c++ - converting -1 to unsigned types

Question

Consider the following code to set all bits of x

unsigned int x = -1;

Is this portable ? It seems to work on at least Visual Studio 2005-2010

Answer 1

Apparently it is:

(4.7) If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2ⁿ where n is the number of bits used to represent the unsigned type). [Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation).

It is guaranteed to be the largest amount possible for that type due to the properties of modulo.

C99 also allows it:

Otherwise, if the newtype is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the newtype until the value is in the range of the newtype. 49)

Which wold also be the largest amount possible.

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Largest amount possible may not be all bits set. Use ~static_cast<unsigned int>(0) for that.