Differences

This shows you the differences between two versions of the page.

--- cast [2016/10/26 13:35]
+++ cast [2022/04/16 12:22] (current)
@@ Line 1: / Line 1: @@
+==== reinterpret_cast ====
+=== What is the proper way to initialize a pointer member in C++ (11)? ===
+Any hint to do proper const static member pointer initialization using C++ 11 and later ?
+Source:
+struct App {
+  static const App* ok;
+  static constexpr App* ko = reinterpret_cast<App*>(0xBADC0DE); // <= FTBFS
+};
+const App* App::ok = reinterpret_cast<App*>(0xBADC0DE);
+Try it on latest gcc or clang it will fail :
+    http://melpon.org/wandbox/permlink/IlXTqNiVoJtnFDg0
+    http://cpp.sh/96mau
+    http://coliru.stacked-crooked.com/a/e031835a72ed2172
+The reason of fail is clear : reinterpret_cast and constexpr are exclusives according to C++11 standard (but was accepted in some compiler).
+    http://eel.is/c++draft/expr.const#2
+    http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf (p125)
+But older 2 steps method still works but not single one (on gcc-6.2.0 at least).
+This code looks a bit odd, I agree on that but how to get it working or clang or latest g++.
+Declaration and init in 2 places can be a workaround but it does not fully explain (init to nullptr is working of course).
+Related errors are :
+  error: 'constexpr' needed for in-class initialization of static data member
+  error: reinterpret_cast from integer to pointer
+More resources:
+    reinterpret_cast<> and portabilty
+   * http://stackoverflow.com/questions/39441338/what-is-the-proper-way-to-initialize-a-pointer-member-in-c-11
+   * http://stackoverflow.com/questions/23401489/reinterpret-cast-and-portabilty#
+</code>
+==== C cast ====
+  /// @author: http://rzr.online.fr/contact.htm
+  #include <cstdlib>
+  // #define FIX // uncomment
+  /// show why anonymous types are evil in C++
+  struct foo {
+  #ifndef FIX
+    struct  {} *m; // anonymous type make cast impossible in C++ (ok on C)
+  #else
+    struct foo_m {} *m; // this way the type is named and can be used
+  #endif
+  };
+  int main(int /* argc*/ , char* /* argv*/ [])
+  {
+    struct foo o; o.m = 0;
+  #ifndef FIX
+    o.m = malloc(sizeof( o.m ));  //error:
+    //  invalid conversion from 'void*' to 'foo::<anonymous struct>*'
+  #else
+    o.m =
+      (struct foo::foo_m*)  //mandatory in c++
+      malloc(sizeof( o.m ));
+  #endif
+    return 0;
+  }
+  //#eof "$Id: rzr --  --  $"
+  * http://bugs.debian.org/447196
+  * http://sourceforge.net/tracker/index.php?func=detail&aid=1816107&group_id=12349&atid=312349
+==== reinterpret_cast ====
+  error: reinterpret_cast from integer to pointer
+  * http://stackoverflow.com/questions/39441338/what-is-the-proper-way-to-intialize-a-pointer-member-in-c-11
+  * http://stackoverflow.com/questions/10369606/constexpr-pointer-value# ConstExpr
+  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63715# ConsT
+  * http://stackoverflow.com/questions/27946241/workaround-for-gcc-4-9-constexpr-bug
+  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63715# CasT
+  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70172# ExpressioN
+  * http://stackoverflow.com/questions/32790106/passing-reference-pointing-to-struct-as-template-parameter
+  * http://stackoverflow.com/questions/13499658/constexpr-initializing-with-pointers# StatiC
+  * http://stackoverflow.com/questions/23401489/reinterpret-cast-and-portabilty# ReinterpretCast
+   error: 'constexpr' needed for in-class initialization of static data member
+@TaG: [[CPlusPlus]] [[gcc]]