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Q&A How to write a macro that discards the const qualifier, for any type?

How to write a macro that discards the const qualifier, for any type? I hope some combination of typeof and a cast will do, but haven't found the combination. I tried this, without luck: #define...

2 answers  ·  posted 2y ago by alx‭  ·  last activity 1y ago by alx‭

#4: Nominated for promotion by user avatar Alexei‭ · 2022-11-06T07:40:39Z (over 1 year ago)
#3: Post edited by user avatar alx‭ · 2022-06-10T15:07:49Z (almost 2 years ago)
__auto_type
  • How to write a macro that discards the const qualifier, for any type?
  • I hope some combination of `typeof` and a cast will do, but haven't found the combination.
  • I tried this, without luck:
  • ```c
  • #define discard_const(_x) ((typeof(_x + 0)) (_x))
  • struct t {
  • char *s;
  • };
  • char *
  • foo(const struct t *r)
  • {
  • return discard_const(r->s);
  • }
  • ```
  • The objective is that if I cast to `(char *)`, I fear that if the type of `s` changes in the future, the cast might silence any warnings, so I want the type to be automatically calculated, and I only want to discard `const`.
  • Of course, with pragmas I may disable any const warnings for that specific line, but a macro that does it within the language would be nicer.
  • How to write a macro that discards the const qualifier, for any type?
  • I hope some combination of `typeof` and a cast will do, but haven't found the combination.
  • I tried this, without luck:
  • ```c
  • #define discard_const(_x) ((typeof(_x + 0)) (_x))
  • struct t {
  • char *s;
  • };
  • char *
  • foo(const struct t *r)
  • {
  • return discard_const(r->s);
  • }
  • ```
  • I also tried: `#define discard_const(_x) ({__auto_type _y = (_x); _y;})`, but not luck
  • The objective is that if I cast to `(char *)`, I fear that if the type of `s` changes in the future, the cast might silence any warnings, so I want the type to be automatically calculated, and I only want to discard `const`.
  • Of course, with pragmas I may disable any const warnings for that specific line, but a macro that does it within the language would be nicer.
#2: Post edited by user avatar alx‭ · 2022-06-10T14:56:00Z (almost 2 years ago)
  • How to write a macro that discards the const qualifier, for any type?
  • I hope some combination of `typeof` and a cast will do, but haven't found the combination.
  • I tried this, without luck:
  • ```c
  • #define discard_const(_x) ((typeof(_x + 0)) (_x))
  • struct t {
  • char *s;
  • };
  • char *foo(const struct t *r)
  • {
  • return discard_const(r->s);
  • }
  • ```
  • The objective is that if I cast to `(char *)`, I fear that if the type of `s` changes in the future, the cast might silence any warnings, so I want the type to be automatically calculated, and I only want to discard `const`.
  • Of course, with pragmas I may disable any const warnings for that specific line, but a macro that does it within the language would be nicer.
  • How to write a macro that discards the const qualifier, for any type?
  • I hope some combination of `typeof` and a cast will do, but haven't found the combination.
  • I tried this, without luck:
  • ```c
  • #define discard_const(_x) ((typeof(_x + 0)) (_x))
  • struct t {
  • char *s;
  • };
  • char *
  • foo(const struct t *r)
  • {
  • return discard_const(r->s);
  • }
  • ```
  • The objective is that if I cast to `(char *)`, I fear that if the type of `s` changes in the future, the cast might silence any warnings, so I want the type to be automatically calculated, and I only want to discard `const`.
  • Of course, with pragmas I may disable any const warnings for that specific line, but a macro that does it within the language would be nicer.
#1: Initial revision by user avatar alx‭ · 2022-06-10T14:55:22Z (almost 2 years ago)
How to write a macro that discards the const qualifier, for any type?
How to write a macro that discards the const qualifier, for any type?

I hope some combination of `typeof` and a cast will do, but haven't found the combination.

I tried this, without luck:

```c
#define discard_const(_x)  ((typeof(_x + 0)) (_x))

struct t {
    char *s;
};

char *foo(const struct t *r)
{
    return discard_const(r->s);
}

```

The objective is that if I cast to `(char *)`, I fear that if the type of `s` changes in the future, the cast might silence any warnings, so I want the type to be automatically calculated, and I only want to discard `const`.

Of course, with pragmas I may disable any const warnings for that specific line, but a macro that does it within the language would be nicer.