未定义的符号链接器错误

我有三个c++文件，我得到一个非常烦人的c++链接器错误。下面是错误:

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)

这是我的文件:

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 */

, 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;
    }
}

似乎没有定义函数日志，但我在debug.cpp文件中定义了它。我已经研究了几个小时了…有人能帮我吗?

另外，如果有帮助的话，这里是Xcode在构建程序时使用的命令:

Ld/Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug/tiled normal x86_64cd/用户/home/文件/瓦出口MACOSX_DEPLOYMENT_TARGET = 10.10/应用程序/Xcode-beta.app/内容/开发/工具链/XcodeDefault。xctoolchain/usr/bin/clang++ -arch x86_64 -isysroot/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/sdk/MacOSX10.11. xctoolchain/usr/bin/clang++ -arch x86_64sdk -L/Users/home/Library/Developer/Xcode/DerivedData/tileed -bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Products/Debug -L/usr/local/lib -F/Users/home/Library/Developer/Xcode/DerivedData/tileed - bhqwfqpsuugzhkagbmrhebngnpb/Build/Products/Debug -filelist/Users/home/Library/Developer/Xcode/DerivedData/tileed - bhqwfqpsuugzhkagbmrhebngnpb/Build/Intermediates/tileed . Build/Debug/tileed . Build/Objects-normal/x86_64/tileed . BuildLinkFileList -mmacosx-version-min=10.10 -stdlib=libc++ - lglfw3.1 -framework OpenGL -Xlinker -dependency_info -Xlinker/Users/home/Library/Developer/Xcode/DerivedData/tiled-bhqwfqpsuugzhkagbmrhebnbgnpb/Build/Intermediates/tiled.build/Debug/tiled.build/Objects-normal/x86_64/tiled_dependency_info.dat -o/Users/home/Library/Developer/Xcode/DerivedData/tiled- bhqwfqpsuugzhkagbmrhebngnpb/Build/Products/Debug/tile