What does ( [ ]( ){ output;}); mean?

Question

Currently using the ESP8266ServerSecure.h library , HelloServerBearSSL example.

What does this mean? Is the output server.send being initialize as an array? I'm really confused.

server.on("/inline", [](){ server.send(200, "text/plain" , "this works as well"); } );

Answer 1

This is a lambda: https://en.cppreference.com/w/cpp/language/lambda

Equivalent of:

struct Callback
{
  void operator()() {
    server.send(200, "text/plain" , "this works as well");
  }
};

server.on("/inline", Callback{});