Few doubts about casting operators in C++

Question

The reinterpret_cast as we know can cast any pointer type to any another pointer type. The question I want to ask regarding this cast operator are:

1. How does reinterpret_cast work, What is the magic(the internal implementation) that allows reinterpret_cast to work?
2. How to ensure safety when using reinterpret_cast? As far as i know, it doesn't guarantee of safe casting, So what precaution to take while using reinterpret_cast?
3. What is the practical usage of this operator. I have not really encountered it in my professional programing experience, wherein I could'nt get around without using this operator.Any practical examples apart from usual int* to char* will be highly helpful and appreciated.

One other Question regarding casting operators in general:
Casting operators(static_cast, dynamic_cast, const_cast, reinterpret_cast) are all called Operators i.e is to the best of my understanding, So is it correct statement to make that casting operators cannot be overloaded unlike most other operators (I am aware not all operators can be overloaded and I am aware of which can't be(except the Q I am asking, Please refrain flaming me on that) Just I had this doubt that since they are operators, what does the standard say about these?

Answer 1

1. There is no magic. reinterpret_cast normally just means (at least try to) treat what you find at this address as if it was the type I've specified. The standard defines little enough about what it does that it could be different from that, but it rarely (if ever) really is.
2. In a few cases, you can get safety from something like a discriminated union. For example, if you're reading network packets, and read enough to see that what you've received is a TCP packet, then you can (fairly) safely do a reinterpret_cast from IPHdr to TCPHdr (or whatever names you happen to have used). The compiler won't (again, normally) do much though -- any safety is up to you to implement and enforce.
3. I've used code like I describe in 2), dealing with different types of network packets.

For your final question: you can overload casting for a class:

class XXX { 
public:
    operator YYY() { return whatever; }
};

This can be used for conversions in general though -- whether done by a static_cast, C-style cast, or even an implicit conversion. C++0x allows you to add an explicit qualifier so it won't be used for implicit conversions, but there's still no way to differentiate between a static_cast and a C-style cast though.

Answer 2

reinterpret_cast generally lets you do some very bad things. In the case of casting a pointer it will permit casting from one type to another which has absolutely no reason to assume this should work. It's like saying "trust me I really want to do this". What exactly this does is unpredictable from one system to the next. On your system it might just copy the bit-patterns, where as on another one it could transform them in some (potentially useful) way.

e.g.

class Foo {
    int a;
};

class Bar {
    int a;
};

int main() {

  Foo a;

  // No inheritance relationship and not void* so must be reinterpret_cast 
  // if you really want to do it
  Bar *b = reinterpret_cast<Bar*>(&a);

  char buffer[sizeof(Bar)];

  Bar *c = reinterpret_cast<Bar*>(buffer); // alignment?

}

Will quite happily let you do that, no matter what the scenario. Sometimes if you're doing low-level manipulation of things this might actually be what you want to do. (Imagine char * of a buffer casting to something user defined type)

Potential pitfalls are huge, even in the simplest case like a buffer, where alignment may well be a problem.

With dlsym() on Linux it's useful to be able to cast void* to a function pointer, which is otherwise undefined behaviour in C++. (Some systems might use separate address spaces or different size pointers!). This can only be done with reinterpret_cast in C++.

Answer 3

First, it's unclear what you mean by "non-standard pointer". I think your premise is flawed. Happily it doesn't seem to affect the questions.

"How does [it] work?" Well, the intent, as you can guess from the name, is to just change the interpretation of a bitpattern, perhaps extending or shorting as appropriate. This is a kind of change of type where the bitpattern is left unchanged but the interpretation and hence conceptual value is changed. And it's in contrast to a kind of change of type where the conceptual value is kept (e.g. int converted to double) while the bitpattern is changed as necessary to keep the conceptual value. But most cases of reinterpret_cast have implementation defined effect, so for those cases your compiler can do whatever it wants -- not necessarily keeping the bitpattern -- as long as it is documented.

"How to ensure safety" That is about knowing what your compiler does, and about avoiding reinterpret_cast. :-)

"What is the practical usage". Mostly it is about recovering type information that's been lost in C-oriented code where void* pointers are used to sort of emulate polymorphism.

