Difference between revisions of "cpp/language/reinterpret cast"

Revision as of 18:51, 12 February 2023

Converts between types by reinterpreting the underlying bit pattern.

Syntax

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.

As with all cast expressions, the result is:

• an lvalue if target-type is an lvalue reference type or an rvalue reference to function type(since C++11);

• a prvalue otherwise.

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

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

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 CWG issue 2051.

Example

#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);
}

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.

See also