Raylib's suggest is like this.

It is intended to make binary file.

saveandload.cpp

// Save integer value to storage file (to defined position)
// NOTE: Storage positions is directly related to file memory layout (4 bytes each integer)
bool SaveStorageValue(unsigned int position, float value, const char* _fileName)
{
    bool success = false;
    unsigned int dataSize = 0;
    unsigned int newDataSize = 0;
    unsigned char *fileData = LoadFileData(_fileName, &dataSize);
    unsigned char *newFileData = NULL;
 
    if (fileData != NULL)
    {
        if (dataSize <= (position*sizeof(float)))
        {
            // Increase data size up to position and store value
            newDataSize = (position + 1)*sizeof(float);
            newFileData = (unsigned char *)RL_REALLOC(fileData, newDataSize);
 
            if (newFileData != NULL)
            {
                // RL_REALLOC succeded
                float *dataPtr = (float *)newFileData;
                dataPtr[position] = value;
            }
            else
            {
                // RL_REALLOC failed
                TraceLog(LOG_WARNING, "FILEIO: [%s] Failed to realloc data (%u), position in bytes (%u) bigger than actual file size", *_fileName, dataSize, position*sizeof(int));
 
                // We store the old size of the file
                newFileData = fileData;
                newDataSize = dataSize;
            }
        }
        else
        {
            // Store the old size of the file
            newFileData = fileData;
            newDataSize = dataSize;
 
            // Replace value on selected position
            float *dataPtr = (float *)newFileData;
            dataPtr[position] = value;
        }
 
        success = SaveFileData(_fileName, newFileData, newDataSize);
        RL_FREE(newFileData);
 
        TraceLog(LOG_INFO, "FILEIO: [%s] Saved storage value: %i", _fileName, value);
    }
    else
    {
        TraceLog(LOG_INFO, "FILEIO: [%s] File created successfully", _fileName);
 
        dataSize = (position + 1)*sizeof(float);
        fileData = (unsigned char *)RL_MALLOC(dataSize);
        float *dataPtr = (float *)fileData;
        dataPtr[position] = value;
 
        success = SaveFileData(_fileName, fileData, dataSize);
        UnloadFileData(fileData);
 
        TraceLog(LOG_INFO, "FILEIO: [%s] Saved storage value: %i", _fileName, value);
    }
 
    return success;
}
 
// Load float value from storage file (from defined position)
// NOTE: If requested position could not be found, value 0 is returned
float LoadStorageValue(unsigned int position, const char* _fileName)
{
    float value = 0;
    unsigned int dataSize = 0;
    unsigned char *fileData = LoadFileData(_fileName, &dataSize);
 
    if (fileData != NULL)
    {
        if (dataSize < (position*sizeof(float))) TraceLog(LOG_WARNING, "FILEIO: [%s] Failed to find storage position: %i", _fileName, position);
        else
        {
            float *dataPtr = (float *)fileData;
            value = dataPtr[position];
        }
 
        UnloadFileData(fileData);
 
       TraceLog(LOG_INFO, "FILEIO: [%s] Loaded storage value: %f", _fileName, value);
    }
 
    return value;
}

But by this way, saved file is binary. I don't like this binary file.

I think nlohmann Json Library is good json parser library for c++.

Just download single json.hpp file from singleinclude

this is required header file

#include "utils/json.hpp"
#include <iostream>
#include <fstream>

and this is my real c++ header file

config.h

#pragma once
#ifndef __CONFIG__
#define __CONFIG__
#include "utils/json.hpp"
#include <iostream>
#include <fstream>
// 로고, 메인메뉴, 게임 화면 등 각각의 씬
enum class Scene 
{
    logo = 0, titleMenu, game, scoreScene, settingScreen
};
 
// 전역적으로 사용할 싱글톤 클래스
class SceneManager 
{
    private:
        SceneManager() {} 
        static SceneManager* instance; 
 
    public:
 
        static SceneManager* GetInstance()
        {
            if (instance != nullptr)
            {
                return instance;
            }else
            {
                instance = new SceneManager();
                return instance;
            }
        }
 
        bool SaveJson();
        bool LoadJson();
        bool gameExit;
        Scene scene;
        Scene lastSce;
        float volume = 1.0f;
        bool isLog = false;
};
 
 #endif

By this way, You can save and load json file

config.cpp

#include "config.h"
 
// Save Setting to json file
bool SceneManager::SaveJson()
{
    using namespace nlohmann;
 
    json j;
    j["Name"] = "DK Flappy Bird!";
    j["SoundVolume"] = volume;
    j["bLog"] = isLog;
 
 
    // write prettified JSON to another file
    std::ofstream o("settings.json");
    if (o.bad())
    {
        return false;
    }
 
    o << std::setw(4) << j << std::endl;
    o.close();
 
    return true;
}
 
bool SceneManager::LoadJson()
{
    using namespace nlohmann;
 
    std::ifstream f("settings.json");
 
    if (f.bad())
    {
        return false;
    }
 
    json data = json::parse(f);
    volume = data["SoundVolume"];
    isLog = data["bLog"];
    f.close();
    return true;
 
}

Raylib Json File save and load