Arrays declared globally gives out of scope error in functions - c++11

Question

I have these arrays declared globally
char str1[] = "6541254939322816220209974565477289648317";
char str2[] = "3142522751761601737419090933147067701840";
int str1_size = strlen(str1);
int str2_size = strlen(str2);
int lcs[str1_size][str2_size];
int arrows[str1_size][str2_size];
But when I access this inside a function, it gives
lcs was not declared in this scope
also
array bound is not an integer constant before ‘]’ token
int lcs[str1_size][str2_size];
What am I doing wrong here ?

Answer 1

First of all
int a[dim];
where dim isn't a compile-time know constant (constexpr by example) isn't C++ standard; maybe it's possible with some extension with some compiler but isn't C++ standard.
So, in your case
int lcs[str1_size][str2_size];
int arrows[str1_size][str2_size];
where str1_size and str2_size are normal (not constexpr, not const initialized with literals) variables, is an error.
If you could redefine str1_size and str2_size as follows
constexpt int str1_size = strlen(str1);
constexpr int str2_size = strlen(str2);
the lcs and arrows definition could works.
Unfortunately (1) str1 and str2 aren't constexpr and (2) std::strlen() isn't constexpr.
But if declare str1/str2 constexpr and write a constexpr alternative to std::strlen...
#include <iostream>
constexpr std::size_t lenStr (char const * str)
{
std::size_t ret{};
while ( *(str++) )
++ret;
return ret;
}
constexpr char str1[] = "6541254939322816220209974565477289648317";
constexpr char str2[] = "3142522751761601737419090933147067701840";
constexpr auto str1_size = lenStr(str1);
constexpr auto str2_size = lenStr(str2);
int lcs[str1_size][str2_size];
int arrows[str1_size][str2_size];
int main ()
{
}
Unfortunately the preceding code require C++14 (in C++11 it's impossible write so complex constexpr functions).
In C++11 you should write lenStr() in a recursive way
constexpr std::size_t lenStr (char const * str, std::size_t ret = 0u)
{ return *str ? lenStr(++str, ++ret) : ret; }