Typical errors of porting C++ code on the 64-bit platform

This kind of errors is separated from the others for better structuring of the account because indexing in arrays with the use of square brackets is just a different record of address arithmetic observed before.

In programming in language C and then C++ a practice formed to use in the constructions of the following kind variables of int/unsigned types:

unsigned Index = 0;
while (MyBigNumberField[Index] != id)
  Index++;

But time passes and everything changes. And now it’s a high time to say - do not do so anymore! Use for indexing (large) arrays memsize types.

The given code won’t process in a 64-bit program an array containing more than UINT_MAX items. After the access to the item with UNIT_MAX index an overflow of the variable Index will occur and we’ll get infinite loop.

To persuade you entirely in the necessity of using only memsize types for indexing and in the expressions of address arithmetic, I’ll give the last example.

class Region {
  float *array;
  int Width, Height, Depth;
  float Region::GetCell(int x, int y, int z) const;
  ...
};

float Region::GetCell(int x, int y, int z) const {
  return array[x + y * Width + z * Width * Height];
}

The given code is taken from a real program of mathematics simulation in which the size of RAM is an important source, and the possibility to use more than 4 Gb of memory on the 64-bit architecture improves the calculation speed greatly. In the programs of this class one-dimensional arrays are often used to save memory while they participate as three-dimensional arrays. For this purpose there are functions alike GetCell which provide access to the necessary items. But the given code will work correctly only with the arrays containing less than INT_MAX items. The reason for that is the use of 32-bit int types for calculation of the items index.

Programmers often make a mistake trying to correct the code in the following way:

float Region::GetCell(int x, int y, int z) const {
  return array[static_cast<ptrdiff_t>(x) + y * Width +
               z * Width * Height];
}

They know that according to C++ rules the expression for calculation of the index will have ptrdiff_t type and hope to avoid the overflow with its help. But the overflow may occur inside the sub-expression "y * Width" or "z * Width * Height" for int type is still used to calculate them.

If you want to correct the code without changing types of the variables participating in the expression you may use the explicit type conversion of every variable memsize type:

float Region::GetCell(int x, int y, int z) const {
  return array[ptrdiff_t(x) +
               ptrdiff_t(y) * ptrdiff_t(Width) +
               ptrdiff_t(z) * ptrdiff_t(Width) *
               ptrdiff_t(Height)];
}

Another solution is to replace types of variables with memsize type:

typedef ptrdiff_t TCoord;
class Region {
  float *array;
  TCoord Width, Height, Depth;
  float Region::GetCell(TCoord x, TCoord y, TCoord z) const;
  ...
};

float Region::GetCell(TCoord x, TCoord y, TCoord z) const {
  return array[x + y * Width + z * Width * Height];
}