Question

Here is my template function

template<typename T> std::stringstream logging_expansion ( T const * value ){
    std::stringstream retval;
    retval = retval << *value;
    return retval;
}

Here is how i call it to use it

logging_expansion( "This is the log comes from the convinient function");

But the linker is telling me that it can not reference the function:

Undefined symbols for architecture x86_64:
  "std::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >     logging_expansion<char>(char const*)", referenced from:
  _main in WirelessAutomationDeviceXpcService.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

You need to provide the implementation of template functions in header files or define the specialization in the header.

I’m assuming you currently have something like:

//header.h
template<typename T> std::stringstream logging_expansion ( T const * value );

//implementation.cpp
#include "header.h"

template<typename T> std::stringstream logging_expansion ( T const * value ){
    std::stringstream retval;
    retval = retval << *value;
    return retval;
}

//main.cpp
#include "header.h"

//....
logging_expansion( "This is the log comes from the convinient function");
//....

So you need to move the implementation to the header:

//header.h

template<typename T> std::stringstream logging_expansion ( T const * value ){
    std::stringstream retval;
    retval = retval << *value;
    return retval;
}