reinterpret_cast conversion

 

C++

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Technical Specifications

 

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reinterpret_cast

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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. 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 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.

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.

Keywords

reinterpret_cast

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 type T (since C++11)

• AliasedType is the (possibly cv-qualified) signed or unsigned variant of DynamicType

• 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 of DynamicType

• AliasedType is char or unsigned 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:

Run this code

#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