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Should I use static_cast or reinterpret_cast when casting a void* to whatever
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I'm reading a book and I found that reinterpret_cast should not be used directly, but rather casting to void* in combination with static_cast:
T1 * p1=...
void *pv=p1;
T2 * p2= static_cast<T2*>(pv);
Instead of:
T1 * p1=...
T2 * p2= reinterpret_cast<T2*>(p1);
However, I can't find an explanation why is this better than the direct cast. I would very appreciate if someone can give me an explanation or point me to the answer.
Thanks in advance
p.s. I know what is reinterpret_cast used for, but I never saw that is used in this way
For types for which such cast is permitted (e.g. if T1 is a POD-type and T2 is unsigned char), the approach with static_cast is well-defined by the Standard.
On the other hand, reinterpret_cast is entirely implementation-defined - the only guarantee that you get for it is that you can cast a pointer type to any other pointer type and then back, and you'll get the original value; and also, you can cast a pointer type to an integral type large enough to hold a pointer value (which varies depending on implementation, and needs not exist at all), and then cast it back, and you'll get the original value.
To be more specific, I'll just quote the relevant parts of the Standard, highlighting important parts:
5.2.10[expr.reinterpret.cast]:
The mapping performed by reinterpret_cast is implementation-defined. [Note: it might, or might not, produce a representation different from the original value.] ... A pointer to an object can be explicitly converted to a pointer to an object of different type.) Except that converting an rvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are object types and where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.
So something like this:
struct pod_t { int x; };
pod_t pod;
char* p = reinterpret_cast<char*>(&pod);
memset(p, 0, sizeof pod);
is effectively unspecified.
Explaining why static_cast works is a bit more tricky. Here's the above code rewritten to use static_cast which I believe is guaranteed to always work as intended by the Standard:
struct pod_t { int x; };
pod_t pod;
char* p = static_cast<char*>(static_cast<void*>(&pod));
memset(p, 0, sizeof pod);
Again, let me quote the sections of the Standard that, together, lead me to conclude that the above should be portable:
3.9[basic.types]:
For any object (other than a base-class subobject) of POD type T, whether or not the object holds a valid value of type T, the underlying bytes (1.7) making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value.
The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).
3.9.2[basic.compound]:
Objects of cv-qualified (3.9.3) or cv-unqualified type void* (pointer to void), can be used to point to objects of unknown type. A void* shall be able to hold any object pointer. A cv-qualified or cv-unqualified (3.9.3) void* shall have the same representation and alignment requirements as a cv-qualified or cv-unqualified char*.
3.10[basic.lval]:
If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined):
...
a char or unsigned char type.
4.10[conv.ptr]:
An rvalue of type “pointer to cv T,” where T is an object type, can be converted to an rvalue of type “pointer to cv void.” The result of converting a “pointer to cv T” to a “pointer to cv void” points to the start of the storage location where the object of type T resides, as if the object is a most derived object (1.8) of type T (that is, not a base class subobject).
5.2.9[expr.static.cast]:
The inverse of any standard conversion sequence (clause 4), other than the lvalue-to-rvalue (4.1), array-topointer (4.2), function-to-pointer (4.3), and boolean (4.12) conversions, can be performed explicitly using static_cast.
[EDIT] On the other hand, we have this gem:
9.2[class.mem]/17:
A pointer to a POD-struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [Note: There might therefore be unnamed padding within a POD-struct object, but not at its beginning, as necessary to achieve appropriate alignment. ]
which seems to imply that reinterpret_cast between pointers somehow implies "same address". Go figure.
There is not the slightest doubt that the intent is that both forms are well defined, but the wording fails to capture that.
Both forms will work in practice.
reinterpret_cast is more explicit about the intent and should be preferred.
The real reason this is so is because of how C++ defines inheritance, and because of member pointers.
With C, pointer is pretty much just an address, as it should be. In C++ it has to be more complex because of some of its features.
Member pointers are really an offset into a class, so casting them is always a disaster using C style.
If you have multiply inherited two virtual objects that also have some concrete parts, that's also a disaster for C style. This is the case in multiple inheritance that causes all the problems, though, so you should not ever want to use this anyway.
Really hopefully you never use these cases in the first place. Also, if you are casting a lot that's another sign you are messing up in in your design.
