在函数中创建新对象 - C++

Question

我正在为 MQTT 连接使用一个库，我想创建一个函数来创建对象而不是在预处理器中设置它。原因是因为我需要在程序启动后抓取它连接到哪个服务器运行.

这是我的 mqttConnection 函数：

void mqttConnection(bool checkConnection)   {

if (!checkConnection)   {
    strcpy(mqttServer, mqttUrl);

    PubSubClient client(mqttServer, 1883, callback, wifiClient); // why would it care if this was here?
}

// Connecting to MQTT broker, loop until connected
while (!client.connected()){
    if(!client.connect("device-name")) {
        Serial.println("MQTT Broker Connection Failed. Trying Again...");
    }
    else {
        Serial.println("MQTT Broker Connection Success!");
        if(client.subscribe(mqttTopic)) {
            Serial.println("Subscription Successful! Woot!");
        }
    }
}

}

这是Class：

#ifndef PubSubClient_h
#define PubSubClient_h

#include <Arduino.h>
#include "Client.h"

// MQTT_MAX_PACKET_SIZE : Maximum packet size
#define MQTT_MAX_PACKET_SIZE 128

// MQTT_KEEPALIVE : keepAlive interval in Seconds
#define MQTT_KEEPALIVE 15

#define MQTTPROTOCOLVERSION 3
#define MQTTCONNECT     1 << 4  // Client request to connect to Server
#define MQTTCONNACK     2 << 4  // Connect Acknowledgment
#define MQTTPUBLISH     3 << 4  // Publish message
#define MQTTPUBACK      4 << 4  // Publish Acknowledgment
#define MQTTPUBREC      5 << 4  // Publish Received (assured delivery part 1)
#define MQTTPUBREL      6 << 4  // Publish Release (assured delivery part 2)
#define MQTTPUBCOMP     7 << 4  // Publish Complete (assured delivery part 3)
#define MQTTSUBSCRIBE   8 << 4  // Client Subscribe request
#define MQTTSUBACK      9 << 4  // Subscribe Acknowledgment
#define MQTTUNSUBSCRIBE 10 << 4 // Client Unsubscribe request
#define MQTTUNSUBACK    11 << 4 // Unsubscribe Acknowledgment
#define MQTTPINGREQ     12 << 4 // PING Request
#define MQTTPINGRESP    13 << 4 // PING Response
#define MQTTDISCONNECT  14 << 4 // Client is Disconnecting
#define MQTTReserved    15 << 4 // Reserved

#define MQTTQOS0        (0 << 1)
#define MQTTQOS1        (1 << 1)
#define MQTTQOS2        (2 << 1)

class PubSubClient {
private:
   Client* _client;
   uint8_t buffer[MQTT_MAX_PACKET_SIZE];
   uint16_t nextMsgId;
   unsigned long lastOutActivity;
   unsigned long lastInActivity;
   bool pingOutstanding;
   void (*callback)(char*,uint8_t*,unsigned int);
   uint16_t readPacket();
   uint8_t readByte();
   boolean write(uint8_t header, uint8_t* buf, uint16_t length);
   uint16_t writeString(char* string, uint8_t* buf, uint16_t pos);
   uint8_t *ip;
   char* domain;
   uint16_t port;
public:
   PubSubClient(Client& client);
   PubSubClient(uint8_t *, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
   PubSubClient(char*, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
   boolean connect(char *);
   boolean connect(char *, char *, char *);
   boolean connect(char *, char *, uint8_t, uint8_t, char *);
   boolean connect(char *, char *, char *, char *, uint8_t, uint8_t, char*);
   void disconnect();
   boolean publish(char *, char *);
   boolean publish(char *, uint8_t *, unsigned int);
   boolean publish(char *, uint8_t *, unsigned int, boolean);
   boolean publish_P(char *, uint8_t *, unsigned int, boolean);
   boolean subscribe(char *);
   boolean loop();
   boolean connected();
};


#endif

我得到的错误是，

'client' was not declared in this scope

为什么它在函数中很重要？

Answer 1

PubSubClient client(mqttServer, 1883, callback, wifiClient);

在 if 语句范围内。由于 C++ 中的范围规则，只能在 if 语句的右大括号之前访问变量。

你可以改变

if (!checkConnection) {
    strcpy(mqttServer, mqttUrl);

    PubSubClient client(mqttServer, 1883, callback, wifiClient); // why would it care if this was here?
}

至

if (!checkConnection) {
    strcpy(mqttServer, mqttUrl);
}
PubSubClient client(mqttServer, 1883, callback, wifiClient);

Answer 2

client 的范围仅限于 if 语句。您需要将其声明移到该范围之外，如下所示：

PubSubClient client(/*Some default arguments*/);

if (!checkConnection) {
   client = client(mqttServer, 1883, callback, wifiClient);
}

while (!client.connected()) {
/*...*/
}

Answer 3

除了有关 client 范围的答案外，如果您使用最新版本的 PubSubClient 库，现在有用于在客户端对象被创建后设置服务器详细信息的函数创建：

PubSubClient client(wifiClient);

// some time later
client.setServer(mqttServer,1883);

这使得在运行时更改服务器详细信息变得更加容易。

完整的 api 文档可在此处获得：http://pubsubclient.knolleary.net/

Answer 4

我可以连接到mQTT服务器，也可以从客户端订阅。但是如何从客户端获取订阅数据

它显示 1

str=client.subscribe(testq);
   if(str>0)
    {

       Serial.println(str);
}

表示订阅成功

但是如何打印订阅的数据

请帮助我

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