When I was reading about the array initialization in this tutorial. I found out this note.
type name [elements];
NOTE: The elements field within square brackets
[], representing the number of elements in the array, must be a constant expression, since arrays are blocks of static memory whose size must be determined at compile time, before the program runs.*
As I know array allocate the memory in the run time. This should be a false note? or what does it means?
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.
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];
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)
elementsand it "worked" you're using a compiler that supports VLAs in C++ (such a gcc). And though it may "work", it isn't standard. The note is accurate. If you want runtime-sizing, use astd::vector<>and heap-managed data.int foo[sz];) with dynamically allocated arrays (int *foo = new foo[sz]). The former must have its size known at compile time; the latter can have its size determined at runtime.std::vector<>in C++, which is often a better alternative. (There's alsostd::array<Type, Count>which is stored likeType[Count]but has.size(), .begin(), .end()etcetera)