The only time I end up casting is with the primitives in areas C++ decides are not the same but where obviously they have to be. For actual objects, any time you want to cast something, start to question your design because you should be 'programming to the interface' most of the time. Of course, you can't change how 3rd party APIs work so you don't always have much choice.
Related
I think that I am misreading the standard quotation, hence I do not fully understand what's the exact intent of the wording.
Firstly, I am already aware of what alignment requirement is, but I can't figure out the exact relation between alignment requirement and casting in general, and what're the points I should care about, regarding alignment requirement, when I perform static_casting or reinterpret_casting. I think now the reader got my first question.
Secondly, there're some words in the standard quotation I spend two days to understand them but I don't. From the paragraph in:
N4885: 7.6.1.9 Static cast [expr.static.cast]
A prvalue of type “pointer to cv1 void” can be converted to a prvalue
of type “pointer to cv2 T”, where T is an object type and cv2 is the
same cv-qualification as, or greater cv-qualification than, cv1. If
the original pointer value represents the address A of a byte in
memory and A does not satisfy the alignment requirement of T, then the
resulting pointer value is unspecified.
Here, they said "if the original pointer value doesn't satisfy the alignment requirement of T, then the resulting pointer value is unspecified". What really does that mean?
What's I can't understand is when does this "original pointer value" satisfies the alignment requirement of T, and when does not, to avoid such unspecified pointer value. I just need someone to explain the bold part from the above quote with simple examples; what I have to know, as a programmer, from that bold part and what I have to avoid. For example:
int i = 12;
double *pd = static_cast<double *>(static_cast<void *>(&i)); // does 'pd' has unspecified address value?.
short *ps = static_cast<short *>(static_cast<void *>(&i)); // does 'ps' has unspecified address value?.
Finally, there's a relatively same sentence I need to understand in:
N4885: 7.6.1.10 Reinterpret cast [expr.reinterpret.cast]
An object pointer can be explicitly converted to an object pointer of
a different type.61 When a prvalue v of object pointer type is
converted to the object pointer type “pointer to cv T”, the result is
static_cast<cv T*>(static_cast<cv void*>(v)). [Note 7: Converting a
pointer of type “pointer to T1” that points to an object of type T1 to
the type “pointer to T2” (where T2 is an object type and the
alignment requirements of T2 are no stricter than those of T1) and
back to its original type yields the original pointer value. — end
note].
What does the standard mean by this sentence "the alignment requirements of T2 are no stricter than those of T1", what does the word "stricter than" mean.
I think if I have this static_assert expression, then maybe the alignment requirements of T2 would not be stricter than those of T1: static_assert(alignof(T1) >= alignof(T2)); Or this assertion is not true for some cases.
int i = 34;
double *pd = reinterpret_cast<double *>(&i); // does 'pd' has unspecified address value?.
short *ps = reinterpret_cast<short *>(&i); // does 'ps' has unspecified address value?.
I am added these example to just clear what my problem lies, not to just answer the questions in the // comments
While common implementations have integer-like pointers (such that reinterpret_cast behaves like memcpy between pointers and integers and arithmetic on the pointers is reflected in the integer values), the standard as usual provides only weak guarantees to support less common architectures where pointers have other formats and/or special registers. As such, it’s impossible to observe alignment of a dynamic pointer value: the unspecified value applies if alignof(expression_type)<alignof(cast_type) unless the pointer refers to an object declared with alignas or to an object whose actual type is more strongly aligned.
This means that double* is a poor type to use for the sort of “temporary pointer storage” for which reinterpret_cast exists; fortunately, most C++ code uses templates (so this doesn’t come up), old C code uses char* (whose alignment is 1), and other code uses void* (which has no alignment), so there’s rarely an actual issue here.
I have stumbled upon a reddit thread in which a user has found an interesting detail of the C++ standard. The thread has not spawned much constructive discussion, therefore I will retell my understanding of the problem here:
OP wants to reimplement memcpy in a standard-compliant way
They attempt to do so by using reinterpret_cast<char*>(&foo), which is an allowed exception to the strict aliasing restrictions, in which reinterpreting as char is allowed to access the "object representation" of an object.
[expr.reinterpret.cast] says that doing so results in static_cast<cv T*>(static_cast<cv void*>(v)), so reinterpret_cast in this case is equivalent to static_cast'ing first to void * and then to char *.
