解决方法如下：

#ifdef __APPLE__
#include "CoreFoundation/CoreFoundation.h"
#endif

// ----------------------------------------------------------------------------
// This makes relative paths work in C++ in Xcode by changing directory to the Resources folder inside the .app bundle
#ifdef __APPLE__    
    CFBundleRef mainBundle = CFBundleGetMainBundle();
    CFURLRef resourcesURL = CFBundleCopyResourcesDirectoryURL(mainBundle);
    char path[PATH_MAX];
    if (!CFURLGetFileSystemRepresentation(resourcesURL, TRUE, (UInt8 *)path, PATH_MAX))
    {
        // error!
    }
    CFRelease(resourcesURL);

    chdir(path);
    std::cout << "Current Path: " << path << std::endl;
#endif
// ----------------------------------------------------------------------------

具体实现文件如下：

LoadFile.hpp

#ifndef LoadFile_hpp
#define LoadFile_hpp

#ifdef __cplusplus
extern"C" {
#endif
    
#include <stdio.h>

#ifdef __APPLE__
#include "CoreFoundation/CoreFoundation.h"
#endif
    
void loadFile();
    
#ifdef __cplusplus
}
#endif

#endif /* LoadFile_hpp */

LoadFile.cpp

#include "LoadFile.hpp"

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

void loadFile()
{
    printf("%s, %d\n", __func__, __LINE__);
    
    ifstream inputFile;
    string countries;
    
    // This makes relative paths work in C++ in Xcode by changing directory to the Resources folder inside the .app bundle
#ifdef __APPLE__
    CFBundleRef mainBundle = CFBundleGetMainBundle();
    CFURLRef resourcesURL = CFBundleCopyResourcesDirectoryURL(mainBundle);
    char path[PATH_MAX];
    if (!CFURLGetFileSystemRepresentation(resourcesURL, TRUE, (UInt8 *)path, PATH_MAX)) {
        // error!
        
    }
    CFRelease(resourcesURL);
    chdir(path);
    
    std::string sourcePath(path);
    
#if TARGET_OS_IPHONE || TARGET_OS_TV
    // /var/containers/Bundle/Application/8A7E3671-5BE7-4C02-867B-58C53A391397/iOSTest.app/model.bundle/load.bin
    std::cout << "Current Path: " << sourcePath << std::endl;
    std::cout << "Current Path: " << (sourcePath + "/model.bundle/load.bin") << std::endl;
    
#elif TARGET_OS_MAC
    // /Users/xxxxxx/Library/Developer/Xcode/DerivedData/MacTest-fnhmrwbqmbccbxhjywjehgdoqfib/Build/Products/Debug/MacTest.app/Contents/Resources/model.bundle/load.bin
    std::cout << "Current Path: " << sourcePath << std::endl;
    std::cout << "Current Path: " << (sourcePath + "/model.bundle/load.bin") << std::endl;
    
#endif
    
#endif

    
    inputFile.open(sourcePath + "/load.bin"); // The name of the file you set up.
    
    while(inputFile >> countries) {
        cout << countries << endl;
    }

    inputFile.close();
}

iOS 工程中，.cpp 文件获取资源文件的路径地址为: /var/containers/Bundle/Application/8A7E3671-5BE7-4C02-867B-58C53A391397/iOSTest.app/model.bundle/load.bin

MacOS 工程中，.cpp 文件获取资源文件的路径地址为: /Users/xxxxxx/Library/Developer/Xcode/DerivedData/MacTest-fnhmrwbqmbccbxhjywjehgdoqfib/Build/Products/Debug/MacTest.app/Contents/Resources/model.bundle/load.bin

sourcePath + /model.bundle/load.bin 中，虽然不同平台的 sourcePath 不同，但是都能获取到 load.bin 文件的绝对路径。所以，不用用宏 "TARGET_OS_IPHONE || TARGET_OS_TV", "TARGET_OS_MAC" 来区别是 iOS 还是 Mac 平台了。

参考资料: Relative Paths Not Working in Xcode C++