This is kind of basic but I can't seem to get a hold on this one. Reference here
Are void *p and const void *p sufficiently different? Why would a function use const void * instead of void *?
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142answers:
3
+6
A:
The reason to use void* at all (whether const or not) is the kind of genericity is provides. It's like a base class: All pointers are void* and can implicitly cast into it, but casts from void* to typed pointers have to be done explicitly and manually.
Usually, C++ has better ways to offer to do this (namely OO and templates), so it doesn't make much sense to use void* at all, except when you're interfacing C. However, if you use it, then const offers just what it offers elsewhere: you need an (additional) const_cast to be able to change the object referred to, so you are less likely to change it accidentally.
Of course, this relies on you not employing C-style casts, but explicit C++ casts. The cast from void* to any T* requires a static_cast, and this doesn't allow removing of const. So you can cast const void* to const char* using static_cast, but not to char*. This would need an additional const_cast.
sbi
2010-09-14 10:11:43
Nice answer, but I think this is not what's asked. It is the difference between the const and non const version.
Diego Sevilla
2010-09-14 10:18:09
I believe cast from `void *` to `T *` requires `static_cast`. `reinterpret_cast` is always implementation defined and should be avoided.
Tadeusz Kopec
2010-09-14 10:21:02
I think OP knows the meaning of `const`, he is just saying that he does not understand what to make of a `const void` specifically. So I think this is a good answer. And a good use of `reinterpret_cast`, by the way.
Gorpik
2010-09-14 11:27:10
Interesting answer!
Chubsdad
2010-09-14 11:33:09
@Diego: I thought I had written that a `const void*` is useful for the same reason any other const is useful: to prevent accidental changes to an object. To be honest, I'm somewhat at a loss as to how to state this more explicitly. Could you please elaborate?
sbi
2010-09-14 13:05:49
The last paragraph is wrong, and you should correct it, conversions from and to `void*` are performed with `static_cast` §5.2.9[expr.static.cast]/10
David Rodríguez - dribeas
2010-09-14 13:55:22
@David: Yes, you're right! It's conversions between different typed pointers, but not conversions between `void*` and typed pointers. I confused this. I corrected that.
sbi
2010-09-14 15:27:55
@sbi: it is just a subtle thing. Yes, you mention that being const is of course to avoid modifying the object passed to the function, but as the question itself was asking just that, that exact phrase was kind of hidden, and the response was a little bit too much elaborated (at least from my point of view). Also, given that void* is usually used to interface C or system calls, usually void* vs. const void* comes just as a hint to tell the caller whether the function is modifying the passed object pointed or not.
Diego Sevilla
2010-09-14 21:41:48
+1
A:
There is a simple difference. As other stated, there is little need to use void* in C++ due to its better type system, templates, etc. However, when interfacing to C or to system calls, you sometimes need a way to specify a value without a known type.
As you ask, the difference between void* and const void* is a hint to show you if the contents of the pointed memory will be modified within the function you're calling, the const meaning it will have a read-only access.
Diego Sevilla
2010-09-14 10:20:16
+1
A:
In c++ a const in front of a pointer says the data at the pointer's address should not be changed. i.e. it stops someone doing this:
int v1 = 3;
int v2 = 4;
const int *pv = &v1;
pv = &v2 // ok;
*pv = 5; // error
You can also make the pointer value itself a const:
int v1 = 3;
int v2 = 4;
int * const pv = &v1;
*pv = 5; // ok
pv = &v2; // error
You can also combine the two:
int v1 = 3;
int v2 = 4;
const int * const pv = &v1;
*pv = 5; // error
pv = &v2; // error
locka
2010-09-14 10:26:55