reinterpret_cast conversion

< cpp‎ | language

Converts between types by reinterpreting the underlying bit pattern.


[edit] Syntax

reinterpret_cast < new_type > ( expression )

Returns a value of type new_type.

[edit] 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.

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. (since C++11)
2) Any pointer can be converted to any integral type large enough to hold the value of the pointer (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 pointer to object of type 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)
6) An lvalue expression of type 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)
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) An rvalue pointer to member function can be converted to pointer to a different member function of a different type. Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely.
11) An rvalue 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 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.

[edit] Keywords


[edit] 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, after stripping away cv-qualifications at every level (but excluding anything inside a function type), they are the same type. 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).

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 an 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).

[edit] Notes

The paragraph defining the strict aliasing rule in the standard contains 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). See core issue 2051.

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 = p1->x; // undefined behavior
p1->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 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

[edit] 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

[edit] 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";
        std::cout << "This system is big-endian\n";
    // type aliasing through reference
    reinterpret_cast<unsigned int&>(i) = 42;
    std::cout << i << '\n';
    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
This system is little-endian

[edit] See also

const_cast conversion adds or removes const[edit]
static_cast conversion performs basic conversions[edit]
dynamic_cast conversion performs checked polymorphic conversions[edit]
explicit casts permissive conversions between types [edit]
standard conversions implicit conversions from one type to another[edit]