KnowItAllWannabe KnowItAllWannabe - 9 days ago 5
C++ Question

Why both runtime-sized arrays and std::dynarray in C++14?

Draft C++14 includes both runtime-sized arrays and the

std::dynarray
container. From what I can tell, the only real difference between the two is that
std::dynarray
has an STL interface (e.g.,
begin
,
end
,
size
, etc.), while runtime-sized arrays do not. So why does C++14 need them both?

I understand that runtime-sized arrays are part of the core language, while
std::dynarray
is part of the standard library, but the proposal for
std::dynarray
makes clear that the authors expect compilers, in many cases, to offer special support for
std::dynarray
so that it can be as efficient as possible, i.e., as efficient as a runtime-sized array. As such, the language/library distinction seems somewhat artificial.

So, again, why does C++14 need both runtime-sized arrays and
std::dynarray
? And given that
std::dynarray
has a richer (STLified) interface, why not just drop runtime-sized arrays, assuming that
std::dynarray
can be implemented with equal runtime efficiency?

Clarification

When I talk about "runtime-sized arrays," I'm referring to a new C++14 core language feature that's described in N3639, not to traditional C arrays or VLAs or anything in C++11.

Answer

N3639 proposes to add local runtime-sized arrays with automatic storage duration to C++.

N2648 says that in keeping with C++ practice, std::dynarrays are usable with more than just automatic variables. But to take advantage of the efficiency stack allocation, we wish to make dynarray optimizable when used as an automatic variable.

In short, C11 style runtime-sized arrays are restricted to being stored on the stack. dynarray is not, but can be optimized when stored on the stack to be as efficient as C11 style runtime-sized arrays (or so is the goal).

C11 style runtime-sized arrays can be a useful syntax still, and the cost to increase intercompilability with C isn't high: the mechanism would have to be implemented for efficient automatic dynarray anyhow. In addition, C11 style runtime-sized arrays are first class citizens, and exist regardless of use of std libraries by the programmer.

There are important differences between actual C11 runtime-sized arrays and C++1y C11-style runtime-sized arrays, not the least of which is the runtime sizeof that actual C11 runtime-sized arrays support. But basic use of it may be compatible.

Note that in the end, neither where added in C++14.