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From this post, I need to export function ci_find_substr, so in my header file, i declare function:

template<typename Twst>
__declspec(dllexport) int ci_find_substr (const Twst& str1,const Twst& str2 ,
    const std::locale& loc = std::locale());

but when compile this code from another project:

int k=ci_find_substr (wstring(mycstring), wstring(L".doc");

I get an error Severity Code Description Project File Line Suppression State

Error LNK2001 unresolved external symbol "int __cdecl ci_find_substr (class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> >,class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> >,class std::locale const &)" (?ci_find_substr @@YAHV?$basic_string@_WU?$char_traits@_W@std@@V?

I also tried the same thing in this post

template<typename Twst>
int ci_find_substr (const Twst& str1,const Twst& str2,
    const std::locale& loc = std::locale());

__declspec(dllexport) int ci_find_substr (const wstring& str1,const wstring& str2,
    const std::locale& loc = std::locale()); // Explicit instantiation

But still no success! Where did I go wrong? How do I fix my code?

Yen Dang
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    Can your template not just be defined in the header file? If it's in the header file it shouldn't need exporting – Tas Jun 28 '21 at 02:30
  • See: [Explicit template instantiation - when is it used?](https://stackoverflow.com/questions/2351148/explicit-template-instantiation-when-is-it-used) – Matt Eding Jun 28 '21 at 02:34

1 Answers1

1

You can't export a template from a DLL, you can only export a specialisation of said template!

#ifdef COMPILING_DLL
// ensure you are declaring 'COMPILING_DLL' somewhen when building your DLL
#define DLL_EXPORT __declspec(dllexport)
#else
// when using the function it must be DLL import!
#define DLL_EXPORT __declspec(dllimport)
#endif


// header
template<typename Twst>
DLL_EXPORT int ci_find_substr (const Twst& str1,const Twst& str2 ,
    const std::locale& loc = std::locale());


// in the source file...
//
// |-- 'template'
// |                   specialisation type--|
// |       |-- DLLEXPORT                    |
// V       V                                V
template DLL_EXPORT int ci_find_substr<std::wstring>(
        const std::wstring& str1, 
        const std::wstring& str2, 
        const std::locale& loc);
robthebloke
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