# V109. Implicit type conversion of return value to memsize type.

The analyzer found a possible error related to the implicit conversion of the return value type. The error may consist in the incorrect determination of the return value.

Let's examine an example.

    extern int Width, Height, Depth;
    size_t GetIndex(int x, int y, int z) {
      return x + y * Width + z * Width * Height;
    }
    ...
    array[GetIndex(x, y, z)] = 0.0f;

If the code deals with large arrays (more than 'INT_MAX' items) it will behave incorrectly and we will address not those items of the array 'array' that we want. But the analyzer won't show a warning message on the line "array[GetIndex(x, y, z)] = 0.0f;" for it is absolutely correct. The analyzer informs about a possible error inside the function and is right for the error is located exactly there and is related to the arithmetic overflow. In spite of the facte that we return the type 'size_t' value the expression "x + y * Width + z * Width * Height" is determined with the use of type 'int'.

To correct the error we should use the explicit conversion of all the variables included into the expression to memsize types.

    extern int Width, Height, Depth;
    size_t GetIndex(int x, int y, int z) {
      return (size_t)(x) +
       (size_t)(y) * (size_t)(Width) +
       (size_t)(z) * (size_t)(Width) * (size_t)(Height);
    }

Another variant of correction is the use of other types for the variables included into the expression.

    extern size_t Width, Height, Depth;
    size_t GetIndex(size_t x, size_t y, size_t z) {
      return x + y * Width + z * Width * Height;
    }

When you are sure that the code is correct and the implicit type conversion does not cause errors while porting to the 64-bit architecture you may use the explicit type conversion so that to avoid showing of the warning messages in this line. For example:

    DWORD_PTR Calc(unsigned a) {
      return (DWORD_PTR)(10 * a);
    }

In case you suspect that the code contains incorrect explicit type conversions to memsize types about which the analyzer does not show warnings you may use the V201.

Additional materials on this topic:

- 64-bit Lessons. Lesson 17. Pattern 9. Mixed arithmetic.