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.

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

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compiler: mingw32-g++

Answer 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 (ie n not being const or constexpr ) although most compilers support it following C rules.