C++ Primer, Fifth Edition

10.3.3. Lambda Captures and Returns

When we define a lambda, the compiler generates a new (unnamed) class type that corresponds to that lambda. We’ll see how these classes are generated in § 14.8.1 (p. 572). For now, what’s useful to understand is that when we pass a lambda to a function, we are defining both a new type and an object of that type: The argument is an unnamed object of this compiler-generated class type. Similarly, when we use auto to define a variable initialized by a lambda, we are defining an object of the type generated from that lambda.

By default, the class generated from a lambda contains a data member corresponding to the variables captured by the lambda. Like the data members of any class, the data members of a lambda ...