C++ - Undefined Symbols Linker Error - Xcode

Question

I have three C++ files and I'm getting a very annoying C++ linker error. Here's the error:

Undefined symbols for architecture x86_64:
  "tiled::debug::log(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
  _main in main.o
  error_callback(int, char const*) in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Here are my files:

main.cpp

#include "debug.hpp"
#include <GLFW/glfw3.h>

using namespace tiled;

void error_callback(int error, const char* description);

int main(int argc, char* argv[])
{
    debug::log("Initializing");
    glfwSetErrorCallback(error_callback);

    debug::log("Initializing GLFW...");
    if (!glfwInit())
    {
        debug::log("Failed to initialize GLFW!");

        glfwTerminate();
        return -1;
    }
    debug::log("Done");

    debug::log("Exiting program!");
    glfwTerminate();
    return 0;
}

void error_callback(int error, const char* description)
{
    std::string error_code;
    error_code.append("GLFW error ");
    error_code.append(std::to_string(error));

    debug::log(error_code);
    debug::log(description);
}

debug.hpp

#ifndef TILED_DEBUG_HPP
#define TILED_DEBUG_HPP

#include <ctime>
#include <fstream>
#include <iostream>
#include <string>
#include <vector>

namespace tiled
{
    class debug {
    private:
        static std::vector<std::string> data;
        static std::ofstream log_file;

    public:
        static void log(const std::string& msg);
        static int write_logs();
    };
}

#endif /* TILED_DEBUG_HPP */

and, debug.cpp

#include "debug.hpp"

namespace tiled
{
    void debug::log (const std::string& msg)
    {
        // Log to the console
        std::cout << msg << std::endl;
        // Add to the data we will put in the log file.
        data.push_back(msg);
    }

    int debug::write_logs()
    {
        std::string data_str;

        /*
         We get the system time here. This is because we want to name each log
         after what time the log file was made.
         */
        std::time_t result = std::time(nullptr);
        std::string time = std::asctime(std::localtime(&result));

        data_str.append("\n");
        data_str.append(time);

        // Put the contents of the data vector into the string we append to file
        for (std::string str : data)
        {
            data_str.append("\n");
            data_str.append(str);
        }

        log_file.open(time.c_str(), std::ios::app);

        if (log_file.is_open())
        {
            log_file << data_str;
            log_file.close();
        }
        else
        {
            std::cout << "Error: Couldn't write log file!" << std::endl;
            return 1;
        }

        return 0;
    }
}

It seems like the function log is not defined, but I defined it in the debug.cpp file. I've been working at this for hours... Can anyone help me?

Also, If it helps here are the commands Xcode is using the build the program:

Ld /Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug/tiled normal x86_64 cd /Users/home/Documents/tiled export MACOSX_DEPLOYMENT_TARGET=10.10 /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -arch x86_64 -isysroot /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -L/Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug -L/usr/local/lib -F/Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug -filelist /Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Intermediates/tiled.build/Debug/tiled.build/Objects-normal/x86_64/tiled.LinkFileList -mmacosx-version-min=10.10 -stdlib=libc++ -lglfw.3.1 -framework OpenGL -Xlinker -dependency_info -Xlinker /Users/home/Library/Developer/Xcode/DerivedData/tiled-bhq wfqpsuugzhkagbmrhebnbgnpb/Build/Intermediates/tiled.build/Debug/tiled.build/Objects-normal/x86_64/tiled_dependency_info.dat -o /Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug/tile

Answer 1

To fix this I got rid of the .cpp file and just made those inline functions. Because this was just a workaround, I'm not going to select this as an answer.