It's not quite the same as default arguments in that default arguments are evaluated at run time, whereas array lengths for C++ (and not C) needs to be a compil

It's not quite the same as default arguments in that default arguments are evaluated at run time, whereas array lengths for C++ (and not C) needs to be a compile time constant expression. #include <stdio.h> // "puts" makes "f" not constexpr. constexpr int f() { return puts("hello"); } // error: size of array 'argv' is not an integral constant-expression int main(int argc, char *argv[f()]) {}