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Pointer arithmetic

The code:

int** a = new int*[2];
a[0] = new int(1);
a[1] = new int(2);
cout << "a[0] " << a[0] << '\n';
cout << "a[1] " << a[1] << '\n';
cout << "a[2] " << a[2] << '\n';
cout << "a[0] + 1 " << a[0] + 1 << '\n';//WHY THIS ISN'T == a[1] ?
cout << "*(a + 1): " << *(a + 1) << '\n'; //WHY THIS IS == a[1] ?
cout << "a[0] - a[1] " << static_cast<int>(a[0] - a[1])<< '\n';//WHY THIS IS == 16 not 4?

cout << sizeof(int**);

Questions are included next to the relevant lines in the code.

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4 answers


a[0] + 1

means 1 is added to the value stored ina[0]

*(a + 1)

means it is 1 * sizeof(int)

added to memory address a and then accesses that address, which means you geta[1]



Read more about pointers here .

As for your last question, there is no guarantee that it will print any particular value, since the two memory addresses should not be contiguous. For me it prints -4

for example.

+9


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a[0]

and a[1]

are contiguous, and both are pointers, but they do not point to contiguous memory areas.

This is because the memory pointers returned by the operator new

are actually unpredictable.

In other words, there is no guarantee (in fact, the exact opposite) that the chunks of memory returned



a[0] = new int(1);
a[1] = new int(2);

are adjacent.

+5


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a[0] + 1

access the first item in and add 1.

a[1]

access the second item in.

*(a + 1)

is synonymous with a[1]

.

static_cast<int>(a[0] - a[1])

subtracts the second element from a from the first element into a.

+1


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  • You take the value at position zero and add 1 to it.
  • You take the beginning of the array, advance the pointer one size of an int pointer, and take the value that the pointer points to [1].
  • It could be anything. You subtract different addresses and pass it to int.
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