Unresolved externals using C++ lambdas

Question

I am getting unresolved externals compile error with following code snippet.

acquire_gray(identity,  []{});

samething happens when I try to use auto

  auto acquire_callback = [](LPBITMAPINFOHEADER pbi, HANDLE hdib)
    {
            printf("Callback\n");
    };

   acquire_gray("",  acquire_callback );

but when I pass in null it compiles

acquire_gray(identity,  NULL);

This is structure of my program

driver.cpp

#include "bridge.h"

void TB_AcquireImagesStart(HANDLE hNamedPipe, const TB_Message request)
{
    acquire_gray("",  []{});
}

bridge.h

template<typename T>
void acquire_gray(const string_t& identity, T& callback);

bridge.cpp

template<typename T>
void acquire_gray(const string_t& identity, T& callback)
{
   callback();
}

So the two exceptions that I am getting are

Error   12  error LNK1120: 1 unresolved externals

Error   11  error LNK2019: unresolved external symbol "void __cdecl acquire_gray<class <lambda_e125ff607fe0339bba6077ce9c14d586> >(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class <lambda_e125ff607fe0339bba6077ce9c14d586> &)" (??$acquire_gray@V<lambda_e125ff607fe0339bba6077ce9c14d586>@@@@YAXABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@AAV<lambda_e125ff607fe0339bba6077ce9c14d586>@@@Z) referenced in function "void __cdecl TB_AcquireImagesStart(void *,class TB_Message)" (?TB_AcquireImagesStart@@YAXPAXVTB_Message@@@Z)

So my question is what is wrong with my code, and how can I fix this, and why is auto not detecting my lambda type.

You cannot put template function definitions in separate `.cpp` files. The compiler won't see them. Try putting the definition of `acquire_gray<>()` in `bridge.h` — Andy Prowl, Mar 08 '13 at 17:30

score 3 · Accepted Answer · answered Mar 08 '13 at 17:32

3

You can't put template definitions in .cpp (well you can, but it only makes sense if you're using them in that compilation unit). After compilation, only template instanciations exist.

answered Mar 08 '13 at 17:32

Cubic

14,902
5
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92

score 2 · Answer 2 · answered Mar 08 '13 at 17:32

2

This is just a common pitfall when using templates. You cannot (or at least should not) separate a template into header (.hpp) and source (.cpp) files. See here for details.

answered Mar 08 '13 at 17:32

Matt Kline

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Unresolved externals using C++ lambdas

2 Answers2