c++ - "Access violation reading error" -

exception thrown @ 0x003d65b6 in new_predator.exe: 0xc0000005: access violation reading location 0x00000000.

i keep getting error , cannot figure out why. here section of code points throwing error:

void simulation::run() {  (int = 0; < 20; i++) {     (int j = 0; j < 20; j++) {         if (world[i][j]->name() == 'a')             world[i][j]->move();      }    }  } 

it points "if (world[i][j]->name() == 'a') line throwing error.

my simulation constructor defined follows:

this private variable in simulation class. critter abstract base class.

critter ***world;  simulation::simulation() {    world = new critter**[20];   (int = 0; < 20; i++) {       world[i] = new critter*[20];   }    (int = 0; < 20; i++)       (int j = 0; j < 20; j++)           world[i][j] = null;   } 

i have display function in sim class defined follows, similiar run function works fine:

 void simulation::display() {  (int = 0; < 20; i++) {     std::cout << std::endl;     (int j = 0; j < 20; j++) {         if (world[i][j] == nullptr)             std::cout << " . ";         else if (world[i][j]->name() == 'a')             std::cout << " o ";         else             std::cout << " x ";     } }  std::cout << std::endl;  } 

i have 2 derived class contain name member function returns char 'a' or 'd'. have ideas happening here? hope have provided enough information.

you must check word[i][j] not nullptr befor trying access it:

for (int = 0; < 20; i++) {     (int j = 0; j < 20; j++) {         if (word[i][j]!=nullptr && world[i][j]->name() == 'a')             world[i][j]->move();     } } 

by way, you're doing such check in display function :

  if (world[i][j] == nullptr)    // yes !!!          std::cout << " . ";   else if (world[i][j]->name() == 'a')   // here we're sure it's not nullptr         ...