View Compilation Error

0_0_14995292_23893.cpp: In function 'int main()':
0_0_14995292_23893.cpp:36:28: error: 'strlen' was not declared in this scope
   Num a(val_1, strlen(num_1));
                            ^
0_0_14995292_23893.cpp: In function 'void turn(char*, int*)':
0_0_14995292_23893.cpp:102:23: error: 'strlen' was not declared in this scope
  for (int i = strlen(a) - 1; i >= 0; i--)
                       ^