reinterpret_cast Operator

 

The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.

The latest version of this topic can be found at reinterpret_cast Operator.

Allows any pointer to be converted into any other pointer type. Also allows any integral type to be converted into any pointer type and vice versa.

reinterpret_cast < type-id > ( expression )  

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.

#include <iostream>  
using namespace std;  
  
// Returns a hash code based on an address  
unsigned short Hash( void *p ) {  
   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;  
}  
  
Output:   
64641  
64645  
64889  
64893  
64881  
64885  
64873  
64877  
64865  
64869  
64857  
64861  
64849  
64853  
64841  
64845  
64833  
64837  
64825  
64829  

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.

Casting Operators
Keywords

Show: