reinterpret_cast conversion

From cppreference.com
< cpp‎ | language
 
C++
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
decltype (C++11)
auto (C++11)
alignas (C++11)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
Classes
Class-specific function properties
explicit (C++11)
static
Special member functions
Templates
Miscellaneous
 
Expressions
General
value categories (lvalue, rvalue, xvalue)
order of evaluation (sequence points)
constant expressions
unevaluated expressions
primary expressions
lambda-expression(C++11)
Literals
integer literals
floating-point literals
boolean literals
character literals including escape sequences
string literals
null pointer literal(C++11)
user-defined literal(C++11)
Operators
Assignment operators: a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b
Increment and decrement: ++a, --a, a++, a--
Arithmetic operators: +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b
Logical operators: a||b, a&&b, !a
Comparison operators: a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b(C++20)
Member access operators: a[b], *a, &a, a->b, a.b, a->*b, a.*b
Other operators: a(...), a,b, a?b:c
new-expression
delete-expression
throw-expression
alignof
sizeof
sizeof...(C++11)
typeid
noexcept(C++11)
Fold expression(C++17)
Alternative representations of operators
Precedence and associativity
Operator overloading
Default comparisons(C++20)
Conversions
Implicit conversions
const_cast
static_cast
reinterpret_cast
dynamic_cast
Explicit conversions (T)a, T(a)
User-defined conversion
 

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 (except when converting between integers and pointers or on obscure architectures where pointer representation depends on its type). It is purely a compile-time directive which instructs the compiler to treat 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.

1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. The resulting value is the same as the value of expression.
2) A pointer can be converted to any integral type large enough to hold all values of its type (e.g. to std::uintptr_t)
3) A value of any integral or enumeration type can be converted to a pointer type. A pointer converted to an integer of sufficient size and back to the same pointer type is guaranteed to have its original value, otherwise the resulting pointer cannot be dereferenced safely (the round-trip conversion in the opposite direction is not guaranteed; the same pointer may have multiple integer representations) The null pointer constant 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.
4) Any value of type 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)
5) Any object pointer type T1* can be converted to another object pointer 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)
6) An lvalue (until C++11)glvalue (since C++11) expression of type T1 can be converted to reference to another type T2. The result is that of *reinterpret_cast<T2*>(p), where p is a pointer of type “pointer to T1” to the object designated by expression. 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)
7) Any pointer to function can be converted to a pointer to a different function type. Calling the function through a pointer to a different function type is undefined, but converting such pointer back to pointer to the original function type yields the pointer to the original function.
8) On some implementations (in particular, on any POSIX compatible system as required by dlsym), a function pointer can be converted to void* or any other 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.
9) The null pointer value of any pointer type can be converted to any other pointer type, resulting in the null pointer value of that type. Note that the null pointer constant 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.
10) A pointer to member function can be converted to pointer to a different member function of a different type. Conversion back to the original type yields the original value, otherwise the resulting pointer cannot be used safely.
11) A pointer to member object of some class 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 back to the original type T1 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 a reference type;
  • an rvalue otherwise.
(until C++11)
  • 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.
(since C++11)

Keywords

reinterpret_cast

Type aliasing

Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type AliasedType, the behavior is undefined unless one of the following is true:

  • AliasedType and DynamicType are similar.
  • AliasedType is the (possibly cv-qualified) signed or unsigned variant of DynamicType.
  • AliasedType is std::byte, (since C++17) char, or unsigned char: this permits examination of the object representation of any object as an array of bytes.

Informally, two types are similar if, ignoring top-level cv-qualification:

  • they are the same type; or
  • they are both pointers, and the pointed-to types are similar; or
  • they are both pointers to member of the same class, and the types of the pointed-to members are similar; or
  • they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar.
(until C++20)
  • they are both arrays of the same size or at least one of them is array of unknown bound, and the array element types are similar.
(since C++20)

For example:

  • const int * volatile * and int * * const are similar;
  • const int (* volatile S::* const)[20] and int (* const S::* volatile)[20] are similar;
  • int (* const *)(int *) and int (* volatile *)(int *) are similar;
  • int (S::*)() const and int (S::*)() are not similar;
  • int (*)(int *) and int (*)(const int *) are not similar;
  • const int (*)(int *) and int (*)(int *) are not similar;
  • int (*)(int * const) and int (*)(int *) are similar (they are the same type);
  • std::pair<int, int> and std::pair<const int, int> are not similar.

This rule enables type-based alias analysis, in which a compiler assumes that the value read through a glvalue of one type is not modified by a write to a glvalue of a different type (subject to the exceptions noted above).

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).

