C ++ Lambda-错误：没有匹配的函数可调用

问题描述

我试图将lambda作为参数传递给函数，但一旦尝试访问在外部声明的lambda内部的变量，构建就会失败：错误：没有匹配的函数可以调用' AWS :: subscribe（char [128]，mainTask（void *）:::< lambda（AWS_IoT_Client *，char *，uint16_t，IoT_Publish_Message_Params *，void *）>）'

I am trying to pass a lambda as parameter to a function but once I attempt to access a variable inside the lambda which was declared outside, the build fails: error: no matching function for call to 'AWS::subscribe(char [128], mainTask(void*)::<lambda(AWS_IoT_Client*, char*, uint16_t, IoT_Publish_Message_Params*, void*)>)'

我以为 [&] 将负责捕获变量。我还尝试了 [=] 以及 [someVar] ， [& someVar] 。

I was thinking that the [&] would take care of capturing variables. I also tried [=] as well as [someVar], [&someVar].

我正在使用C ++ 11。

I'm using C++11.

char someVar[128];

aws->subscribe(
  topic,
  [&] (AWS_IoT_Client *pClient, char *topicName, uint16_t topicNameLen, IoT_Publish_Message_Params *params, void *pData) {
    char *text = (char *)params->payload;
    sprintf(someVar, "%s", text);
  }
);

从AWS库中：

void AWS::subscribe(const char *topic,
                    pApplicationHandler_t iot_subscribe_callback_handler) {
  m_error =
      ::aws_iot_mqtt_subscribe(&m_client, topic, (uint16_t)std::strlen(topic),
                              QOS1, iot_subscribe_callback_handler, NULL);

}

推荐答案

问题是 AWS :: subscribe 函数需要函数指针，而不是lambda。无需捕获的lambda可以转换为函数指针，但是带有捕获（即状态）的lambda不能转换。

The issue is that the AWS::subscribe function expects a function pointer, not a lambda. Capture-less lambdas can be converted to function pointers, but lambdas with captures (i.e. state) cannot.

您可以在签名中看到常规解决方案：有一个 void * 参数，您应该将所有特定于回调的数据打包到其中。大概这是 aws_iot_mqtt_subscribe 的最后一个参数，您当前将其设置为 NULL （最好使用 nullptr btw）。

You can see the "conventional" solution to this already in the signature: There is a void* parameter that you should pack all your callback-specific data into. Presumably this is the last argument of aws_iot_mqtt_subscribe that you currently set to NULL (prefer using nullptr btw).

这比使用lambda更为丑陋，但这基本上是C兼容库接口的唯一选择：

This is uglier than using lambdas, but it's basically the only option for C-compatible library interfaces:

// Your callback (could also be a capture-less lambda):
void callbackFunc(/* etc. */, void *pData)
{
    std::string* someVarPtr = static_cast<std::string*>(pData);
    char *text = (char *)params->payload;
    sprintf(*someVarPtr, "%s", text);
}

// To subscribe:
std::string someVar;
void* callbackData = &someVar; // Or a struct containing e.g. pointers to all your data.
aws_iot_mqtt_subscribe(/* etc. */, callbackFunc, callbackData);

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