Why can't an array's size be defined with a variable?

Answer 1

you can if you declare that variable constant.

const int validsize = 6;
int myarray[validsize];

or with a function if you declare it constexpr:

constexpr int f() { return 5; }
int myarray[f()];

Answer 2

This is because you allocate it on the stack or on the static / global space. For these the compiler has to know their size at compile time.
Truly dynamic allocation is done on the heap via "new" operator.

Answer 3

Okey thx both of you!