const

The const keyword indicates a variable that is to be constant.

Two primary applications:

• to indicate a constant "parameter"
• to indicate which function arguments might be modified

     int foo( const double *input_variable, double *output_variable )

Most modern code is written to be const-correct.

Note the similarity in purpose to FORTRAN 90 intents IN and OUT.

Casting

non-const to const is always permitted: the other way is always a problem.

Confusions

Note that it is easy (in C) and possible in C++ to override constness. It should be taken as a guarantee, and a contract.

One hears the objection to const that because it can be overridden, it is useless.

That is a confusion of the purpose of the const paradigm. It's not about certainty or security. It's about responsibility. By calling a function that takes a non-const pointer argument, you are giving it the responsibility of changing the data.