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Merge pull request #917 from scraimer/master

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C.148 adding Reason and Example
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Andrew Pardoe 2017-05-08 11:08:12 -07:00 committed by GitHub
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@ -7296,15 +7296,41 @@ Casting to a reference expresses that you intend to end up with a valid object,
##### Reason ##### Reason
??? The `dynamic_cast` conversion allows to test whether a pointer is pointing at a polymorphic object that has a given class in its hierarchy. Since failure to find the class merely returns a null value, it can be tested during run-time. This allows writing code that can choose alternative paths depending on the results.
Contrast with [C.147](#Rh-ptr-cast), where failure is an error, and should not be used for conditional execution.
##### Example ##### Example
??? The example below describes the `add` method of a `Shape_owner` that takes ownership of constructed `Shape` objects. The objects are also sorted into views, according to their geometric attributes.
In this example, `Shape` does not inherit from `Geometric_attributes`. Only its subclasses do.
void add(Shape* const item)
{
// Ownership is always taken
owned_shapes.emplace_back(item);
// Check the Geometric_attributes and add the shape to none/one/some/all of the views
if (auto even = dynamic_cast<Even_sided*>(item))
{
view_of_evens.emplace_back(even);
}
if (auto trisym = dynamic_cast<Trilaterally_symmetrical*>(item))
{
view_of_trisyms.emplace_back(trisym);
}
}
##### Notes
A failure to find the required class will cause `dynamic_cast` to return a null value, and de-referencing a null-valued pointer will lead to undefined behavior.
Therefore the result of the `dynamic_cast` should always be treated as if it may contain a null value, and tested.
##### Enforcement ##### Enforcement
??? * (Complex) Unless there is a null test on the result of a `dynamic_cast` of a pointer type, warn upon dereference of the pointer.
### <a name="Rh-smart"></a>C.149: Use `unique_ptr` or `shared_ptr` to avoid forgetting to `delete` objects created using `new` ### <a name="Rh-smart"></a>C.149: Use `unique_ptr` or `shared_ptr` to avoid forgetting to `delete` objects created using `new`