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In the C++ Without Fear: A Beginner's Guide That Makes You Feel Smart book, and in chapter (8), it mentions the following about reinterpret_cast
....converts from one pointer type (int) to another (char*). Because the cast changes the way the data pointed to is interpreted, it is called reinterpret_cast, as opposed to static_cast.*
Can you describe this paragraph here? Especially the reason for the way the operation is named?
Thanks.
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The reinterpret_cast allows the pointer to be treated as an integral type. The result is then bit-shifted and XORed with itself to produce a unique index (unique to a high degree of probability). The index is then truncated by a standard C-style cast to the return type of the function.
The reinterpret_cast operator performs potentially unsafe type casts. It is most often used to cast a pointer to a different pointer type. Casting a pointer to a different pointer and back is usually safe and yields the original value.
static_cast only allows conversions like int to float or base class pointer to derived class pointer. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert pointers to/from integers or to allow some kind of low level memory manipulation.
Anyway, the consequence of this is, that reinterpret_cast<> is portable as long as you do not rely on the byte order in any way. Your example code does not rely on byte order, it treats all bytes the same (setting them to zero), so that code is portable.
Basically, reinterpret_cast reinterprets the bit pattern at a specific location as a different type.
See for example here: http://publib.boulder.ibm.com/infocenter/lnxpcomp/v7v91/index.jsp?topic=%2Fcom.ibm.vacpp7l.doc%2Flanguage%2Fref%2Fclrc05keyword_reinterpret_cast.htm
The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument.
A static cast converts the argument instead of just reinterpreting it. You can try this out by static_casting an int to float and reinterpret_casting an int to float. The result will be totally different.
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There's nothing fancy here. it's really just intended to reinterpret something.
From standard 5.3.10, reinterpret_cast is aimed to cater the following cases:
- A pointer can be explicitly converted to any integral type large enough to hold it.
- A value of integral type or enumeration type can be explicitly converted to a pointer.
- A pointer to a function can be explicitly converted to a pointer to a function of a different type.
- A pointer to an object can be explicitly converted to a pointer to a different object type.
- Converting a pointer to a function into a pointer to an object type or vice versa is conditionally-supported.
- The null pointer value (4.10) is converted to the null pointer value of the destination type.
- A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types.
- An lvalue expression of type T1 can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_cast. That is, a reference cast reinterpret_cast < T& >(x) has the same effect as the conversion *reinterpret_cast< T* >(&x) with the built-in & and * operators (and similarly for reinterpret_cast< T&& >(x)).
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