Question

I was curious to know what would happen if I assign a negative value to an unsigned variable.

The code will look somewhat like this.

unsigned int nVal = 0;
nVal = -5;

It didn’t give me any compiler error. When I ran the program the nVal was assigned a strange value! Could it be that some 2’s complement value gets assigned to nVal?

Answer 1

For the official answer – Section 4.7 conv.integral for conversion from signed integral types.

"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). —end note ]

This essentially means that if the underlying architecture stores in a method that is not Two’s Complement (like Signed Magnitude, or One’s Complement), that the conversion to unsigned must behave as if it was Two’s Complement.

(C++20 and later require that signed integers use 2’s complement, but earlier versions of the standard allowed those other representations.)

The "… congruent …" part means that you add or subtract 2ⁿ until the value is in the value-range of the unsigned type. For 2’s complement, this means doing 2’s complement sign-extension or truncation. For the same width, the bit-pattern is unchanged because adding 2ⁿ is a no-op: the low n bits of 2ⁿ are all zero. 2’s complement addition/subtraction are the same bitwise operation as unsigned, which is what makes it special.

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PS. conversion from floating point to unsigned works differently: it’s undefined behavior if the value is out of range for the unsigned type (if it’s negative after truncation to integer or too large). Modulo reduction only happens for signed integer to unsigned integer.