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c++comparisonunsigned

What does this do? if(unsigned(x-dx)<unsigned(size))

I found this in a piece of code I am trying to understand. It is probably some clever trick, but I don't get it. When is 

    unsigned(x-dx)<unsigned(size)

true? x, dx, and size are all (signed) int. From the context I guess, that x-dx should not be smaller than zero and not be larger than size. But I don't understand the unsigned trick.

Could someone explain it or tell where I can find more info about it?

thx in advance

Solution

  • This code determines if x - dx is in the range [0 ... size): if x - dx is negative, it becomes a large positive value when cast to unsigned. Large in this context means, a value larger than any positive value of the type [signed] int. That is, the comparison

    unsigned(x - dx) < unsigned(size)

    is true if x - dx is non-negative and smaller than size.