C++ Type punning conversion


A pointer (resp. reference) to an object type can be converted to a pointer (resp. reference) to any other object type using reinterpret_cast. This does not call any constructors or conversion functions.

int x = 42;
char* p = static_cast<char*>(&x);      // error: static_cast cannot perform this conversion
char* p = reinterpret_cast<char*>(&x); // OK
*p = 'z';                              // maybe this modifies x (see below)

The result of reinterpret_cast represents the same address as the operand, provided that the address is appropriately aligned for the destination type. Otherwise, the result is unspecified.

int x = 42;
char& r = reinterpret_cast<char&>(x);
const void* px = &x;
const void* pr = &r;
assert(px == pr); // should never fire

The result of reinterpret_cast is unspecified, except that a pointer (resp. reference) will survive a round trip from the source type to the destination type and back, as long as the destination type's alignment requirement is not stricter than that of the source type.

int x = 123;
unsigned int& r1 = reinterpret_cast<unsigned int&>(x);
int& r2 = reinterpret_cast<int&>(r1);
r2 = 456; // sets x to 456

On most implementations, reinterpret_cast does not change the address, but this requirement was not standardized until C++11.

reinterpret_cast can also be used to convert from one pointer-to-data-member type to another, or one pointer-to-member-function type to another.

Use of reinterpret_cast is considered dangerous because reading or writing through a pointer or reference obtained using reinterpret_cast may trigger undefined behaviour when the source and destination types are unrelated.