Const POD Default Constructor

[ax3l]

Fixes clang(-tidy) errors of the kind [clang-diagnostic-error]:
"default initialization of an object of const type '...' without a user-provided default constructor":

First approach before discussion:

const typename Difference::template GetDifference<0> Dx;
// to
const typename Difference::template GetDifference<0> Dx{};

Final approach: add explicit constructors to POD classes that are used as const:

    struct GetDifference
    {
        static constexpr uint32_t direction = T_direction;

        HDINLINE GetDifference()
        {
        }
        // ...
    };

[psychocoderHPC]

Based on this stackoverflow post we should add a explicit default constructor to GetDifference, the reult of traits::FieldPositionand ShiftMeIfYouCan.
The reason is that a type without a constructor is a POD data type and can be uninitialized. An uninitialized const variable make no sense. With the current PR we defining from the outside that we a fine with that uninitialized type but this requires internal knowledge about the implementation.

We should add a default constructor e.g. GetDifference() = default;

[ax3l]

thanks!

I thinks it's actually exactly the opposite implementation wise, instead of explicitly not adding a constructor in the current class with = default; we just add one with GetDifference(){} instead. (As the SO answer references, this is weird for a class with only static constexpr members but currently part of the C++ standard...)

the problem seems to be, that POD classes do not get a default constructor in C++ and a const POD without constructor makes no sense as it will always stay uninitialized (also, the D in POD is thought to store data...)

[psychocoderHPC]

In c++11 it should be FieldPosition = default; Please to not add a prefix qualifier else we will get nvcc or clang warnings.

[psychocoderHPC]

Ohh missread your comment. = default is equal to the old c++98 way.
Whe can check it in a mini ap with std::is_pod.

[psychocoderHPC]

offline discussed: = default is not equal to a hand written empty constructor :-(

[ax3l]

I tried but = default did not work :( See link above

[ax3l]

Observed today: this is a clang 3.8 weirdness and gone in 3.9+:

Has problems to compile with the error message above:

#include <string>
#include <iostream>

constexpr float f = 4.5;

struct A
{
    static constexpr float value = f;

    float
    operator()() const
    {
        return f;
    }
};

int main()
{
    A const a;
    std::cout << a() << std::endl;

    return 0;
}

We should just require clang 3.9+ (also supported in CUDA 8.0+)

[sbastrakov]

Yuck! Even a trivial example with const object + automatic default constructor does not work in clang 3.8.

[ax3l]

Yep, also tried it online: https://godbolt.org/g/sfhPXz

[sbastrakov]

Omg, it was actually a standard-conforming behaviour, which then was marked as a defect: http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253

[ax3l]

omg -.-