Difference between revisions of "cpp/utility/launder"

Revision as of 08:49, 3 December 2021

Obtains a pointer to the object located at the address represented by p.

Formally, given

• the pointer p represents the address A of a byte in memory
• an object X is located at the address A
• X is within its lifetime
• the type of X is the same as T, ignoring cv-qualifiers at every level
• every byte that would be reachable through the result is reachable through p (bytes are reachable through a pointer that points to an object Y if those bytes are within the storage of an object Z that is pointer-interconvertible with Y, or within the immediately enclosing array of which Z is an element).

Then std::launder(p) returns a value of type T* that points to the object X. Otherwise, the behavior is undefined.

The program is ill-formed if T is a function type or (possibly cv-qualified) void.

std::launder may be used in a core constant expression if and only if the (converted) value of its argument may be used in place of the function invocation. In other words, std::launder does not relax restrictions in constant evaluation.

Notes

std::launder has no effect on its argument. Its return value must be used to access the object. Thus, it's always an error to discard the return value.

Typical uses of std::launder include:

• Obtaining a pointer to an object created in the storage of an existing object of the same type, where pointers to the old object cannot be reused (for instance, because either object is a base class subobject);
• Obtaining a pointer to an object created by placement new from a pointer to an object providing storage for that object.

The reachability restriction ensures that std::launder cannot be used to access bytes not accessible through the original pointer, thereby interfering with the compiler's escape analysis.

int x[10];
auto p = std::launder(reinterpret_cast<int(*)[10]>(&x[0])); // OK
 
int x2[2][10];
auto p2 = std::launder(reinterpret_cast<int(*)[10]>(&x2[0][0])); 
// Undefined behavior: x2[1] would be reachable through the resulting pointer to x2[0]
// but is not reachable from the source
 
struct X { int a[10]; } x3, x4[2]; // standard layout; assume no padding
auto p3 = std::launder(reinterpret_cast<int(*)[10]>(&x3.a[0])); // OK
auto p4 = std::launder(reinterpret_cast<int(*)[10]>(&x4[0].a[0])); 
// Undefined behavior: x4[1] would be reachable through the resulting pointer to x4[0].a 
// (which is pointer-interconvertible with x4[0]) but is not reachable from the source 
 
struct Y { int a[10]; double y; } x5;
auto p5 = std::launder(reinterpret_cast<int(*)[10]>(&x5.a[0])); 
// Undefined behavior: x5.y would be reachable through the resulting pointer to x5.a
// but is not reachable from the source

Example

#include <new>
#include <cstddef>
#include <cassert>
 
struct X {
  const int n; // note: X has a const member
  int m;
};
 
struct Y {
  int z;
};
 
struct A { 
    virtual int transmogrify();
};
 
struct B : A {
    int transmogrify() override { new(this) A; return 2; }
};
 
int A::transmogrify() { new(this) B; return 1; }
 
static_assert(sizeof(B) == sizeof(A));
 
int main()
{
  // Case 1: the new object failed to be transparently replaceable because it is a
  // base subobject but the old object is a complete object.
  A i;
  int n = i.transmogrify();
  // int m = i.transmogrify(); // undefined behavior
  int m = std::launder(&i)->transmogrify(); // OK
  assert(m + n == 3);
 
  // Case 2: access to a new object whose storage is provided by a byte array through
  // a pointer to the array.
  alignas(Y) std::byte s[sizeof(Y)];
  Y* q = new(&s) Y{2};
  const int f = reinterpret_cast<Y*>(&s)->z; // Class member access is undefined behavior:
                                             // reinterpret_cast<Y*>(&s) has value
                                             // "pointer to s" and does not
                                             // point to a Y object 
  const int g = q->z; // OK
  const int h = std::launder(reinterpret_cast<Y*>(&s))->z; // OK
 
  [f, g, h] {}; // suppresses "unused-variable" warnings; see also [[maybe_unused]].
}

Defect reports

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