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reinterpret_cast Operator

Grammar

postfix-expression:
reinterpret_cast < type-id > ( expression )

The reinterpret_cast operator allows any pointer to be converted into any other pointer type. It also allows any integral type to be converted into any pointer type and vice versa. Misuse of the reinterpret_cast operator can easily be unsafe. Unless the desired conversion is inherently low-level, you should use one of the other cast operators.

The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe.

The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Other uses are, at best, nonportable.

The reinterpret_cast operator cannot cast away the const, volatile, or __unaligned attributes. See const_cast Operator for information on removing these attributes.

The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type.

One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index.

// expre_reinterpret_cast_Operator.cpp
// compile with: /EHsc
#include <iostream>

unsigned short Hash( void *p )
// Returns a hash code based on an address
{
   unsigned int val = reinterpret_cast<unsigned int>( p );
   return ( unsigned short )( val ^ (val >> 16));
}

using namespace std;
int main()
{
   int a[20];
   for ( int i = 0; i < 20; i++ )
      cout << Hash( a + i ) << endl;
}

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.

See Also

Casting Operators | C++ Keywords

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