How to export an instantiation of a Standard Template Library (STL) class and a class that contains a data member that is an STL object (168958)

MORE INFORMATION

Beginning with Visual C++ 5.0, it is possible to force an instantiation of a template class and export the instantiation. To export a template class instantiation, use the following syntax:

To Export an STL Class

1. In both the DLL and the .exe file, link with the same DLL version of the C run time. Either link both with Msvcrt.lib (release build) or link both with Msvcrtd.lib (debug build).
2. In the DLL, provide the __declspec specifier in the template instantiation declaration to export the STL class instantiation from the DLL.
3. In the .exe file, provide the extern and __declspec specifiers in the template instantiation declaration to import the class from the DLL. This results in a warning C4231 "nonstandard extension used : 'extern' before template explicit instantiation." You can ignore this warning.

To Export a Class Containing a Data Member that Is an STL Object

1. In both the DLL and the .exe file, link with the same DLL version of the C run time. Either link both with Msvcrt.lib (release build) or link both with Msvcrtd.lib (debug build).
2. In the DLL, provide the __declspec specifier in the template instantiation declaration to export the STL class instantiation from the DLL.
   
   NOTE: You cannot skip step 2. You must export the instantiation of the STL class that you use to create the data member.
3. In the DLL, provide the __declspec specifier in the declaration of the class to export the class from the DLL.
4. In the .exe file, provide the __declspec specifier in the declaration of the class to import the class from the DLL.
   
   If the class you are exporting has one or more base classes, then you must export the base classes as well. If the class you are exporting contains data members that are of class type, then you must export the classes of the data members as well.

NOTE: Some STL classes use other STL classes. These other classes must also be exported. The classes that must be exported are listed in compiler warnings if you compile with a warning level lower than 1; that is, /W2, /W3, or /W4. Warning level 4 generates a lot of warning messages for STL headers and is not currently recommended for that reason.

Some STL classes contain nested classes. These classes can not be exported. For instance, deque contains a nested class deque::iterator. If you export deque, you will get a warning that you must export deque::iterator. If you export deque::iterator, you get a warning that you must export deque. This is caused by a designed limitation that once a template class is instantiated, it can not be re-instantiated and exported. The only STL container that can currently be exported is vector. The other containers (that is, map, set, queue, list, deque) all contain nested classes and cannot be exported.

When you export an STL container parameterized with a user-defined type (UDT), you must define the operators < and == for your UDT. For example, if you export vector<MyClass>, you must define MyClass::operator < and MyClass operator ==. This is because all STL container classes have member comparison operators that require the existence of the operators < and == for the contained type. Normally, these are not instantiated because they are not used. When you instantiate an instance of a template class, all member functions are generated. Because the STL container classes have member functions that use the operators < and == for the contained type, you must implement them. If comparing objects of your UDT does not make sense, you can define the comparison operators to simply return "true."

When the symbol _DLL is defined during compiling (this symbol is implicitly defined when compiling with /MD or /MDd to link with the DLL version of the C Runtime), the following STL classes, and various global operators and functions that operate on these classes, are already exported by the C Runtime DLL. Therefore, you cannot export them from your DLL. This should not cause a problem for the executable program that imports your class as long as it also uses the DLL version of the C run time:

Header      STL template class
------------------------------
<IOSFWD>    basic_ios
<IOSFWD>    <IOSFWD>
<IOSFWD>    basic_istream
<IOSFWD>    basic_string (also typedef'd as string and wstring)
<IOSFWD>    complex
<LOCALE>    messages
<XLOCALE>   codecvt
<XLOCALE>   ctype
<XLOCMON>   moneypunct
<XLOCMON>   money_get
<XLOCMON>   money_put
<XLOCNUM>   numpunct
<XLOCTIME>  time_get
<XLOCTIME>  time_put
<XSTRING>   basic_string (also typedef'd as string and wstring)
				

For specific details on which template parameters are used and which global functions and operators are declared, please see the relevant header file.

Sample Code

   // -------------------------------------------
   // MYHEADER.H
   //disable warnings on 255 char debug symbols
    #pragma warning (disable : 4786)
   //disable warnings on extern before template instantiation
    #pragma warning (disable : 4231)

    #include <vector>

    // Provide the storage class specifier (extern for an .exe file, null
    // for DLL) and the __declspec specifier (dllimport for .an .exe file,
    // dllexport for DLL).
    // You must define EXP_STL when compiling the DLL.
    // You can now use this header file in both the .exe file and DLL - a
    // much safer means of using common declarations than two different
    // header files.
    #ifdef EXP_STL
    #    define DECLSPECIFIER __declspec(dllexport)
    #    define EXPIMP_TEMPLATE
    #else
    #    define DECLSPECIFIER __declspec(dllimport)
    #    define EXPIMP_TEMPLATE extern
    #endif

    // Instantiate classes vector<int> and vector<char>
    // This does not create an object. It only forces the generation of all
    // of the members of classes vector<int> and vector<char>. It exports
    // them from the DLL and imports them into the .exe file.
    EXPIMP_TEMPLATE template class DECLSPECIFIER std::vector<int>;
    EXPIMP_TEMPLATE template class DECLSPECIFIER std::vector<char>;

    // Declare/Define a class that contains both a static and non-static
    // data member of an STL object.
    // Note that the two template instantiations above are required for
    // the data members to be accessible. If the instantiations above are
    // omitted, you may experience an access violation.
    // Note that since you are exporting a vector of MyClass, you must
    // provide implementations for the operator < and the operator ==.
    class DECLSPECIFIER MyClass
    {
    public:
        std::vector<int> VectorOfInts;
        static std::vector<char> StaticVectorOfChars;

    public:
        bool operator < (const MyClass > c) const
        {
            return VectorOfInts < c. VectorOfInts;
        }
        bool operator == (const MyClass > c) const
        {
            return VectorOfInts == c. VectorOfInts;
        }
    };

    // Instantiate the class vector<MyClass>
    // This does not create an object. It only forces the generation of
    // all of the members of the class vector<MyClass>. It exports them
    // from the DLL and imports them into the .exe file.
    EXPIMP_TEMPLATE template class DECLSPECIFIER std::vector<MyClass>;

    // -------------------------------------------
    // Compile options needed: /GX /LDd /MDd /D"EXP_STL"
    //                     or: /GX /LD  /MD  /D"EXP_STL"
    // DLL.CPP

    #include "MyHeader.h"
    std::vector<char> MyClass::StaticVectorOfChars;

    // -------------------------------------------
    // Compile options needed: /GX /MDd
    //                     or: /GX /MD
    // EXE.CPP

    #include <iostream>
    #include "MyHeader.h"

    int main ()
    {
        MyClass x;

        for (int i=0; i<5; i++) x.VectorOfInts.push_back(i);
        for (char j=0; j<5; j++) x.StaticVectorOfChars.push_back('a' + j);

        std::vector<int>::iterator vii = x.VectorOfInts.begin();
        while (vii != x.VectorOfInts.end())
        {
            std::cout << *vii;
            std::cout << " displayed from x.VectorOfInts" << std::endl;
            vii++;
        }
        std::vector<char>::iterator vci = x.StaticVectorOfChars.begin();
        while (vci != x.StaticVectorOfChars.end())
        {
            std::cout << *vci;
            std::cout << " displayed from MyClass::StaticVectorOfChars";
            std::cout << std::endl;
            vci++;
        }

        std::vector<MyClass> vy;
        for (i=0; i=5; i++) vy.push_back(MyClass());

        return 1;
    }