posix_tutorial

MMKV for POSIX

MMKV is an efficient, small, easy-to-use mobile key-value storage framework used in the WeChat application. It's currently available on both Android, iOS/macOS, Win32 and POSIX.

Tutorial

You can use MMKV as you go. All changes are saved immediately, no save, no sync calls needed.

Configuration

• Setup MMKV on App startup, say in your main(), add these code:

  #include "MMKV.h"
  
  int main() {
      std::string rootDir = getYourAppDocumentDir();
      MMKV::initializeMMKV(rootDir);
      //...
  }

CRUD Operations

• MMKV has a global instance, that can be used directly:

  auto mmkv = MMKV::defaultMMKV();
  
  mmkv->set(true, "bool");
  std::cout << "bool = " << mmkv->getBool("bool") << std::endl;
  
  mmkv->set(1024, "int32");
  std::cout << "int32 = " << mmkv->getInt32("int32") << std::endl;
  
  mmkv->set(numeric_limits<uint32_t>::max(), "uint32");
  std::cout << "uint32 = " << mmkv->getUInt32("uint32") << std::endl;
  
  mmkv->set(numeric_limits<int64_t>::min(), "int64");
  std::cout << "int64 = " << mmkv->getInt64("int64") << std::endl;
  
  mmkv->set(numeric_limits<uint64_t>::max(), "uint64");
  std::cout << "uint64 = " << mmkv->getUInt64("uint64") << std::endl;
  
  mmkv->set(3.14f, "float");
  std::cout << "float = " << mmkv->getFloat("float") << std::endl;
  
  mmkv->set(numeric_limits<double>::max(), "double");
  std::cout << "double = " << mmkv->getDouble("double") << std::endl;
  
  mmkv->set("Hello, MMKV for Win32", "string");
  std::string result;
  if (mmkv->getString("string", result)) {
     std::cout << "string = " << result << std::endl;
  }

  As you can see, MMKV is quite easy to use.

• Deleting & Querying:

  auto kv = MMKV::defaultMMKV();
       
  mmkv->removeValueForKey("bool");
  std::cout << "bool = " << mmkv->getBool("bool") << std::endl;
  
  mmkv->removeValuesForKeys({"int32", "int64"});
  std::cout << "allKeys: ";
  for (auto &key : mmkv->allKeys()) {
      std::cout << key << ", ";
  }
  std::cout << std::endl;
        
  bool hasBool = mmkv->containsKey("bool");

• If different modules/logics need isolated storage, you can also create your own MMKV instance separately:

  auto mmkv = MMKV::mmkvWithID("MyID");
  mmkv->set(true, "bool");

• If multi-process accessing is needed，you can set MMKV_MULTI_PROCESS on MMKV initialization:

  auto mmkv = MMKV::mmkvWithID("InterProcessKV", MMKV_MULTI_PROCESS);
  mmkv->set(true, "bool");

Supported Types

• Primitive Types:

  • bool, int32, int64, uint32, uint64, float, double

• Classes & Collections:

  • std::string, std::vector<std::string>, mmkv::MMBuffer

Logs

• By default, MMKV prints log to stdout, which is not convenient for diagnosing online issues. You can setup MMKV log redirecting on App startup. Implement callback function with type mmkv::LogHandler(), add some code like these:

  void MyLogHandler(MMKVLogLevel level, const char *file, int line, const char *function, const string &message) {
      auto desc = [level] {
          switch (level) {
              case MMKVLogDebug:
                  return "D";
              case MMKVLogInfo:
                  return "I";
              case MMKVLogWarning:
                  return "W";
              case MMKVLogError:
                  return "E";
              default:
                  return "N";
          }
      }();
      printf("[%s] <%s:%d::%s> %s\n", desc, file, line, function, message.c_str());
  }
  int main() {
      std::string rootDir = getYourAppDocumentDir();
      MMKV::initializeMMKV(rootDir);
      MMKV::registerLogHandler(MyLogHandler);
      //...
  }

  As for a logging tool, we recommand using xlog, which also comes from WeChat team.

• You can turn off MMKV's logging once and for all (which we strongly disrecommend).
  You should never turn MMKV's log off unless you have very strong evidence that it makes your App slow.

  MMKV::setLogLevel(MMKVLogNone);

Next

• MMKV for Python