Const correctness in C vs C++

Question

I understand what const correctness means and my question is not about what const correctness is. So I am not expecting an explanation or C++-FAQ links for that.

My questions are:

• What are the semantic differences between const in C and const in C++? and
• What is the reason for the difference?

Quotes from the respective standards which make the differences clear would be nice to have.

I regularly switch between C and C++ and I would like to know the important points that one should keep in mind while doing so.

I don't seem to remember the reason for these (special thanks if you can provide a reasoning) but from the top of my mind, I can remember:

• const variables in C++ have internal linkage by default, while in C they have default external linkage;
• const objects can be used as compile-time values in C++, but cannot be used as compile-time values in C;
• Pointers to string literals must be an char const* in C++ but in C it can be char*.

What am I missing?

Answer 1

In addition to the differences you cite, and the library differences that Steve Jessop mentions,

char* p1;
char const* const* p2 = &p1;

is legal in C++, but not in C. Historically, this is because C originally allowed:

char* p1;
char const** p2 = &p1;

Shortly before the standard was adopted, someone realized that this punched a hole in const safety (since *p2 can now be assigned a char const*, which results in p1 being assigned a char const*); with no real time to analyse the problem in depth, the C committee banned any additional const other than top level const. (I.e. &p1 can be assigned to a char ** or a char **const, but not to a char const** nor a char const* const*.) The C++ committee did the further analysis, realized that the problem was only present when a const level was followed by a non-const level, and worked out the necessary wording. (See §4.4/4 in the standard.)