undefined | Better HN

0 pointsteddyh5y ago0 comments

> For example, D enables a "pointer to immutable data" type, while C does not.

Wait, isn’t that what

  const int *x;

does? I.e. a pointer to a constant.

0 comments

WalterBright5y ago

In C, the value pointed to by `x` cannot be changed via a write through `x`, but if there is another pointer to that value, it can be changed through that.

In D, `immutable(int)* x` cannot be changed by any reference to the value.

flohofwoe5y ago

Isn't that what 'restrict' is for? It's a new type of footgun of course because the compiler doesn't detect if the value is actually accessed through another pointer, but by simply ignoring that possibility via 'restrict', the compiler should have the same optimization opportunities, no?

WalterBright5y ago

> but by simply ignoring that possibility via 'restrict',

If you deceive the compiler like that, it is entitled to hand you a broken executable :-/

Besides, I forgot to mention that D immutable data is inherently thread safe, no synchronization required. And you can have as many live references to it as you like.

Immutable data is part of D's support for functional programming.

1 more reply

adrianN5y ago

restrict is used so rarely in C code that the Rust team hit a lot of bugs when they tried to give LLVM all the aliasing info the Rust compiler has.

teddyhOP5y ago

You’d think that C could do any appropriate optimizations as well as D, since the C compiler (in theory) should know whether there can exist (in any given program) any writeable pointers to the same value or if there only can exist pointers to const.

marton785y ago

Only inside the same compilation unit.

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innocenat5y ago

Pointer arithmetic would surely make all value mutable. Or even simple array access.

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knome5y ago

    const int world = 42;
    const int const * const hello = &world ;

Apparently, you can be very const and `gcc -Wall -Wextra -std=c99` won't raise any complaints.

teddyhOP5y ago

Firstly, isn’t that a syntax error? There’s a stray “const” in there. You probably meant

  const int * const hello = &world;

Secondly, what should the compiler complain about? You have a const int, and then a const pointer to const int, pointing to that first const int. What’s the problem?

Thirdly, the latest C version supported by GCC is “-std=c17”.

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jcelerier5y ago

> In D, `immutable(int)* x` cannot be changed by any reference to the value.

even if I use a magnetic needle to change the bits in my RAM ? :)

petertodd5y ago

There's an easy answer to that: using a magnetic needle to change the bits in your RAM violated the assumptions of the compiler, leading to undefined behaviour. What exactly will happen in that circumstance is just that: undefined.

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0 comments

WalterBright5y ago

In C, the value pointed to by `x` cannot be changed via a write through `x`, but if there is another pointer to that value, it can be changed through that.

In D, `immutable(int)* x` cannot be changed by any reference to the value.

flohofwoe5y ago

WalterBright5y ago

> but by simply ignoring that possibility via 'restrict',

If you deceive the compiler like that, it is entitled to hand you a broken executable :-/

Besides, I forgot to mention that D immutable data is inherently thread safe, no synchronization required. And you can have as many live references to it as you like.

Immutable data is part of D's support for functional programming.

1 more reply

adrianN5y ago

restrict is used so rarely in C code that the Rust team hit a lot of bugs when they tried to give LLVM all the aliasing info the Rust compiler has.

teddyhOP5y ago

marton785y ago

Only inside the same compilation unit.

1 more reply

innocenat5y ago

Pointer arithmetic would surely make all value mutable. Or even simple array access.

2 more replies

knome5y ago

    const int world = 42;
    const int const * const hello = &world ;

Apparently, you can be very const and `gcc -Wall -Wextra -std=c99` won't raise any complaints.

teddyhOP5y ago

Firstly, isn’t that a syntax error? There’s a stray “const” in there. You probably meant

  const int * const hello = &world;

Secondly, what should the compiler complain about? You have a const int, and then a const pointer to const int, pointing to that first const int. What’s the problem?

Thirdly, the latest C version supported by GCC is “-std=c17”.

1 more reply

jcelerier5y ago

> In D, `immutable(int)* x` cannot be changed by any reference to the value.

even if I use a magnetic needle to change the bits in my RAM ? :)

petertodd5y ago

1 more reply

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