Unresolved external symbol error on variadic template c++ due to usage of unsigned int

Question

I've declared a simple function with variadic template.

template<typename ...Args>
void Log(const LogLevel level, const char * format, Args ...args);

Upon calling it in the following manner -

Log(LogLevel::debug,
        R"(starting x, %d pending call for "%s" with param "%s")",
        id, first.c_str(),
       second.c_str())

where the variable types are : id (unsigned int), first (std::string) , second (std::string)

I'm getting the following error:

Error   LNK2001 unresolved external symbol "public: void __cdecl Log<unsigned int,char const *,char const *>(enum LogLevel,char const *,unsigned int,char const *,char const *)"

When I remove the unsigned int argument from the function call - the error disappears. AFAIK the variadic template does support different types... so what am I missing?

"I've declared a simple function with variadic template." And where and how did you define it? — O'Neil, Dec 19 '17 at 16:23
If your template function *definition* is *not* in a header file, see [this](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file). Otherwise, post a [mcve]. — n. 1.8e9-where's-my-share m., Dec 19 '17 at 16:27

max66 · Accepted Answer · 2017-12-19T16:17:16.803

It's a linker error, so (I suppose) you have declared the template function in a header file and defined it in a c++ (not header) file.

If you use the template function that receive the unsigned int in a different c++ file, the compiler doesn't know which versions of the function to implement.

Simple solution: declare and define the template functions/classes/structs in the headers.

If I'm wrong... please prepare a minimal example to replicate the error.

Unresolved external symbol error on variadic template c++ due to usage of unsigned int

1 Answers1