Notes

Assuming that alignment requirements are met, a reinterpret_cast does not change the value of a pointer outside of a few limited cases dealing with pointer-interconvertible objects: 

struct S1 { int a; } s1;
struct S2 { int a; private: int b; } s2; // not standard-layout
union U { int a; double b; } u = {0};
int arr[2];
 
int* p1 = reinterpret_cast<int*>(&s1); // value of p1 is "pointer to s1.a" because
                                       // s1.a and s1 are pointer-interconvertible
 
int* p2 = reinterpret_cast<int*>(&s2); // value of p2 is unchanged by reinterpret_cast
                                       // and is "pointer to s2". 
 
int* p3 = reinterpret_cast<int*>(&u);  // value of p3 is "pointer to u.a":
                                       // u.a and u are pointer-interconvertible
 
double* p4 = reinterpret_cast<double*>(p3); // value of p4 is "pointer to u.b": u.a and
                                            // u.b are pointer-interconvertible because
                                            // both are pointer-interconvertible with u
 
int* p5 = reinterpret_cast<int*>(&arr); // value of p5 is unchanged by reinterpret_cast
                                        // and is "pointer to arr"

Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through a reinterpret_cast - results in undefined behavior: 

struct S { int x; };
struct T { int x; int f(); };
struct S1 : S {};    // standard-layout
struct ST : S, T {}; // not standard-layout
 
S s = {};
auto p = reinterpret_cast<T*>(&s); // value of p is "pointer to s"
auto i = p->x; // class member access expression is undefined behavior;
               // s is not a T object
p->x = 1; // undefined behavior
p->f();   // undefined behavior
 
S1 s1 = {};
auto p1 = reinterpret_cast<S*>(&s1); // value of p1 is "pointer to the S subobject of s1"
auto i = p1->x; // OK
p1->x = 1;      // OK
 
ST st = {};
auto p2 = reinterpret_cast<S*>(&st); // value of p2 is "pointer to st"
auto i = p2->x; // undefined behavior
p2->x = 1;      // undefined behavior

Many compilers issue "strict aliasing" warnings in such cases, even though technically such constructs run afoul of something other than the paragraph commonly known as the "strict aliasing rule".

The purpose of strict aliasing and related rules is to enable type-based alias analysis, which would be decimated if a program can validly create a situation where two pointers to unrelated types (e.g., an int* and a float*) could simultaneously exist and both can be used to load or store the same memory (see this email on SG12 reflector). Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior.

When it is needed to interpret the bytes of an object as a value of a different type, std::memcpy or std::bit_cast (since C++20)can be used: 

double d = 0.1;
std::int64_t n;
static_assert(sizeof n == sizeof d);
// n = *reinterpret_cast<std::int64_t*>(&d); // Undefined behavior
std::memcpy(&n, &d, sizeof d);               // OK
n = std::bit_cast<std::int64_t>(d);          // also OK

If the implementation provides std::intptr_t and/or std::uintptr_t, then a cast from a pointer to an object type or cv void to these types is always well-defined. However, this is not guaranteed for a function pointer.

(since C++11)

The paragraph defining the strict aliasing rule in the standard used to contain two additional bullets partially inherited from C:

  • 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).
  • AliasedType is a (possibly cv-qualified) base class of DynamicType.

These bullets describe situations that cannot arise in C++ and therefore are omitted from the discussion above. In C, aggregate copy and assignment access the aggregate object as a whole. But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., via unsigned char). These bullets were eventually removed via CWG2051.

Example

Demonstrates some uses of reinterpret_cast:

Run this code
#include <cstdint>
#include <cassert>
#include <iostream>
 
int f() { return 42; }
 
int main()
{
    int i = 7;
 
    // pointer to integer and back
    std::uintptr_t v1 = reinterpret_cast<std::uintptr_t>(&i); // static_cast is an error
    std::cout << "The value of &i is " << std::showbase << 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);
    std::cout << (p2[0] == '\x7' ? "This system is little-endian\n"
                                 : "This system is big-endian\n");
 
    // type aliasing through reference
    reinterpret_cast<unsigned int&>(i) = 42;
    std::cout << i << '\n';
 
    [[maybe_unused]] const int &const_iref = i;
    // int &iref = reinterpret_cast<int&>(
    //     const_iref); // compiler error - can't get rid of const
    // Must use const_cast instead: int &iref = const_cast<int&>(const_iref);
}

Possible output: 

The value of &i is 0x7fff352c3580
42
This system is little-endian
42

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 195 C++98 conversion between function pointers
and object pointers not allowed 		made conditionally-supported
CWG 658 C++98 the result of pointer conversions was unspecified
(except for conversions back to the original type) 		specification provided for pointers
whose pointed-to types satisfy
the alignment requirements
CWG 799 C++98 it was unclear which identity conversion
can be done by reinterpret_cast 		made clear
CWG 1268 C++11 reinterpret_cast could only cast lvalues to reference types xvalues also allowed

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
(C++20)
 reinterpret the object representation of one type as that of another
(function template)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/reinterpret_cast&oldid=140728"