[英]C++ Lambda - error: no matching function for call to
我试图将lambda作为参数传递给函数,但是一旦我尝试访问在外部声明的lambda内部的变量,构建就会失败: 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*)>)'
我当时以为[&]
将负责捕获变量。 我也尝试过[=]
以及[someVar]
, [&someVar]
。
我正在使用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);
}
问题在于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,但是具有捕获(即状态)的lambda不能。
您可以在签名中看到针对此问题的“常规”解决方案:这里有一个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)。
这比使用lambda更为丑陋,但它基本上是C兼容库接口的唯一选择:
// 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);
void AWS::subscribe(const char *topic,
pApplicationHandler_t iot_subscribe_callback_handler,
void* ptr) {
m_error =
::aws_iot_mqtt_subscribe(&m_client, topic, (uint16_t)std::strlen(topic),
QOS1, iot_subscribe_callback_handler, ptr);
}
然后是一个实用程序类型:
namespace utils {
template<class F>
struct c_style_callback_t {
F f;
template<class...Args>
static void(*get_callback())(Args..., void*) {
return [](Args...args, void* fptr)->void {
(*static_cast<F*>(fptr))(std::forward<Args>(args)...);
};
}
void* get_pvoid() {
return std::addressof(f);
}
};
template<class F>
c_style_callback_t< std::decay_t<F> >
c_style_callback( F&& f ) { return {std::forward<F>(f)}; }
}
现在我们可以做:
auto task = utils::c_style_callback(
[&] (AWS_IoT_Client *pClient, char *topicName, uint16_t topicNameLen, IoT_Publish_Message_Params *params) {
char *text = (char *)params->payload;
sprintf(someVar, "%s", text);
}
);
aws->subscribe(
topic,
task.get_callback<AWS_IoT_Client*, char*, uint16_t, IoT_Publish_Message_Params*>(),
task.get_pvoid()
);
现场例子 。
据了解,您可以修改AWS
类,我可以这样做:
template<class Handler>
void AWS::subscribe(const char *topic,
Handler iot_subscribe_callback_handler) {
auto task = utils::c_style_callback(iot_subscribe_callback_handler);
m_error =
::aws_iot_mqtt_subscribe(
&m_client,
topic,
(uint16_t)std::strlen(topic),
QOS1,
task.get_callback<AWS_IoT_Client*, char*, uint16_t, IoT_Publish_Message_Params*>(),
task.get_pvoid()
);
}
现在, subscribe
接受带有签名void(AWS_IoT_Client*, char*, uint16_t, IoT_Publish_Message_Params*)
。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.