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....;

1 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....;

// ...

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.

  • 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
    – Mr.B
    Apr 27, 2020 at 10:19
  • 2
    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.
    – Majenko
    Apr 27, 2020 at 10:22
  • Thanks Mr.@Majenko thats useful
    – Mr.B
    Apr 27, 2020 at 11:17
  • 2
    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.
    – HTNW
    Apr 27, 2020 at 18:48
  • 1
    @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.
    – Cort Ammon
    Apr 28, 2020 at 0:45

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