[expr.static.cast] in combination with [basic.compound]
A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. [...] if the original pointer value points to an object a, and there is an object b of type T (ignoring cv-qualification) that is pointer-interconvertible with a, the result is a pointer to b. [...] [emphasis mine]
Consider now the following union class:
union Foo{
char c;
int i;
};
// the OP has used union, but iiuc,
// it can also be a struct for the problem to arise.
OP has thus come to the conclusion that reinterpreting a Foo* as char* in this case yields a pointer pointing to the first char member of the union (or its object representation), rather than to the object representation of the union itself, i.e. it points only to the member. While this appears superficially to be the same, and corresponds to the same memory address, the standard seems to differentiate between the "value" of a pointer and its corresponding address, in that on the abstract C++ machine, a pointer belongs to a certain object only. Incrementing it beyond that object (compare with end() of an array) is undefined behavior.
OP thus argues that if the standard forces the char* to be associated with the objects's first member instead of the object representation of the whole union object, dereferencing it after one incrementation is UB, which allows a compiler to optimize as if it were impossible for the resultant char* to ever access the following bytes of the int member. This implies that it is not possible to legally access the complete object representation of a class object which is pointer-interconvertible with a char member.
The same would, if I understand correctly apply if "union" was simply replaced with "struct", but I have taken this example from the original thread.
What do you think? Is this a standard defect? Is it a misinterpretation?
This video, linked in the comments (now chat) by #KonradRudolph is likely the answer to the problem.
At around the 40min mark, Timur Doumler, who is a member of the ISO C++ commitee, discusses the possibility of accessing byte representations. The summary is that any attempt of accessing byte representation except memcpy is UB. The situation in the OP does not even arise without making use of UB because the very act of using a pointer to an object like an array, or doing any pointer arithmetic on it is UB, as these operations are only well-defined when dealing with actual array objects, as far as the abstract machine is concerned.
Also, while reinterpreting a pointer as a char* does not on its own violate aliasing rules, there is technically no guarantee that the resulting char* will point to the first byte of the object.
The only legal way of accessing byte representations is to memcpy the object into a char array. This means that reimplementing memcpy is impossible.
Timur Doumler additionally describes this as a wording defect that will hopefully be fixed in C++23 and presents a paper that proposes a fix to this.
Here's an example:
#include <cstddef>
#include <iostream>
struct A
{
char padding[7];
int x;
};
constexpr int offset = offsetof(A, x);
int main()
{
A a;
a.x = 42;
char *ptr = (char *)&a;
std::cout << *(int *)(ptr + offset) << '\n'; // Well-defined or not?
}
I always assumed that it's well-defined (otherwise what would be the point of offsetof), but wasn't sure.
Recently I was told that it's in fact UB, so I want to figure it out once and for all.
Does the example above cause UB or not? If you modify the class to not be standard-layout, does it affect the result?
And if it's UB, are there any workarounds for it (e.g. applying std::launder)?
This entire topic seems to be moot and underspecified.
Here's some information I was able to find:
Is adding to a “char *” pointer UB, when it doesn't actually point to a char array? - In 2011, CWG confirmed that we're allowed to examine the representation of a standard-layout object through an unsigned char pointer.
Unclear if a char pointer can be used insteaed, common sense says it can.
Unclear if staring from C++17 std::launder needs to be applied to the result of the (unsigned char *) cast. Given that it would be a breaking change, it's probably unnecessarly, at least in practice.
Unclear why C++17 changed offsetof to conditionally-support non-standard-layout types (used to be UB). It seems to imply that if an implementation supports that, then it also lets you examine the representation of non-standard-layout objects through unsigned char *.
Do we need to use std::launder when doing pointer arithmetic within a standard-layout object (e.g., with offsetof)? - A question similar to this one. No definitive answer was given.
Here I will refer to C++20 (draft) wording, because one relevant editorial issue was fixed between C++17 and C++20 and also it is possible to refer to specific sentences in HTML version of the C++20 draft, but otherwise there is nothing new in comparison to C++17.
At first, definitions of pointer values [basic.compound]/3:
Every value of pointer type is one of the following:
— a pointer to an object or function (the pointer is said to point to the object or function), or
— a pointer past the end of an object ([expr.add]), or
— the null pointer value for that type, or
— an invalid pointer value.
