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I am working in ESP8266 AsyncWebserver library and have been using this [](parameter) as an argument to some functions as shown below but does not really know what it means. Being a neophyte I am curious to know what does this convention means, its functionalities and how to use it. Hopefully experts here would help me with this.
server.on("/page",HTTP_GET,[](AsyncWebServerRequest * request){
some code....;
request->getParam("Param1")->value();
});
That is called a lambda expression and it's a way of including the content of a function as a parameter anonymously instead of writing an actual function and using the name of it.
It's shorthand for:
void myCallback(AsyncWebServerRequest * request) {
// some code....;
request->getParam("Param1")->value();
}
// ...
server.on("/page", HTTP_GET, myCallback);
It does have a few benefits over traditional functions at the expense of readability, such as the ability to "capture" local variables from the surrounding scope.
server.on appears in can be "captured" by placing it between the [...], so you can then use it from within the lambda expression itself.
struct unnamed_lambda_helper { /* captured local variables are data members of this class */ auto operator()(/* arguments here */) -> /* return type here, if specified */ { /* body here */ } };, and then the expression creates an instance of this class.