Why doesn't C++ support dynamic arrays on the stack? [closed]

Answer 1

This functionality largely duplicates that of std::vector, except that it consumes a more limited resource (stack vs heap space). As such, there is not really any need for it in C++, semantics-wise.

One could argue that on-stack allocation can improve efficiency (particularly in the face of multiple threads); however, this can also be achieved in C++ using custom allocators to build a private memory pool, either on the stack or heap. This is again more flexible than placing memory on the stack, and indeed you could create a custom allocator that carves chunks out of an on-stack memory buffer easily enough. It's not exactly the same as dynamic array semantics, but the existence of custom allocators and STL containers covers most use cases you'd want stack allocation.

Answer 2

I think, it's because C++ provides superior solutions: std::vector<T> and std::array<T,N> (C++11); though the latter is not dynamic as such but it's superior to raw arrays. You can always know the size, no matter which function you pass the vector or array.

Since C cannot provide these solutions, C99 came up with Variable Length Array (VLA). It has the same problem as regular arrays: it decays into a pointer on passing it to function, and you no longer know the size of the array.

And as Florian Weimer asked here at comp.std.c++ that if C++0x allows VLA, then what would the following code mean?

int vla[n]; //n is known at runtime!
std::vector<decltype(vla)> v; //what does this mean?

How is the compiler going to instantiate the vector template at compile-time when it's type argument depends on n which is known at runtime?