PSA: Value-initialization is not merely default-construction

At the Varna WG21 meeting, Giuseppe D’Angelo presented his P2782 “Type trait to detect if value initialization can be achieved by zero-filling”. The intent of this new type trait is to tell vector’s implementor whether

std::vector<T> v;

can just do a memset of all the T objects to put them into their correctly constructed state. By “correctly constructed state,” I mean the state a T would be in after

::new (p) T(); // value-initialization

The syntax T() causes a kind of initialization called value-initialization. Here’s a public service announcement (not for Giuseppe, who knows; but perhaps for you, dear reader; and probably for me again six months from now) —

Value-initialization is not merely default-construction!

Value-initialization for a type with a defaulted default constructor does zero-initialization first, then calls the default constructor. The effects are most visible for primitive scalar types:

int i;          // default-initialized, garbage value
int j = int();  // value-initialized, always zero

int is both trivially default-constructible (because you can default-initialize an int by default-initializing each of the unsigned chars in its object representation to garbage), and trivially value-initializable (because you can value-initialize an int by value-initializing each of the unsigned chars in its object representation to zero).

Yet “trivially default-constructible” and “trivially value-initializable” are 100% orthogonal properties. Here’s a constructive proof:

struct T;
struct S1 {
    explicit S1() = default;
    int T::*mf;
struct S2 {
    explicit S2() {}
    int T::*mf;

Struct S1 is trivially default-constructible, but it is not trivially value-initializable. Struct S2 is trivially value-initializable, but it is not trivially default-constructible.

Lacking a compiler builtin to query trivial value-initializability, we prove our assertion empirically by writing a little function and seeing how the compiler optimizes it.

S1 f1() { return S1(); }
S2 f2() { return S2(); }

clang++ -O2 produces:

  movq $-1, %rax

The object representation of a value-initialized S1() is all-bits-one, but the object representation of a value-initialized S2() is not just all-bits-zero — it’s actually indeterminate! We might call that “even better than trivial.”

To focus on “better than triviality” would be a mistake: only contrived types like S2 partake of it. In fact we would expect __is_trivially_value_initializable(S2) to report false in practice, because S2’s default constructor is user-provided and thus not readily inspectable by the front-end. Its triviality is discovered only by the optimizer.

Posted 2023-06-22