Converts between types by reinterpreting the underlying bit pattern.
Syntax
reinterpret_cast < new_type > ( expression ) |
Returns a value of type new_type
.
Explanation
Unlike static_cast
, but like const_cast
, the reinterpret_cast
expression does not compile to any CPU instructions. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.
Only the following conversions can be done with reinterpret_cast
, except when such conversions would cast away constness or volatility.
expression
. (since C++11) std::uintptr_t
)NULL
or integer zero is not guaranteed to yield the null pointer value of the target type; static_cast or implicit conversion should be used for this purpose.std::nullptr_t
, including nullptr
can be converted to any integral type as if it were (void*)0
, but no value, not even nullptr
can be converted to std::nullptr_t
: static_cast
should be used for that purpose. (since C++11) T1
can be converted to pointer to object of another type cv T2
. This is exactly equivalent to static_cast<cv T2*>(static_cast<cv void*>(expression))
(which implies that if T2
's alignment requirement is not stricter than T1
's, the value of the pointer does not change and conversion of the resulting pointer back to its original type yields the original value). In any case, the resulting pointer may only be dereferenced safely if allowed by the type aliasing rules (see below) T1
can be converted to reference to another type T2
. The result is an lvalue or xvalue referring to the same object as the original lvalue, but with a different type. No temporary is created, no copy is made, no constructors or conversion functions are called. The resulting reference can only be accessed safely if allowed by the type aliasing rules (see below)void*
or any another object pointer, or vice versa. If the implementation supports conversion in both directions, conversion to the original type yields the original value, otherwise the resulting pointer cannot be dereferenced or called safely.nullptr
or any other value of type std::nullptr_t
cannot be converted to a pointer with reinterpret_cast
: implicit conversion or static_cast
should be used for this purpose.T1
can be converted to a pointer to another member object of another class T2
. If T2
's alignment is not stricter than T1
's, conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely.As with all cast expressions, the result is:
- an lvalue if new_type is an lvalue reference type or an rvalue reference to function type;
- an xvalue if new_type is an rvalue reference to object type;
- a prvalue otherwise.
Keywords
Type aliasing
When a pointer or reference to object whose dynamic type is DynamicType
is reinterpret_cast
(or C-style cast) to a pointer or reference to object of a different type AliasedType
, the cast always succeeds, but the resulting pointer or reference may only be used to access the object if both DynamicType
and AliasedType
are standard-layout types and one of the following is true:
-
AliasedType
is the (possibly cv-qualified)DynamicType
-
AliasedType
and {DynamicType
are both (possibly multi-level, possibly cv-qualified at each level) pointers to the same typeT
(since C++11) -
AliasedType
is the (possibly cv-qualified) signed or unsigned variant ofDynamicType
-
AliasedType
is an aggregate type or a union type which holds one of the aforementioned types as an element or non-static member (including, recursively, elements of subaggregates and non-static data members of the contained unions): this makes it safe to obtain a usable pointer to a struct or union given a pointer to its non-static member or element. -
AliasedType
is a (possibly cv-qualified) base class ofDynamicType
-
AliasedType
ischar
orunsigned char
: this permits examination of the object representation of any object as an array of unsigned char.
If AliasedType
does not satisfy these requirements, accessing the object through the new pointer or reference invokes undefined behavior. This is known as the strict aliasing rule and applies to both C++ and C programming languages.
Note that many C++ compilers relax this rule, as a non-standard language extension, to allow wrong-type access through the inactive member of a union (such access is not undefined in C).
Also note that this set of rules is more strict than the equivalent rules in the C programming language: C allows access through a pointer to any compatible type. C++ has no compatible types and does not allow access through a pointer or reference to a layout-compatible type if it doesn't satisfy any of the rules listed above (although access to its member may be allowed).
Notes
C++98 did not allow conversion between function pointers and void*
, this was corrected by DR CWG195.
Example
Demonstrates some uses of reinterpret_cast:
#include <cstdint> #include <cassert> #include <iostream> int f() { return 42; } int main() { int i = 7; // pointer to integer and back uintptr_t v1 = reinterpret_cast<uintptr_t>(&i); // static_cast is an error std::cout << "The value of &i is 0x" << std::hex << v1 << '\n'; int* p1 = reinterpret_cast<int*>(v1); assert(p1 == &i); // pointer to function to another and back void(*fp1)() = reinterpret_cast<void(*)()>(f); // fp1(); undefined behavior int(*fp2)() = reinterpret_cast<int(*)()>(fp1); std::cout << std::dec << fp2() << '\n'; // safe // type aliasing through pointer char* p2 = reinterpret_cast<char*>(&i); if(p2[0] == '\x7') std::cout << "This system is little-endian\n"; else std::cout << "This system is big-endian\n"; // type aliasing through reference reinterpret_cast<unsigned int&>(i) = 42; std::cout << i << '\n'; }
Possible output:
The value of &i is 0x7fff352c3580 42 This system is little-endian 42
See also
const_cast conversion | adds or removes const |
static_cast conversion | performs basic conversions |
dynamic_cast conversion | performs checked polymorphic conversions |
explicit casts | permissive conversions between types |
standard conversions | implicit conversions from one type to another |
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