doc, wifi: (fixes #1158) Reintroduce WifiPpdu's destructor due to PIMPL

Also add reference explaining why declaring a default destructor in the header is a problem

doc/contributing/source/coding-style.rst

@@ -1512,6 +1512,16 @@ for more details.
 
 - Avoid declaring trivial destructors, to optimize performance.
 
+.. _When an empty destructor is required: https://andreasfertig.com/blog/2023/12/when-an-empty-destructor-is-required/
+
+- When declaring default destructors with ``~Class() = default;``, be aware
+  that classes derived from ``SimpleRefCount<T>`` must have this declaration
+  on the source file (``.cc``). The header file (``.h``) should contain
+  the plain destructor declaration ``~Class();``. This is due to PIMPL's
+  opaque pointer, as explained in Andrea Fertig's blog post
+  `When an empty destructor is required`_.
+  See class WifiPpdu's destructor for an example.
+
 C++ standard
 ============
 

src/wifi/model/wifi-ppdu.cc

@@ -100,6 +100,8 @@ WifiPpdu::WifiPpdu(const WifiConstPsduMap& psdus,
     m_psdus = psdus;
 }
 
+WifiPpdu::~WifiPpdu() = default;
+
 const WifiTxVector&
 WifiPpdu::GetTxVector() const
 {

src/wifi/model/wifi-ppdu.h

@@ -73,7 +73,7 @@ class WifiPpdu : public SimpleRefCount<WifiPpdu>
     /**
      * Destructor for WifiPpdu.
      */
-    virtual ~WifiPpdu() = default;
+    virtual ~WifiPpdu();
 
     /**
      * Get the TXVECTOR used to send the PPDU.