I am trying std::enable_if for the first time and struggling. Any guidance would be appreciated.
As a toy example, here is a simple static vector class, for which I want to define a copy constructor, but the behaviour depends on the relative sizes of the vectors:
- just copy data into a smaller or same-sized vector
- copy data into a larger vector and then pad the rest with zeroes
So the vector class is:
template <size_t _Size>
class Vector
{
double _data[_Size];
public:
Vector()
{
std::fill(_data, _data + _Size, 0.0);
}
const double* data() const
{
return _data;
}
...
};
The copy constructor should support something like this, copying the first 2 elements of v3 into v2:
Vector<3> v3;
Vector<2> v2(v3);
I tried a copy constructor for behaviour 1. like this, which compiles:
template <size_t _OtherSize,
typename = typename std::enable_if_t<_Size <= _OtherSize>>
Vector(const Vector<_OtherSize>& v) : Vector()
{
std::copy(v.data(), v.data() + _Size, _data);
}
but the compiler cannot distinguish this from behaviour 2. even though the enable_if conditions are mutually exclusive.
template <size_t _OtherSize,
typename = typename std::enable_if_t<_OtherSize < _Size>>
Vector(const Vector<_OtherSize>& v) : Vector()
{
std::copy(v.data(), v.data() + _OtherSize, _data);
std::fill(_data + _OtherSize, _data + _Size, 0.0);
}
I also tried putting enable_if in the argument instead, but it couldn't deduce the value of _OtherSize:
template <size_t _OtherSize>
Vector(const typename std::enable_if_t<_Size <= _OtherSize,
Vector<_OtherSize>> & v)
: Vector()
{
std::copy(v.data(), v.data() + _Size, _data);
}
What is the best way to do this (using enable_if, not a simple if statement)?
Thanks