What does []() argument in function mean?

Question

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();
});

Answer 1

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.

• Mr.B: Thanks Mr.@Majenko, I will look further about this topic. So in the above example instead of creating a new function as a callback they used lambda function on the go to use locally within the function scope itself Am I right. Im just curious to know can we use the variables declared in server.on function inside the lambda
• Majenko: All is explained in the link. But any variable that is defined within the function that server.on appears in can be "captured" by placing it between the [...], so you can then use it from within the lambda expression itself.
• Mr.B: Thanks Mr.@Majenko thats useful
• HTNW: More generally, the lambda is actually shorthand for a class declaration somewhere in the enclosing scope, 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.
• Cort Ammon: @PeterMortensen It is a C++11 feature (meaning older compilers may not support it). Of course, other languages do have their own version of this. To the best of my knowledge, C does not.