函数C++中的Auto变量出错

Question

我在C++中的函数之间传递值时遇到问题。我在下面添加代码。在mqttReceive中，接收JSON中的MQTT消息，并在send((中再次发送该消息，以便在void send(((中接收。然而，我已经尝试将接收到的消息声明为auto，但它不起作用。我缺少什么？

cpp:

void MqttApplication::mqttReceive()
{
    try {
        
        mqttClient->start_consuming();
        mqttClient->subscribe(TOPIC, QOS)->wait();
        
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
        return;
    }
    
    while (true) {
        auto msg = mqttClient->consume_message();   
        
        try {               
            send(msg);
        }
        catch (const mqtt::exception& exc) {
            cerr << exc.what() << endl;
            return;
        }
        if (msg->get_topic() == "command" &&
                msg->to_string() == "exit") {
            cout << "Exit command received" << endl;
            break;
        }
        cout << msg->get_topic() << ": " << msg->to_string() << endl;
    }
}

void MqttApplication::send(auto msg)
{
    ...
}

hpp:

class MqttApplication : public Application
    {
    private:    
    
        void send(const auto msg);
    
        void mqttReceive();
        

错误：

In file included from /home/mqtt_application.cpp:1:
/home/mqtt_application.hpp:26:24: warning: use of ‘auto’ in parameter declaration only available with ‘-fconcepts-ts’
   26 |     void send(const auto& msg) override;
      |                        ^~~~
/home/mqtt_application.hpp:26:35: error: member template ‘void MqttApplication::send(const auto:1&)’ may not have virt-specifiers
   26 |     void send(const auto& msg) override;
      |                                   ^~~~~~~~
/home/mqtt_application.cpp:320:34: warning: use of ‘auto’ in parameter declaration only available with ‘-fconcepts-ts’
  320 | void MqttApplication::send(auto& msg)
      |                                  ^~~~
/home/mqtt_application.cpp:320:6: error: no declaration matches ‘void MqttApplication::send(auto:2&)’
  320 | void MqttApplication::send(auto& msg)
      |      ^~~~~~~~~~~~~~~~~~
In file included from /home/mqtt_application.cpp:1:
/home/mqtt_application.hpp:26:10: note: candidate is: ‘template<class auto:1> void MqttApplication::send(const auto:1&)’
   26 |     void send(const auto& msg) override;
      |          ^~~~~~~
In file included from /home/mqtt_application.cpp:1:
/home/mqtt_application.hpp:15:7: note: ‘class MqttApplication’ defined here
   15 | class MqttApplication : public Application
      |       ^~~~~~~~~~~~~~~~~~
tools/mqtt.dir/build.make:62: recipe for target 'tools/mqtt.dir/mqtt_application.cpp.o' failed
make[2]: *** [tools/mqtt.dir/mqtt_application.cpp.o] Error 1
CMakeFiles/Makefile2:834: recipe for target 'tools/mqtt.dir/all' failed
make[1]: *** [tools/mqtt.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2

我正在用C++14进行编译。我尝试过所有类型的配置，字符串、int等。输入是一个常规的JSON字符串。感谢

Answer 1

使用auto作为参数是C++20的一项功能，一些编译器在早期版本中支持它作为扩展，但在那里不支持ISO。

Answer 2

函数模板的语法需要C++20。对于C++17及更旧版本，您需要更改以下内容：

void MqttApplication::send(auto msg)

至：

template <typename T>
void MqttApplication::send(T msg)

两者都是等效的，但C++20版本更短。请参见缩写函数模板。

Answer 3

我想知道你预计会发生什么？用"；自动；在函数声明中，编译器无法知道类型是什么，因此拒绝编译。这与"；auto x=3"其中编译器知道x的类型应该与3的类型相同。

如果你的编译器理解概念，你也理解概念，那么你就可以构建你的代码，但这需要你理解一个全新的C++特性。