using std::function in a library

[JMJAtlanta]

I am attempting to use a std::function that SHOULD be in a library, but it does not seem to work.

I then dumbed it down to just attempting to link 2 object files, but it still does not work.

Here are the details:

my_func.cpp

#include <functional>
#include <string>

const std::function<std::string(const char*)> use_std_function = [](const char* str) { return std::string(str); };

const std::string use_function(const char* str)
{
   return std::string(str);
}

test.cpp

#include <string>
#include <functional>
#include <iostream>

const extern std::function<std::string(const char*)> use_std_function;

const extern std::string use_function(const char*);

int main(int argc, char** argv)
{
   const char* input = "Hello, world!";
   {
      std::string output = (use_function)(input);
      if (output != input)
         std::cerr << "Output does not equal input after use_function";
   }
   {
      std::string output = (use_std_function)(input);
      if (output != input)
         std::cerr << "Output does not equal input after use_std_function";
   }
   return 0;
}

Command line:

g++ test.cpp my_func.cpp

Results:

/usr/bin/ld: /tmp/ccc0sxWt.o: in function `main':
test.cpp:(.text+0x7e): undefined reference to `use_std_function[abi:cxx11]'
collect2: error: ld returned 1 exit status

It seems use_function is working, as if I comment out use_std_function it compiles and runs. But I am not sure what I am doing wrong with the std::function. Any help is appreciated.

Additional note: #including my_func.cpp as if it were a header works, but is not what I am after.

[dutch]

The problem is with the initial "const". Get rid of it and it works.

[JLBorges]

> I am attempting to use a std::function that SHOULD be in a library, but it does not seem to work.

The variable is of a const-qualified type; declare it as extern so that it has external linkage.

// declaration (typically in a header)
extern const std::function<std::string(const char*)> use_std_function ;

// definition ( in .cpp )
extern const std::function<std::string(const char*)> use_std_function = [](const char* str) { return std::string(str); };

[JMJAtlanta]

Thank you!

I will read up on extern. I always had the impression it was nothing more than a linker hint, and normally not required.