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> #define sizeof(x) (size)sizeof(x) Undefined behavior[1] > #define assert(c) while (!(c)) __builtin_unreachable() Undefined behavior[1] > I’ll cast awa

by thetic 3y ago

> #define sizeof(x) (size)sizeof(x) Undefined behavior[1] > #define assert(c) while (!(c)) __builtin_unreachable() Undefined behavior[1] > I’ll cast away the const if needed. Undefined behavior[2] > The assignments are separated by sequence points, giving them an explicit order. I don't believe assignments are sequence points and only the function call is. [1] https://en.cppreference.com/w/c/language/identifier#Reserved_identifiers https://en.cppreference.com/w/c/language/identifier#Reserved... [2] https://en.cppreference.com/w/c/language/const https://en.cppreference.com/w/c/language/const

WalterBright 3y ago
> #define assert(c) while (!(c)) __builtin_unreachable() And people keep telling me that nobody uses the C preprocessor to define their own syntax any more!
dundarious 3y ago
> > I’ll cast away the const if needed. > Undefined behavior[2] To be clear, it's only UB if the object was defined const, which is the case given he wrote: > One small exception: I still like it as a hint to place static tables in read-only memory closer to the code. I’ll cast away the const if needed. So you are correct on this point. Funnily enough, such objects are relatively rare IME, so I had to double-check to see that he was advocating it specifically in the rare case where it must not be applied.
comex 3y ago
> Undefined behavior[2] Given that this particular undefined behavior usually causes crashes in practice, I expect the author is talking about casting away the const but not actually writing to the pointer. Which is legal.
- paulddraper 3y ago
  Or a common situation is mutable -> const -> mutable. And that is legal.
- thetic 3y ago
  The legal cases in which he needs to cast away const could be avoided if the arguments to called functions were appropriately qualified.
  - LegionMammal978 3y ago
    He never said he needs to cast away const to do what he is attempting to do, he just said that he wants to cast away const to reduce clutter, even though the program would have the same semantics as if he kept the const.
    - thetic 3y ago
      If only there were a way to indicate the function argument isn't mutated. </s> My spidey senses tingle whenever I see const-ness cast away because it almost always means something is wrong. Either a function is missing a qualifier on an argument, or something very unsafe is happening. Why force callers to cast away const-ness in hopes that everything will be fine when you can just write the correct function signature.
LegionMammal978 3y ago
>> I’ll cast away the const if needed. > Undefined behavior[2] How so? As the page you linked mentions, simply casting 'const T *' to regular 'T *' is well-defined; it's only modifying a const object through the pointer that's UB (C17 6.7.3/7). > I don't believe assignments are sequence points and only the function call is. Assigments within expressions don't create sequence points. However, the expression of an expression statement is a full expression (i.e., not a subexpression of another expression), and there is a sequence point between each pair of full expressions (C17 6.8/4). In other words, the semicolons create sequence points.
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by thetic 3y ago

WalterBright 3y ago

> #define assert(c) while (!(c)) __builtin_unreachable() And people keep telling me that nobody uses the C preprocessor to define their own syntax any more!

dundarious 3y ago

> > I’ll cast away the const if needed. > Undefined behavior[2] To be clear, it's only UB if the object was defined const, which is the case given he wrote: > One small exception: I still like it as a hint to place static tables in read-only memory closer to the code. I’ll cast away the const if needed. So you are correct on this point. Funnily enough, such objects are relatively rare IME, so I had to double-check to see that he was advocating it specifically in the rare case where it must not be applied.

comex 3y ago

> Undefined behavior[2] Given that this particular undefined behavior usually causes crashes in practice, I expect the author is talking about casting away the const but not actually writing to the pointer. Which is legal.

paulddraper 3y ago
Or a common situation is mutable -> const -> mutable. And that is legal.
thetic 3y ago
The legal cases in which he needs to cast away const could be avoided if the arguments to called functions were appropriately qualified.
- LegionMammal978 3y ago
  He never said he needs to cast away const to do what he is attempting to do, he just said that he wants to cast away const to reduce clutter, even though the program would have the same semantics as if he kept the const.
  - thetic 3y ago
    If only there were a way to indicate the function argument isn't mutated. </s> My spidey senses tingle whenever I see const-ness cast away because it almost always means something is wrong. Either a function is missing a qualifier on an argument, or something very unsafe is happening. Why force callers to cast away const-ness in hopes that everything will be fine when you can just write the correct function signature.

LegionMammal978 3y ago

>> I’ll cast away the const if needed. > Undefined behavior[2] How so? As the page you linked mentions, simply casting 'const T *' to regular 'T *' is well-defined; it's only modifying a const object through the pointer that's UB (C17 6.7.3/7). > I don't believe assignments are sequence points and only the function call is. Assigments within expressions don't create sequence points. However, the expression of an expression statement is a full expression (i.e., not a subexpression of another expression), and there is a sequence point between each pair of full expressions (C17 6.8/4). In other words, the semicolons create sequence points.

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