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Compiler Warning (level 2) C4251

  • Grein

'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 nonstatic 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.

To minimize the possibility of data corruption when exporting a class declared as __declspec(dllexport), 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.
  • 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 by:

  • Defining your class to have virtual functions.
  • Defining a virtual destructor.
  • Defining functions to instantiate and delete instances of the type.

You can ignore C4251 if your class is derived from a type in the C++ Standard Library, you're compiling a debug release (/MTd), and the compiler error message refers to _Container_base.

Think carefully about adding __declspec(dllexport) or __declspec(dllimport) to a class because it's almost always not the right choice and it can make maintenance more difficult because it makes changing implementation details harder.

Example

// C4251.cpp
// 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:

// C4251_fixed.cpp
// Compile with /std:c++20 /EHsc /W2 /c C4251-fixed.cpp
#include <vector>
 
class X
{
public:
    __declspec(dllexport) X();
    __declspec(dllexport) ~X();
 
    __declspec(dllexport) void do_something();
 
private:
    void do_something_else();
    std::vector<int> data;
};