Meaning of const

gistfile1.cpp

/* SAVE THIS FILE AS test.X */

#ifndef __cplusplus
#  include <stdio.h>
#  define PRINT_LANG printf("C\n");
#else
#  include <iostream>
#  define PRINT_LANG cout << "C++" << endl;
#endif

#ifdef __cplusplus
extern "C" {
    namespace std {
#endif

int main(int argc, char **argv)
{
    typedef struct {
        int test_int;
    } tst;

    PRINT_LANG;

    tst test = { 23 };

    tst *                   vvv = NULL;
    tst * const             vvc = &test;
    tst const *             vcv = NULL;
    tst const * const       vcc = NULL;
    const tst *             cvv = NULL;
    const tst * const       cvc = NULL;
 // const tst const *       ccv = NULL; // ERROR: Double const --> vcv
 // const tst const * const ccc = NULL; // ERROR: Double const --> vcc

    vvv = &test;
#ifdef WITH_ERRS
    vvc = &test; //ERROR
#endif
    vcv = &test;
#ifdef WITH_ERRS
    vcc = &test; //ERROR
#endif
    cvv = &test;
#ifdef WITH_ERRS
    cvc = &test; //ERROR
 // ccv = &test; // --> vcv
 // ccc = &test; // --> vcc
#endif

    *vvv = test;
    *vvc = test;
#ifdef WITH_ERRS
    *vcv = test; //ERROR
    *vcc = test; //ERROR
    *cvv = test; //ERROR
    *cvc = test; //ERROR
 // *ccv = test; // --> vcv
 // *ccc = test; // --> vcc
#endif

    vvv->test_int = 42;
    vvc->test_int = 42;
#ifdef WITH_ERRS
    vcv->test_int = 42; //ERROR
    vcc->test_int = 42; //ERROR
    cvv->test_int = 42; //ERROR
    cvc->test_int = 42; //ERROR
#endif
 // ccv->test_int = 42; // --> vcv
 // ccc->test_int = 42; // --> vcc

    return 0;
}

#ifdef __cplusplus
    }
}
#endif

HOWTO.md

Run them:

ln -s test.X test.c
ln -s test.X test.cpp

$ gcc -std=c99 test.c && ./a.out
C
$ clang -std=c99 test.c && ./a.out
C
$ g++ test.cpp && ./a.out
C++

Compare the errors:

$ diff -u <(clang -DWITH_ERRS test.c 2>&1 | grep -Eo ':[0-9]+:') <(g++ -DWITH_ERRS test.cpp 2>&1 | grep -Eo ':[0-9]+:') | wc -l
       0