Generalize const_cast(), and use it more.

[alejandro-colomar]

---

Revisions:

v1b

• Rebase

$ git rd 
1:  ce42f9144 = 1:  2c3f13ec1 lib/cast.h: const_cast(): Pass through other types
2:  76d74d912 = 2:  fea4e1cea lib/atoi/a2i.h: Use const_cast() instead of a raw cast
3:  64085c62c = 3:  e92c7e61f lib/string/: Use QChar_of() to simplify some macros

v1c

• Rebase

$ git rd 
1:  2c3f13ec1 = 1:  f03098a32 lib/cast.h: const_cast(): Pass through other types
2:  fea4e1cea = 2:  74b8aa829 lib/atoi/a2i.h: Use const_cast() instead of a raw cast
3:  e92c7e61f = 3:  226fc059e lib/string/: Use QChar_of() to simplify some macros

v1d

• Rebase

$ git rd 
1:  f03098a32 = 1:  36b190225 lib/cast.h: const_cast(): Pass through other types
2:  74b8aa829 ! 2:  92d03967c lib/atoi/a2i.h: Use const_cast() instead of a raw cast
    @@ lib/atoi/a2i.h
                unsigned long long: strtou_noneg                      \
     -  )(s, (char **) endp_, base, min_, max_, &status);             \
     +  )(s, const_cast(char **, endp_), base, min_, max_, &status);  \
    -                                                                       \
    +                                                                 \
        if (status != 0)                                              \
                errno = status;                                       \
3:  226fc059e ! 3:  b55a55aeb lib/string/: Use QChar_of() to simplify some macros
    @@ lib/string/strcmp/strcaseprefix.h
      #define strcaseprefix(s, prefix)                                      \
     -({                                                                    \
     -  const char  *p_;                                              \
    --                                                                      \
    +-                                                                \
     -  p_ = strcaseprefix_(s, prefix);                               \
    --                                                                      \
    +-                                                                \
     -  _Generic(s,                                                   \
     -          const char *:                     p_,                 \
     -          const void *:                     p_,                 \
    @@ lib/string/strcmp/strprefix.h
      #define strprefix(s, prefix)                                          \
     -({                                                                    \
     -  const char  *p_;                                              \
    --                                                                      \
    +-                                                                \
     -  p_ = strprefix_(s, prefix);                                   \
    --                                                                      \
    +-                                                                \
     -  _Generic(s,                                                   \
     -          const char *:                     p_,                 \
     -          const void *:                     p_,                 \

v1e

• Rebase

$ git rd 
1:  36b190225452 = 1:  dbedfb1d387a lib/cast.h: const_cast(): Pass through other types
2:  92d03967cf26 = 2:  ec48e1275c11 lib/atoi/a2i.h: Use const_cast() instead of a raw cast
3:  b55a55aebc5d = 3:  95240c20c060 lib/string/: Use QChar_of() to simplify some macros

v1f

• Rebase

$ git rd 
1:  dbedfb1d = 1:  c2bb0e09 lib/cast.h: const_cast(): Pass through other types
2:  ec48e127 = 2:  e4cdcf42 lib/atoi/a2i.h: Use const_cast() instead of a raw cast
3:  95240c20 = 3:  4f994530 lib/string/: Use QChar_of() to simplify some macros

v1g

• Rebase

$ git rd 
1:  c2bb0e09 = 1:  087bcb12 lib/cast.h: const_cast(): Pass through other types
2:  e4cdcf42 = 2:  555dbd8f lib/atoi/a2i.h: Use const_cast() instead of a raw cast
3:  4f994530 = 3:  e37687a5 lib/string/: Use QChar_of() to simplify some macros

[alejandro-colomar]

Cc: @kees

[kees]

A down-side of this change is that if T is ever already const-qualified, this code will fail to build (on at least Clang), as const const ... is not a valid type qualifier. The original _Generic() macros handled this by matching const with a pass-thru, and only adding const when it was missing.

[alejandro-colomar]

@kees

$ git log --pretty=reference lib/cast.h
eb71706b (*/: Fix including <config.h> as system header, 2025-07-15)
69f74dbf (lib/cast.h: const_cast(): Reimplement with _Generic(3), 2024-05-15)
e9fc8fc7 (lib/cast.h: const_cast(): Add macro for dropping 'const', 2024-01-07)

By "the original _Generic() macros" you mean this, right?

$ git show 69f74dbf:lib/cast.h | grepc -h const_cast
#define const_cast(T, p)  _Generic(p, const T:  (T) (p))

69f74db (2024-06-04; "lib/cast.h: const_cast(): Reimplement with _Generic(3)")

I think that would also fail for const char *, as it would expand const T as const const char *, right?