Cheers & hth.,

Answer 4

1. reinterpret_cast only works on pointers. The way it works is that it leaves the value of the pointer alone and changes the assumed type information about it. It says, "I know these types are not equivalent, but I want you to just pretend this is now a pointer to T2." Of course, this can cause any number of problems if you use the T2 pointer and it does not point to a T2.

2. There are very few guarantees about reinterpret_cast, which is why it is to be so avoided. You're really only allowed to cast from T1 to T2 and then back to T1 and know that, given some assumptions, that the final result will be the same as what you started with.

3. The only one I can think of is casting a char* to an unsigned char*. I know that the underlying representation is the same in my implementation so I know the cast is safe. I can't use a static cast though because it's a pointer to a buffer. In reality, you'll find very little legitimate use of reinterpret_cast in the real world.

Yes, they are operators. AFAIK you can't override them.

Answer 5

One "practical" use of reinterpret_cast.

I have a class where the members are not meant to be read. Example below

class ClassWithHiddenVariables
{
private:
    int a;
    double m;
public:
    void SetVariables(int s, int d)
    {
        a = s;
        m = d;
    }
}; 

This class is used in a thousand places in an application without a problem.

Now, because of some reason I want see the members in one specific part. However, I don't want to touch the existing class.So break the rules as follows.

Create another class with the same bit pattern and public visibility. Here the original class contains an int and double.

class ExposeAnotherClass
{
public:
    int a_exposed;
    double m_exposed;
};

When you want to see members of the ClassWithHiddenVariables object, use reinterpret_cast to cast to ExposeAnotherClass. Example follows

ClassWithHiddenVariables obj;
obj.SetVariables(10, 20.02);    
ExposeAnotherClass *ptrExposedClass;
ptrExposedClass = reinterpret_cast<ExposeAnotherClass*>(&obj);  
cout<<ptrExposedClass->a_exposed<<"\n"<<ptrExposedClass->m_exposed;

I don't think this situation ever occurs in real world. But this is just an explanation of reinterpret_cast which considers objects as bit patterns.

Answer 6

reinterpret_cast tells the compiler "shut up, it's a variable of type T*" and there's no safety unless it is really a variable of type T*. On most implementations just nothing is done - the same value in the variable is passed to the destination.

Your class can have conversion operators to any type T* and those conversions will either be invokde implicitly under certain conditions or you can invoke them explicitly using static_cast.

Answer 7

I've used reinterpret_cast a lot in Windows programming. Message handling uses WPARAM and LPARAM parameters that need casting to the correct types.

Answer 8

The reinterpret_cast as we know can cast any non-standard pointer to another non-standard pointer.

Almost, but not exactly. For example, you can't use reinterpret_cast to cast a const int* to an int*. For that, you need const_cast.

How does reinterpret_cast work, What is the magic(the internal implementation) that allows reinterpret_cast to work?

There's no magic at all. Ultimately, all data is just bytes. The C++ type system is merely an abstraction layer which tells the compiler how to "interpret" each byte. A reinterpret_cast is similar to a plain C-cast, in that it simply says "to hell with the type system: interpret these bytes as type X instead of type Y!"

How to ensure safety when using reinterpret_cast? As far as i know, it doesn't guarantee of safe casting, So what precaution to take while using reinterpret_cast?

Well, reinterpret_cast is inherently dangerous. You shouldn't use it unless you really know what you're doing. Try to use static_cast instead. The C++ type system will protect you from doing anything too dangerous if you use static_cast.

What is the practical usage of this operator. I have not really encountered it in my professional programing experience, wherein I could'nt get around without using this operator.Any practical examples apart from usual int* to char* will be highly helpful and appreciated.

It has many uses, but usually these uses are somewhat "advanced". For example, if you are creating a memory pool of linked blocks, and storing pointers to free blocks on the blocks themselves, you'll need to reinterpret_cast a block from a T* to a T** to interpret the block as a pointer to the next block, rather than a block itself.

Answer 9

reinterpret_cast is pretty equivalent to a C-style cast. It doesn't guarentee anything; it's there to allow you to do what you need to, in the hopes that you know what you're doing.

If you're looking to ensure safety, use dynamic_cast, as that's what it does. If the cast cannot be completed safely, dynamic_cast returns NULL or nullptr (C++0x).

Casting using the "casting operators" such as static_cast, dynamic_cast, etc.. cannot be overloaded. Straight conversions can, such as:

class Degrees
{
public:
    operator double() { ... }
};