Square bracket operator for xgenerator not working

[martinRenou]

The square bracket operator for xgenerator is not working like other implementations (for example xarray), the following code:

auto a = xt::eye<int>(9);
std::cout << "a (xgenerator): ";
for (std::size_t i = 0; i < 9; ++i)
{
    std::cout << a[i] << ", ";
}
std::cout << std::endl;

auto b = xt::xarray<int>({{1, 0, 0},
                          {0, 1, 0},
                          {0, 0, 1}});
std::cout << "b (xarray): ";
for (std::size_t i = 0; i < 9; ++i)
{
    std::cout << b[i] << ", ";
}
std::cout << std::endl;

outputs:

a (xgenerator): 1, 0, 0, 0, 0, 0, 0, 0, 0, 
b (xarray): 1, 0, 0, 0, 1, 0, 0, 0, 1,

[SylvainCorlay]

The behavior for when you pass a lesser number of argument than the actual dimension is still considered undefined.

Although eventually, we intend on having the same behavior for all expressions as for xarray, that is to make it equivalent to prepend zeros until you match the dimension.

[martinRenou]

@wolfv gave me the same answer on friday, but I think it's weird to say that it's an "undefined behavior" while it's explicitly defined, for example in xcontainer:

template <class D>
inline auto xcontainer<D>::operator[](size_type i) -> reference
{
    return operator()(i);
}

[JohanMabille]

Having a defined behavior in some particular cases is not contradictory with general undefined behavior.
Undefined behavior here is more like "implementation of the target general behavior not finished, so we cannot guarantee the same behavior for all kinds of expressions".

[SylvainCorlay]

I don't think that we have defined that behavior even for arrays. We just started changing the behavior of expressions to match the desired one, and are not done with it.