half.h default user-provided constructor breaks c++ semantics (value/zero initialization vs default initialization)

[geotz]

The user-provided default constructor for half in half.h does not respect value/zero initialization which makes the type awkward and/or buggy to work with, especially in generic template code that works with other number types (e.g. floats).

Expected behavior (as with float):

using T = float;
std::vector<T> a(5);	// initialized with 0’s
T b[5];			// uninitialized 
T c[5] = {};		// initialized with 0’s
T d;			// uninitialized

Current behavior (half):

using T = half;
std::vector<T> a(5);	// uninitialized
T b[5];			// uninitialized 
T c[5] = {}; 		// uninitialized
T d; 			// uninitialized

Proposed solution (C++11)
In class definition (half.h) define the default constructor as an explicitly defaulted one:

class half
{
  public:

    //-------------
    // Constructors
    //-------------

    half () = default;
// ...

in order to correctly perform value initialization or default initialization (which actually does nothing). The unsigned short member's 0 value coincides with 16-bit floating point value 0. With the proposed solution, we have the same behavior as with floats.

[meshula]

This seems reasonable.

[milasudril]

I would be happy to see this fix merged soon. This will also make the following compile:

static_assert(std::is_trivially_default_constructible_v<half>);