Now, lets see what happens in the (char *)&a expression.
Let me not prove that a is an lvalue denoting the object of type A, and I will say «the object a» to refer to this object.
The meaning of the &a subexpression is covered in [expr.unary.op]/(3.2):
if the operand is an lvalue of type T, the resulting expression is a prvalue of type “pointer to T” whose result is a pointer to the designated object
So, &a is a prvalue of type A* with the value «pointer to (the object) a».
Now, the cast in (char *)&a is equivalent to reinterpret_cast<char*>(&a), which is defined as static_cast<char*>(static_cast<void*>(&a)) ([expr.reinterpret.cast]/7).
Cast to void* doesn't change the pointer value ([conv.ptr]/2):
A prvalue of type “pointer to cv T”, where T is an object type, can be converted to a prvalue of type “pointer to cv void”. The pointer value ([basic.compound]) is unchanged by this conversion.
i.e. it is still «pointer to (the object) a».
[expr.static.cast]/13 covers the outer static_cast<char*>(...):
A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.
If the original pointer value represents the address A of a byte in memory and A does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified.
Otherwise, if the original pointer value points to an object a, and there is an object b of type T (ignoring cv-qualification) that is pointer-interconvertible with a, the result is a pointer to b.
Otherwise, the pointer value is unchanged by the conversion.
There is no object of type char which is pointer-interconvertible with the object a ([basic.compound]/4):
Two objects a and b are pointer-interconvertible if:
— they are the same object, or
— one is a union object and the other is a non-static data member of that object ([class.union]), or
— one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or
— there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.
which means that the static_cast<char*>(...) doesn't change the pointer value and it is the same as in its operand, namely: «pointer to a».
So, (char *)&a is a prvalue of type char* whose value is «pointer to a». This value is stored into char* ptr variable. Then, when you try to do pointer arithmetic with such a value, namely ptr + offset, you step into [expr.add]/6:
For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T and the array element type are not similar, the behavior is undefined.
For the purposes of pointer arithmetic, the object a is considered to be an element of an array A[1] ([basic.compound]/3), so the array element type is A, the type of the pointer expression P is «pointer to char», char and A are not similar types (see [conv.qual]/2), so the behavior is undefined.
This question, and the other one about launder, both seem to me to boil down to interpretation of the last sentence of C++17 [expr.static.cast]/13, which covers what happens for static_cast<T *> applied to an operand of pointer to unrelated type which is correctly aligned:
A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T ”,
[...]
Otherwise, the pointer value is unchanged by the conversion.
Some posters appear to take this to mean that the result of the cast cannot point to an object of type T, and consequently that reinterpret_cast with pointers or references can only be used on pointer-interconvertible types.
But I don't see that as justified, and (this is a reductio ad absurdum argument) that position would also imply:
The resolution to CWG1314 is overturned.
Inspecting any byte of a standard-layout object is not possible (since casting to unsigned char * or whatever character type supposedly cannot be used to access that byte).
The strict aliasing rule would be redundant since the only way to actually achieve such aliasing is to use such casts.
There would be no normative text to justify the note "[Note: Converting a prvalue of type “pointer to T1 ” to the type “pointer to T2 ” (where T1 and T2 are object types and where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value. —end note ]"
offsetof is useless (and the C++17 changes to it were therefore redundant)
It seems like a more sensible interpretation to me that this sentence means the result of the cast points to the same byte in memory as the operand. (As opposed to pointing to some other byte, as can happen for some pointer casts not covered by this sentence). Saying "the value is unchanged" does not mean "the type is unchanged", e.g. we describe conversion from int to long as preserving the value.
Also, I guess this may be controversial to some but I am taking as axiomatic that if a pointer's value is the address of an object, then the pointer points to that object, unless the Standard specifically excludes the case.
This is consistent with the text of [basic.compound]/3 which says the converse, i.e. that if a pointer points to an object, then its value is the address of the object.
There doesn't seem to be any other explicit statement defining when a pointer can or cannot be said to point to an object, but basic.compound/3 says that all pointers must be one of four cases (points to an object, points past the end, null, invalid).
