C++ vs C Const-Correctness – Key Differences Explained

c++const-correctnessconstantslanguage-design

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?

Best Answer

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.)

Related Solutions

C++ Constants – ‘const int’ vs. ‘int const’ as Function Parameters

The trick is to read the declaration backwards (right-to-left):

const int a = 1; // read as "a is an integer which is constant"
int const a = 1; // read as "a is a constant integer"

Both are the same thing. Therefore:

a = 2; // Can't do because a is constant

The reading backwards trick especially comes in handy when you're dealing with more complex declarations such as:

const char *s;      // read as "s is a pointer to a char that is constant"
char c;
char *const t = &c; // read as "t is a constant pointer to a char"

*s = 'A'; // Can't do because the char is constant
s++;      // Can do because the pointer isn't constant
*t = 'A'; // Can do because the char isn't constant
t++;      // Can't do because the pointer is constant

C vs C++ – How Does ‘const’ Differ?

const in C cannot be used to build constant expressions.

For example :

#include <stdio.h>
int main()
{
   int i = 2;
   const int C = 2;
   switch(i)
   {
      case C  : printf("Hello") ;
      break;

      default : printf("World");
   }
}

doesn't work in C because case label does not reduce to an integer constant.

Best Answer

Related Solutions

C++ Constants – ‘const int’ vs. ‘int const’ as Function Parameters

C vs C++ – How Does ‘const’ Differ?

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