templated classes like the following.
  template <typename T>
  class OwnedValue {
  public:
    const T getValue() const { return _val; }
  private:
    T _val;
  };

  // Code, that yields a false positive.
  OwnedValue<gsl::owner<int*>> Owner(new int(42)); // Type deduction yield T -> int * 
  // False positive, getValue returns int* and not gsl::owner<int*>
  gsl::owner<int*> OwnedInt = Owner.getValue(); 

Another limitation of the current implementation is only the type based checking.
Suppose you have code like the following:

.. code-block:: c++

  // Two owners with assigned resources
  gsl::owner<int*> Owner1 = new int(42); 
  gsl::owner<int*> Owner2 = new int(42);

  Owner2 = Owner1; // Conceptual Leak of initial resource of Owner2!
  Owner1 = nullptr;

The semantic of a ``gsl::owner<T*>`` is mostly like a ``std::unique_ptr<T>``, therefore
assignment of two ``gsl::owner<T*>`` is considered a move, which requires that the 
resource ``Owner2`` must have been released before the assignment.
This kind of condition could be catched in later improvements of this check with 
flowsensitive analysis. Currently, the `Clang Static Analyzer` catches this bug
for dynamic memory, but not for general types of resources.