How to implement fill constructor and range constructor for sequence containers unambiguously

问题

Sequence containers need to have fill constructors and range constructors, i.e. these must both work, assuming MyContainer models a sequence container whose value_type is int and size_type is std::size_t:

// (1) Constructs a MyContainer containing the number '42' 4 times.
MyContainer<int> c = MyContainer<int>(4, 42);

// (2) Constructs a MyContainer containing the elements in the range (array.begin(), array.end())
std::array<int, 4> array = {1, 2, 3, 4};
MyContainer<int> c2 = MyContainer<int>(array.begin(), array.end());

Trouble is, I'm not sure how to implement these two constructors. These signatures don't work:

template<typename T>
MyContainer<T>::MyContainer(const MyContainer::size_type n, const MyContainer::value_type& val);

template<typename T>
template<typename OtherIterator>
MyContainer<T>::MyContainer(OtherIterator i, OtherIterator j);

In this case, an instantiation like in example 1 above selects the range constructor instead of fill constructor, since 4 is an int, not a size_type. It works if I pass in 4u, but if I understand the requirements correctly, any positive integer should work.

If I template the fill constructor on the size type to allow other integers, the call is ambiguous when value_type is the same as the integer type used.

I had a look at the Visual C++ implementation for std::vector and they use some special magic to only enable the range constructor when the template argument is an iterator (_Is_iterator<_Iter>). I can't find any way of implementing this with standard C++.

So my question is... how do I make it work?

Note: I am not using a C++11 compiler, and boost is not an option.

回答1:

I think you've got the solution space right: Either disambiguate the call by passing in only explicitly size_t-typed ns, or use SFINAE to only apply the range constructor to actual iterators. I'll note, however, that there's nothing "magic" (that is, nothing based on implementation-specific extensions) about MSVC's _Is_iterator. The source is available, and it's basically just a static test that the type isn't an integral type. There's a whole lot of boilerplate code backing it up, but it's all standard C++.

A third option, of course, would be to add another fill constructor overload which takes a signed size.

来源：https://stackoverflow.com/questions/21042872/how-to-implement-fill-constructor-and-range-constructor-for-sequence-containers