进一步熟悉nullptr

#ifndef emptyString
#define emptyString F("")
#endif

void foo(const char *payload = nullptr) {
  if (payload == nullptr) {
    Serial.println("emptyString");
  } else {
    Serial.print("payload: ");
    Serial.println(payload);
  }
}

void setup(void)
{
  // start serial port
  Serial.begin(9600);
  Serial.println("Dallas Temperature IC Control Library Demo");
}

/*
 * Main function, get and show the temperature
 */
void loop(void)
{ 
  // call sensors.requestTemperatures() to issue a global temperature 
  foo();
  foo("request to all devices on the bus");
  delay(1000);
}
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程序写入ESP32C3以后，串口吐的内容是
emptyString
payload: request to all devices on the bus
emptyString
payload: request to all devices on the bus
emptyString
payload: request to all devices on the bus




#define emptyString F("")

定义空字符串


foo(const char *payload = nullptr)
定义入参payload，默认值是nullptr


foo();
让入参使用默认值，打印emptyString


foo("request to all devices on the bus");
打印payload: request to all devices on the bus


对语法记不清，最好是写出来验证一下

#ifndef emptyString
#define emptyString F("")
#endif

String foo(const char *payload = nullptr) {
  if (payload == nullptr) {
    return emptyString;
    Serial.println("emptyString");
  }
  
  Serial.print("payload: ");
  Serial.println(payload);

  return "foo";
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Serial begin...");

  foo();
  foo("Serial Test...");
}

void loop() {
  // put your main code here, to run repeatedly:

}
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有兴趣可以运行下这个代码，有没有什么问题呢？