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POWhen exporting STL std::basic_string template from DLL, I get a LNK2005 error
 

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  1. POWhen exporting STL std::basic_string template from DLL, I get a LNK2005 error
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    <p>OK, so I've read several question and articles about this topic, and I feel like I understand the basics, but I'm still having trouble.</p> <p>I have a DLL that exports a class that has a std::string as a member. My main program contains classes that also have strings, and it uses the DLL.</p> <p>If I compile the DLL in VS2010, I get the following warnings:</p> <pre><code>warning C4251: 'MyClass::data' : class 'std::basic_string&lt;_Elem,_Traits,_Ax&gt;' needs to have dll-interface to be used by clients of class 'MyClass' </code></pre> <p>When I compile the EXE, I get the same warnings, but there are no errors, and the program compiles and runs. In reality, it's a large project, so I get like 40 warnings, and I'm not too keen on that. (As a side-observation, these warnings are not present when compiled with VS2008)</p> <p>So, I read about that warning, and it lead me to this MS article: <a href="http://support.microsoft.com/default.aspx?scid=KB;EN-US;168958" rel="noreferrer">http://support.microsoft.com/default.aspx?scid=KB;EN-US;168958</a> which tells how to export a STL template from a DLL to satisfy the warnings I was getting.</p> <p>The problem is, when I add the following lines to remove the warnings:</p> <pre><code>EXPIMP_TEMPLATE template class DECLSPECIFIER std::allocator&lt;char&gt;; EXPIMP_TEMPLATE template class DECLSPECIFIER std::basic_string&lt; char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;; </code></pre> <p>the DLL compiles with no warnings, but when I compile my EXE, the linker throws a fit:</p> <pre><code>2&gt;SampleDLL.lib(SampleDLL.dll) : error LNK2005: "public: __thiscall std::basic_string&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt;::~basic_string&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt;(void)" (??1?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) already defined in OtherClass.obj 2&gt;SampleDLL.lib(SampleDLL.dll) : error LNK2005: "public: unsigned int __thiscall std::basic_string&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt;::size(void)const " (?size@?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QBEIXZ) already defined in OtherClass.obj </code></pre> <p>Both the DLL and the EXE are compiled with the same code generation options. I can use MT on both or MD, and the results are the same.</p> <p>I am including the code from a minimized sample program in case I left anything out above.</p> <p><strong>My main question: Can I fix the LNK2005 errors, or is it safe to just ignore the C4251 warnings?</strong></p> <p><strong>Edit:</strong> So I've read a little more, and it looks like if the std::string that the DLL class uses is a private variable that is only accessed by member functions, it may be safe to ignore the warning... Any comments on this? Is this a step in the right direction?</p> <p>DLL code: </p> <pre><code>#pragma once #include &lt;exception&gt; #include &lt;string&gt; #ifdef SAMPLEDLL_EXPORTS # define DECLSPECIFIER __declspec(dllexport) # define EXPIMP_TEMPLATE #else # define DECLSPECIFIER __declspec(dllimport) # define EXPIMP_TEMPLATE extern #endif //disable warnings on extern before template instantiation (per MS KB article) #pragma warning (disable : 4231) //std::basic_string depends on this allocator, so it must also be exported. EXPIMP_TEMPLATE template class DECLSPECIFIER std::allocator&lt;char&gt;; //std::string is a typedef, so you cannot export it. You must export std::basic_string EXPIMP_TEMPLATE template class DECLSPECIFIER std::basic_string&lt; char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;; #pragma warning (default : 4231) class DECLSPECIFIER MyClass { public: std::string getData(); //returns 'data', body in CPP file private: std::string data; int data2; }; //in SampleDLL.cpp file... std::string MyClass::getData() { return data; } </code></pre> <p>EXE code: </p> <pre><code>#include &lt;iostream&gt; #include "SampleDLL.h" using namespace std; void main() { MyClass class1; cout &lt;&lt; class1.getData() &lt;&lt; endl; } </code></pre>
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    1. COpossible duplicate of [Exporting classes containing std:: objects (vector, map, etc) from a dll](http://stackoverflow.com/questions/767579/exporting-classes-containing-std-objects-vector-map-etc-from-a-dll)
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    2. COMark: I did read that question, but it didn't seem to have an answer for the issue I'm seeing because it does not mention the additional LNK2005 problem I now have.
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