================
@@ -0,0 +1,82 @@
+.. title:: clang-tidy - bugprone-crtp-constructor-accessibility
+
+bugprone-crtp-constructor-accessibility
+=======================================
+
+Finds Curiously Recurring Template Pattern used in an error-prone way.
+
+The CRTP is an idiom, in which a class derives from a template class, where
+itself is the template argument. It should be ensured that if a class is
+intended to be a base class in this idiom, it can only be instantiated if
+the derived class is it's template argument.
+
+Example:
+
+.. code-block:: c++
+
+ template <typename T> class CRTP {
+ private:
+ CRTP() = default;
+ friend T;
+ };
+
+ class Derived : CRTP<Derived> {};
+
+Below can be seen some common mistakes that will allow the breaking of the
idiom.
+
+If the constructor of a class intended to be used in a CRTP is public, then
+it allows users to construct that class on its own.
+
+Example:
+
+.. code-block:: c++
+
+ template <typename T> class CRTP {
+ public:
+ CRTP() = default;
+ };
+
+ class Good : CRTP<Good> {};
+ Good GoodInstance;
+
+ CRTP<int> BadInstance;
+
+If the constructor is protected, the possibility of an accidental instantiation
+is prevented, however it can fade an error, when a different class is used as
+the template parameter instead of the derived one.
+
+Example:
+
+.. code-block:: c++
+
+ template <typename T> class CRTP {
+ protected:
+ CRTP() = default;
+ };
+
+ class Good : CRTP<Good> {};
+ Good GoodInstance;
+
+ class Bad : CRTP<Good> {};
+ Bad BadInstance;
+
+To ensure that no accidental instantiation happens, the best practice is to
make
+the constructor private and declare the derived class as friend. Note that as
a tradeoff,
+this also gives the derived class access to every other private members of the
CRTP.
+
+Example:
+
+.. code-block:: c++
+
+ template <typename T> class CRTP {
+ CRTP() = default;
+ friend T;
+ };
+
+ class Good : CRTP<Good> {};
+ Good GoodInstance;
+
+ class Bad : CRTP<Good> {};
+ Bad CompileTimeError;
+
+ CRTP<int> AlsoCompileTimeError;
----------------
isuckatcs wrote:
The issue with previous standards is the friend declaration. See [my previous
comment](https://github.com/llvm/llvm-project/pull/82403#discussion_r1500091382)
for more details.
https://github.com/llvm/llvm-project/pull/82403
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