The reinterpret_cast operator (C++ only)

A reinterpret_cast operator handles conversions between unrelated types.

reinterpret_cast operator syntax

1  reinterpret_cast < Type > ( expression )

C++11With the right angle bracket feature, you may specify a template_id as Type in the reinterpret_cast operator with the >> token in place of two consecutive > tokens. For details, see Class templates (C++ only).C++11

The result of reinterpret_cast<Type>(expression) belongs to one of the following value categories:
  • If Type is an lvalue reference type C++11or an rvalue reference to a function typeC++11, reinterpret_cast<Type>(expression) is an lvalue.
  • C++11If Type is an rvalue reference to an object type, reinterpret_cast<Type>(expression) is an xvalue.C++11
  • In all other cases, reinterpret_cast<Type>(expression) is a C++11(prvalue)C++11 rvalue.
The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument. You cannot cast away a const or volatile qualification. You can explicitly perform the following conversions:
  • A pointer to any integral type large enough to hold it
  • A value of integral or enumeration type to a pointer
  • A pointer to a function to a pointer to a function of a different type
  • A pointer to an object to a pointer to an object of a different type
  • A pointer to a member to a pointer to a member of a different class or type, if the types of the members are both function types or object types

A null pointer value is converted to the null pointer value of the destination type.

Given a type T and an lvalue expression x, the following two expressions for lvalue references have different syntax but the same semantics:
  • reinterpret_cast<T&>(x)
  • *reinterpret_cast<T*>(&(x))
C++11Given a type T and an lvalue expression x, the following two expressions for rvalue references have different syntax but the same semantics:
  • reinterpret_cast<T&&>(x)
  • static_cast<T&&>(*reinterpret_cast<T*>(&(x)))

Reinterpreting one type of pointer as an incompatible type of pointer is usually invalid. The reinterpret_cast operator, as well as the other named cast operators, is more easily spotted than C-style casts, and highlights the paradox of a strongly typed language that allows explicit casts.

The C++ compiler detects and quietly fixes most but not all violations. It is important to remember that even though a program compiles, its source code may not be completely correct. On some platforms, performance optimizations are predicated on strict adherence to standard aliasing rules. Although the C++ compiler tries to help with type-based aliasing violations, it cannot detect all possible cases.

The following example violates the aliasing rule, but executes as expected when compiled unoptimized in C++ or in K&R C or with NOANSIALIAS. It also successfully compiles optimized in C++ with ANSIALIAS, but does not necessarily execute as expected. The offending line 7 causes an old or uninitialized value for x to be printed.

1  extern int y = 7.;
3  int main() {
4       float x;
5       int i;
6       x = y;
7       i = *(int *) &x;
8       printf("i=%d. x=%f.\n", i, x);
9  }

The next code example contains an incorrect cast that the compiler cannot even detect because the cast is across two different files.

1  /* separately compiled file 1 */
2      extern float f;
3      extern int * int_pointer_to_f = (int *) &f; /* suspicious cast */
5  /* separately compiled file 2 */
6      extern float f;
7      extern int * int_pointer_to_f;
8      f = 1.0;
9      int i = *int_pointer_to_f;           /* no suspicious cast but wrong */
In line 8, there is no way for the compiler to know that f = 1.0 is storing into the same object that int i = *int_pointer_to_f is loading from.