The line `#define INFLUXDB_TOKEN` is a preprocessor directive in C...

The line #define INFLUXDB_TOKEN is a preprocessor directive in C or C++ that defines the preprocessor macro INFLUXDB_TOKEN. Without providing a value after the macro name, it essentially acts as a placeholder for conditional compilation or symbolic purposes.

Here’s what it does:

1. Defines the macro INFLUXDB_TOKEN: This makes the macro exist. The macro is not assigned any value, but its presence can be checked using conditional preprocessing directives like #ifdef or #ifndef.

2. Used for conditional compilation: The presence of INFLUXDB_TOKEN can be used to conditionally include or exclude portions of code. For example:

   #ifdef INFLUXDB_TOKEN
       // Code that depends on INFLUXDB_TOKEN being defined
   #endif
   

In this specific case, it's likely related to handling authentication or interaction with InfluxDB (a time-series database), as the macro name suggests it might represent a token for accessing an InfluxDB instance. However, since it is not assigned a value here, it only enables conditional code execution depending on whether it is defined or not.