compile-time array initialization with non-const variable size

Question

I having a confusion about what is going on behind the scene of array initialization.

int n= 3;
int a[n]; // compile succeeds

but,

int n = 3;
int a[n] = {1, 2, 3};   // compile error

error message from codeblock:

error: variable-sized object 'a' may not be initialized

My understanding is: first snippet, n elements are allocated but uninitialized. Second one allocates n elements and initializes them to {1, 2, 3}. Both do almost the same thing, but why second one causes error.

Please clarify my understanding or leave me some resources (I've tried, but couldn't find the close answer).

compiler: mingw32-g++

What is the error? What compiler are you using? int a[n]; is not standard and should not compile. — NathanOliver
– NathanOliver, Commented Mar 15, 2016 at 16:23

ftynse · Accepted Answer · 2016-03-15 16:28:57Z

1

In C99, it's explicitly forbidden by the standard (6.7.8p3)

The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.

Even though we can see that n is a constant value, it is not marked as such.

AFAIK, C++ standard does not allow for variable-length arrays (i.e. n not being const or constexpr) although most compilers support it following C rules.

answered Mar 15, 2016 at 16:28

ftynse

7974 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

compile-time array initialization with non-const variable size

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related