improve C4251

TylerMSFT · TylerMSFT · commit 746fe09659de · 2023-12-01T16:38:49.000-08:00
diff --git a/docs/error-messages/compiler-warnings/compiler-warning-level-1-c4251.md b/docs/error-messages/compiler-warnings/compiler-warning-level-1-c4251.md
@@ -1,40 +1,69 @@
 ---
 description: "Learn more about: Compiler Warning (level 1) C4251"
 title: "Compiler Warning (level 1) C4251"
-ms.date: 02/22/2022
+ms.date: 12/01/2023
 f1_keywords: ["C4251"]
 helpviewer_keywords: ["C4251"]
-ms.assetid: a9992038-f0c2-4fc4-a9be-4509442cbc1e
 ---
 # Compiler Warning (level 1) C4251
 
 > '*type*' : class '*type1*' needs to have dll-interface to be used by clients of class '*type2*'
 
 ## Remarks
 
+This warning happens if a class is marked with `__declspec(dllexport)` or `__declspec(dllimport)` and a non-static data member that is a member of the class or a member of one of its base classes, has a type that is a class type that isn't marked with `__declspec(dllexport)` or `__declspec(dllimport)`. See [Example](#example).
+
 To minimize the possibility of data corruption when exporting a class declared as [`__declspec(dllexport)`](../../cpp/dllexport-dllimport.md), ensure that:
 
 - All your static data is accessed through functions that are exported from the DLL.
-
 - No inlined methods of your class can modify static data.
-
 - No inlined methods of your class use CRT functions or other library functions that use static data. For more information, see [Potential errors passing CRT objects across DLL boundaries](../../c-runtime-library/potential-errors-passing-crt-objects-across-dll-boundaries.md).
-
 - No methods of your class (whether inlined or not) can use types where the instantiation in the EXE and DLL have static data differences.
 
-You can avoid issues when exporting a class from a DLL: Define your class to have virtual functions, a virtual destructor, and functions to instantiate and delete objects of the type. You can then just call virtual functions on the type.
+You can avoid issues when exporting a class from a DLL by defining your class to have virtual functions, a virtual destructor, and functions to instantiate and delete objects of the type. You can then call virtual functions on the type.
 
 C4251 can be ignored if your class is derived from a type in the C++ Standard Library, you're compiling a debug release (**`/MTd`**), and where the compiler error message refers to `_Container_base`.
 
-## Example
+Think carefully about adding `__declspec(dllexport)` or `__declspec(dllimport)` to a class because it is almost always not the right choice and it can make maintenance more difficult because it makes changing implementation details harder.
 
-This sample exports a specialized class `VecWrapper` derived from `std::vector`.
+## Example
 
 ```cpp
 // C4251.cpp
-// compile with: /EHsc /MTd /W2 /c
+// Compile with /std:c++20 /EHsc /W2 /c C4251.cpp
+#include <vector>
+ 
+class __declspec(dllexport) X
+{
+public:
+    X();
+    ~X();
+ 
+    void do_something();
+ 
+private:
+    void do_something_else();
+    std::vector<int> data; // warning c4251
+};
+```
+
+To fix this warning, don't mark the class with `__declspec(dllexport)` or `__declspec(dllimport)`. Instead, only mark the methods that are used directly by a client. For example:
+
+```cpp
+// C4251_fixed.cpp
+// Compile with /std:c++20 /EHsc /W2 /c C4251-fixed.cpp
 #include <vector>
-using namespace std;
-class Node;
-class __declspec(dllexport) VecWrapper : vector<Node *> {};   // C4251
+ 
+class X
+{
+public:
+    __declspec(dllexport) X();
+    __declspec(dllexport) ~X();
+ 
+    __declspec(dllexport) void do_something();
+ 
+private:
+    void do_something_else();
+    std::vector<int> data;
+};
 ```
