variable-length std::array like

Question

As my usually used C++ compilers allow variable-length arrays (e.g. arrays depending on runtime size), I wonder if there is something like std::array with variable size? Of course std::vector is of variable size, but it allocates on heap, and reallocates on need.

I like to have a stack allocated array with size defined at runtime. Is there any std-template that may feature this? Maybe using std::vector with a fixed maximum size?

Answer 1

There are two proposals currently being worked on to bring run-time fixed size arrays to C++ which may be of interest to you:

• Runtime-sized arrays with automatic storage duration. This would make runtime sized arrays a language feature (like in C11). So you could do:

    void foo(std::size_t size) {     int arr[size];   } 

• C++ Dynamic Arrays. This would bring a new container to the library, std::dynarray, which is given a fixed size at construction. It is intended to be optimized to be allocated on the stack when possible.

    void foo(std::size_t size) {     std::dynarray<int> arr(size);   } 

These are both being worked on as part of an Array Extensions Technical Specification, which will be released alongside C++14.

UPDATE: std::dynarray is not implemented yet(25Aug2021).please refer to What is the status on dynarrays?

Answer 2

As Daniel stated in the comment, size of the std::array is specified as a template parameter, so it cannot be set during runtime.

You can though construct std::vector by passing the minimum capacity through the constructor parameter:

#include <vector>  int main(int argc, char * argv[]) {     std::vector<int> a;     a.reserve(5);     std::cout << a.capacity() << "\n";     std::cout << a.size();      getchar(); } 

But. Still vector's contents will be stored on the heap, not on the stack. The problem is, that compiler has to know, how much space should be allocated for the function prior to its execution, so it is simply not possible to store variable-length data on the stack.