`SWIG/Examples/tcl/constants/`

## Wrapping C Constants

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When SWIG encounters C preprocessor macros and C declarations that look like constants, it creates Tcl variables with an identical value. Click here to see a SWIG interface with some constant declarations in it.

## Accessing Constants from Tcl

Click here to see a script that prints out the values of the constants contained in the above file.

## Key points

- The values of preprocessor macros are converted into Tcl read-only variables.
- Types are inferred by syntax (e.g., "3" is an integer and "3.5" is a float).
- Character constants such as 'x' are converted into strings.
- C string literals such as "Hello World" are converted into strings.
- Macros that are not fully defined are simply ignored. For example:

  >     #define EXTERN extern

  is ignored because SWIG has no idea what type of variable this would be.

- Expressions are allowed provided that all of their components are defined. Otherwise, the constant is ignored.
- Certain C declarations involving 'const' are also turned into Tcl constants.
- The constants that appear in a SWIG interface file do not have to appear in any sort of matching C source file since the creation of a constant does not require linkage to a stored value (i.e., a value held in a C global variable or memory location).
- Since constants are turned into Tcl variables, you have to use the global statement when accessing from a procedure. For example:

  >     proc foo {} {
  >         global ICONST          # Some C constant
  >         puts $ICONST
  >     }