Const and Optimization in C
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Const and Optimization in C
1–10 of 44 posts
Re: Const and Optimization in C
#2So if I understand the author, const function arguments can just have const-ness cast away inside the function, so it's really much closer to a type hint than anything else. However, if a variable is declared const, casting away const-ness is undefined. Is this about right? Yikes that's complex. I suppose the moral is casting away const-ness is a terrible idea.
Re: Const and Optimization in C
#3One of the subtler distinctions between C and D is D does allow this optimization. This occasionally engenders debate about which is better.
Re: Const and Optimization in C
#4[deleted]
Re: Const and Optimization in C
#5So if I understand the author, const function arguments can just have const-ness cast away inside the function, so it's really much closer to a type hint than anything else. However, if a variable is declared const, casting away const-ness is undefined. Is this about right? Yikes that's complex. I suppose the moral is casting away const-ness is a terrible idea.
The key distinction is not variable vs function argument, but rather const variable vs pointer-to-const.
If you have "const int x" (as a local variable, global variable, or function parameter) then there is no valid way to modify x's value. But if you have "const int* y" (as a local variable, global variable, or function parameter), you can always cast away const and mutate as long as the original variable (the one "y" points to) wasn't declared const.
Re: Const and Optimization in C
#6So if I understand the author, const function arguments can just have const-ness cast away inside the function, so it's really much closer to a type hint than anything else. However, if a variable is declared const, casting away const-ness is undefined. Is this about right? Yikes that's complex. I suppose the moral is casting away const-ness is a terrible idea.
[deleted]
Re: Const and Optimization in C
#7The C99 specification, in §6.7.3¶5, has one sentence just for this:
If an attempt is made to modify an object defined with a
const-qualified type through use of an lvalue with non-
const-qualified type, the behavior is undefined.
void foo(const int *readonly_x) {
int *x = (int *)readonly_x; // cast away const
(*x)++;
}
The original x wasn’t const-qualified, so this rule didn’t apply. And there aren’t any rules against casting away const to modify an object that isn’t itself const.
Does the original x matter here, or just the readonly_x variable that is in scope of the function? In the language of the spec, are x and readonly_x the same object? Or two different objects of different types that happen to point to the same address? Is it certain that const-qualifiers on function arguments can be legally be ignored inside that function?
Re: Const and Optimization in C
#8So if I understand the author, const function arguments can just have const-ness cast away inside the function, so it's really much closer to a type hint than anything else. However, if a variable is declared const, casting away const-ness is undefined. Is this about right? Yikes that's complex. I suppose the moral is casting away const-ness is a terrible idea.
"casting away const-ness is undefined"
No, it's not. That's the whole point why compiler can't optimize it away. You can often see ("char *") casts from static "strings" in legacy APIs and libraries calls, because original authors didn't know or didn't care how to use const correctly (or at all). I, personally, use const a lot throughout my C code, when you get it, it makes debugging so much easier.
Re: Const and Optimization in C
#9So if I understand the author, const function arguments can just have const-ness cast away inside the function, so it's really much closer to a type hint than anything else. However, if a variable is declared const, casting away const-ness is undefined. Is this about right? Yikes that's complex. I suppose the moral is casting away const-ness is a terrible idea.
"casting away const-ness is undefined" No, it's not. That's the whole point why compiler can't optimize it away. You can often see ("char *") casts from static "strings" in legacy APIs and libraries calls, because original authors didn't know or didn't care how to use const correctly (or at all). I, personally, use const a lot throughout my C code, when you get it, it makes debugging so much easier.
you skipped the first clause of my sentence --
if a variable is declared const, casting away const-ness is undefined
is that not true?
Re: Const and Optimization in C
#10The C99 specification, in §6.7.3¶5, has one sentence just for this: If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non- const-qualified type, the behavior is undefined. This is a separate question than whether the compiler can rely on the function prototype, but doesn't his definition of bar() invoke undefined behavior by this rule? void foo(const int *read…
`readonly_x` is a non-constant object of type 'pointer to const int'. If `readonly_x` itself was const-qualified, it'd look like this: `const int* const readonly_x`.
This is a distinct object from whatever pointer the caller is passing in. But none of this matters to the special const rule -- that rule is not talking about the pointer object, but about the target object that the pointer is pointing to.