Question

我使用库进行MQTT连接，我想创建一个创建对象的函数，而不是在预处理器中设置它。原因是因为我需要在程序开始运行后抓住它连接的服务器。

这是我的mqttConnection函数：

ReturnFullTimeZoneData="false"

这是班级：

#include "soapH.h"
SOAP_NMAC struct Namespace namespaces[] =
{
    {"SOAP-ENV", "http://schemas.xmlsoap.org/soap/envelope/", "http://www.w3.org/*/soap-envelope", NULL},
    {"SOAP-ENC", "http://schemas.xmlsoap.org/soap/encoding/", "http://www.w3.org/*/soap-encoding", NULL},
    {"xsi", "http://www.w3.org/2001/XMLSchema-instance", "http://www.w3.org/*/XMLSchema-instance", NULL},
    {"xsd", "http://www.w3.org/2001/XMLSchema", "http://www.w3.org/*/XMLSchema", NULL},
    {"ns1", "http://schemas.microsoft.com/exchange/services/2006/types", NULL, NULL},
    {"ews", "http://schemas.microsoft.com/exchange/services/2006/messages", NULL, NULL},
    {NULL, NULL, NULL, NULL}
};

我得到的错误是，

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!");
        }
    }
}

}

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语句的右括号之前访问该变量。

您可以更改

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

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

要

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

Answer 2

除了关于client范围的答案之外，如果您使用最新版本的PubSubClient库，现在有了在创建客户端对象后设置服务器详细信息的功能：

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。

此处提供完整的api文档：http://pubsubclient.knolleary.net/

Answer 3

client的范围仅限于if声明。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 4

我可以连接mQTT服务器，也可以从客户端订阅。但如何从客户端获取订阅数据

显示1

slidingTabLayout.setDistributeEvenly(true);

表示订阅成功

但如何打印订阅数据

KIndly帮帮我

在函数中创建一个新对象 - C ++

4 个答案: