Segmentation error when accessing a dynamic array

Question

Hello! It seems to be a simple question, but I could not find the cause of the bug:

class World{ public: ... Block ***getTerrain(){ return &this->terrain; }; ... private: Block **terrain; }

There is a class that has an array of objects. When accessing an array inside a class (like this->terrain[i][j].getType() ), everything is fine, but when I try to use the link returned by the getTerrain function ( this->world->getTerrain()[i][j]->getType() ), a segmentation error occurs. What is the problem?

By the way, when I refer to null elements ( this->world->getTerrain()[0][0]->getType() ), everything goes fine.

Accepted Answer · 2016-04-18T14:40:57

Your code does not do the same thing.

In the first case, you access the object

 terrain[i][j]

in the second - to

 *((&terrain)[i][j])

The first from the address pointed to by terrain moves to i * sizeof(Block*) , finds the pointer there, and moves from it to j * sizeof(Block) .

The second one from the address pointed to by terrain moves to i * sizeof(Block**) , finds the pointer there, and moves from it to j * sizeof(Block*) .

Most likely, sizeof(Block**) and sizeof(Block*) are equal (because the pointer to the data), but sizeof(Block*) and sizeof(Block) are not.

Try this:

 Block** terrain = *(this->world->getTerrain()); terrain[i][j].getType();

Well, or you can remake the getTerrain function getTerrain that it does not return a double pointer, not triple.

But in that case, the changes in the array will not be visible from the class, right?
Only if you want to change the terrain field itself, it will not be visible, since you get a copy of the terrain .
And the values indicated by terrain are the same in both cases.

Segmentation error when accessing a dynamic array

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