Examples of excluded cases include:
The use case of std::launder specifically addresses a situation where there was such language ruling out the use of the un-laundered pointer.
A past-the-end pointer does not point to an object. (basic.compound/3)
I'm dealing with a special case where I can't use dynamic_cast directly because the object is a void*. Is using first static_cast on it and then dynamic_cast (on the result of the static_cast) bad practice? Is it wrong?
Here's an example of what I'm talking about:
MyClass* CastVoidPtr(void* pVoidPtr)
{
// casting it to MyClass so we could use dynamic_cast
MyClass* pTemp = static_cast<MyClass*>(pVoidPtr);
// returning the actual result that will make sure that we have a MyClass object or a nullptr
return dynamic_cast<MyClass*>(pTemp);
}
This would depend on how the pointer got to be a void* to begin with. If it was cast to the void* from the same type as be cast to (here MyClass*), then yes, this cast is fine and works as expected;
From cppreference on static_cast:
A prvalue of type pointer to void (possibly cv-qualified) can be converted to pointer to any type. If the value of the original pointer satisfies the alignment requirement of the target type, then the resulting pointer value is unchanged, otherwise it is unspecified. Conversion of any pointer to pointer to void and back to pointer to the original (or more cv-qualified) type preserves its original value.
Using static_cast in this way is essentially saying to the compiler "I know it is this type - trust me", and the compiler obliges.
The dynamic_cast can then be evaluated after this. It is typically used to cast to a more derived type. Here, you are casting to the same type - it is not doing anything particularly useful. If the type was a more derived type (e.g. MySpecialisedClass), then it would be fine.
As it stands, the function can be simplified to:
MyClass* CastVoidPtr(void* pVoidPtr)
{
return static_cast<MyClass*>(pVoidPtr);
}
Or to simply use a naked static_cast<>.
A side note; worth mentioning here for completeness is that reinterpret_cast has similar functionality;
Any pointer to object of type T1 can be converted to pointer to object of another type cv T2. This is exactly equivalent to static_cast<cv T2*>(static_cast<cv void*>(expression)) (which implies that if T2's alignment requirement is not stricter than T1's, the value of the pointer does not change and conversion of the resulting pointer back to its original type yields the original value). In any case, the resulting pointer may only be dereferenced safely if allowed by the type aliasing rules ...
According to strict aliasing rules:
struct B { virtual ~B() {} };
struct D : public B { };
D d;
char *c = reinterpret_cast<char*>(&d);
A char* to any object of different type is valid. But now the question is, will it point to the same address of &d? what is the guarantee made by C++ Standard that it will return the same address?
c and &d do indeed have the same value, and if you reinterpret-cast c back to a D* you get a valid pointer that you may dereference. Furthermore, you can treat c as (pointer to the first element of) an opaque array char[sizeof(D)] -- this is indeed the main purpose of casting pointers to char pointers: To allow (de)serialization (e.g. ofile.write(c, sizeof(D));), although you should generally only do this for primitive types (and arrays thereof), since the binary layout of of compound types is not generally specified in a portable fashion.
As #Oli rightly points out and would like me to reinforce, you should really never serialize compound types as a whole. The result will almost never be deserializable, since the implementation of polymorphic classes and padding between data fields is not specified and not accessible to you.
Note that reinterpret_cast<char*>(static_cast<B*>(&d)) may be treated as an opaque array char[sizeof(B)] by similar reasoning.
Section 5.2.10, point 7 of the 2003 C++ Standard says:
A pointer to an object can be explicitly converted to a pointer to an
object of different type. Except that converting an rvalue of type
“pointer to T1” to the type “pointer to T2” (where T1 and T2 are
object types and where the alignment requirements of T2 are no
stricter than those of T1) and back to its original type yields the
original pointer value, the result of such a pointer conversion is
unspecified.
If by "same address" you mean "original pointer value," then this entry says "yes."
The intent is clear (and not something that needs to be debated):
reinterpret_cast never changes the value of an address, unless the target type cannot represent all address values (like a small integer type, on a pointer type with intrinsic alignment: f.ex. a pointer that can only represent even addresses, or pointers to object and pointers to functions cannot be mixed...).
The wording of the standard fails to capture that, but that doesn't mean there is a real practical issue here.
char *c = reinterpret_cast<char*>(&d);
c will point to the first byte of d, always.