Is the size of an array determined at compile time?

Question

When I was reading about the array initialization in this tutorial. I found out this note.

type name [elements];

NOTE: The e

Answer 1

The declaration T a[N] requires that N be a converted constant expression.

Converted constant expression is an expression implicitly converted to prvalue of type T, where the converted expression is a core constant expression. If the literal constant expression has class type, it is contextually implicitly converted to the expected integral or unscoped enumeration type with a constexpr user-defined conversion function.

An int literal such as 5 is a prvalue, so can be used in the declaration T a[5], but an lvalue, for example int n = 5 cannot be used in the declaration T a[n], unless the lvalue under-goes an implicit lvalue-to-rvalue conversion where the lvalue:

a) has integral or enumeration type, is non-volatile const, and is initialized with a constant expression, or an array of such (including a string literal)

b) has literal type and refers to a non-volatile object defined with constexpr or to its non-mutable subobject

c) has literal type and refers to a non-volatile temporary, initialized with a constant expression

Therefore the following are valid:

const int n = 5; 
int a[n]; 
constexpr int n = 5; 
int a[n];

Answer 2

Please check if the following answers help in giving you clarity about this.

Static array vs. dynamic array in C++

Static arrays are created on the stack, and necessarily have a fixed size (the size of the stack needs to be known going into a function): int foo[10];

Dynamic arrays are created on the heap. They can have any size, but you need to allocate and free them yourself since they're not part of the stack frame: int* foo = new int[10]; delete[] foo;

You don't need to deal with the memory management of a static array, but they get destroyed when the function they're in ends

Array size at run time without dynamic allocation is allowed?

C99 standard (http://en.wikipedia.org/wiki/C99) supports variable sized arrays on the stack. Some of the compilers might implement these standards and support variable sized arrays.

Answer 3

You may use :

int array[42];

but not

int n;
std::cin >> n;
int array[n]; // Not standard C++

the later is supported as extension by some compiler as VLA (Variable length array)