Combining std::vector Default and Fill Constructor

Question

I had a fairly quick question. The std::vector provides the following two constructors:

explicit vector( const Allocator& alloc = Allocator());      // default constructor

explicit vector( size_type count,                            // fill constructor
             const T& value = T(),
             const Allocator& alloc = Allocator());

Is there any particular reason the default constructor is not implemented with a default value of 0 for the first parameter in the fill constructor? I can imagine there must be a reason but I can't immediately see one.

Answer 1

Because you then can't pass just an allocator, without providing the count or default element (aka value) as well?

Putting count to 0 will result in ambiguity error.

Would be much more simpler if C++ had named params like Python. Boost has such a library, but again that incurs some runtime overhead :( (can't remember how much right now) I often used this Lib in testing, but not where the performance is important...

Answer 2

The reason is that the constructors place different requirements on the type contained in the vector. To use the second one, the type must copy-constructible, and if you use the default argument for value, it must be default-constructible too. The first constructor places no such requirements on the contained type.

Note that the constructors you're showing in the question only existed until C++11. There, it was sufficient to differentiate those two scenarios (since any type stored in a std::vector had to be copy-constructible). C++11 introduced move semantics, and the second constructor was split further:

explicit vector(size_type count);

vector(
  size_type count,
  const T& value,
  const Allocator& alloc = Allocator()
);

That's because std::vector no longer requires its contained types to be copy-constructibe; move-constructibility is enough. Therefore the count-only constructor requires default constructibility (but not copy constructibility), and the count + prototype constructor requires copy constructibility (but not default constructibility).

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The evolution of std::vector constructors is really quite complex. See their page on cppreference to see how much they've evolved. That evolution includes adding an optional allocator parameter to the count-only constructor in C++14, which was (I assume) mistakenly omitted.