A lambda is an anonymous function construct: it supplies callable behavior without a program-level name. In the macro pattern shown here, a capture list reads values from the surrounding context, an empty parameter list follows, and a trailing () invokes the lambda immediately. section 18.4
Use these rules to read the construct:
- Write captures inside [ ... ]; this is the part that can read surrounding variables. section 18.4
- Specify a capture as an identifier, an equals sign, and an initializer; its semantics resemble auto type inference with const qualification. section 18.4
- Call a no-argument lambda by placing () after its body; execution ends at the body’s end or at return. section 18.4
- Treat anonymous functions as an extension to the C standard rather than portable standard-C syntax. section 18.5
Immediate lambda for swapping
A SWAP macro captures pointers to its two operands, then immediately invokes a lambda whose body exchanges the pointed-to values. The captures serve as the lambda’s effective inputs while the empty parameter list keeps the call argument-free. section 18.4
