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In this line:
double my_double_value = -my_size_t_value*cash;According to operator precedence first -my_size_t_value is calculated, and according to the C++ rules done in "unsigned mode" because size_t is unsigned.
So an expression that negates a small size_t will produce a large positive value - for example: negating a size_t with value 2 produces the value std::numeric_limits<size_t>::max()-1 which, for a 32-bit size_t, is 4294967294 and for 64-bit size_t is 18446744073709551614
(the latter matches your case according to the final output of 1.84467e+22).
Only then the result is converted to double for multiplying by cash.
So it is equivalent to:
double my_double_value = (-my_size_t_value)*cash;If you change it to:
double my_double_value = -(my_size_t_value*cash);You will get the expected result because now first the multiplication will be done, and for that my_size_t_value will be converted to double.
See live demo.
Using int (which is signed) instead of size_t (unsigned) gives you the expected result because now the unary negation does what you expect.
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6 Comments
2025-11-26T08:38:35.907Z+00:00
Nice! (although it doesn't explain why -my_size_t_value - before the static_cast<double> - results in a value which might not be intuitive).
2025-11-26T10:07:22.673Z+00:00
2s complement describes a representation of signed integers, not unsigned ones.
2025-11-26T11:48:34.09Z+00:00
I meant that a value that would have been a small negatives value (in 2s complement) will denote a high positive one as an unsigned. Editted to rephrase, but fill free to edit if you can phrase it better.
2025-11-26T11:55:03.613Z+00:00
OK; an edit suggested. I decided it was worth being specific on what is happening for the OP. Feel free to tweak.
2025-11-26T12:19:58.907Z+00:00
@Peter I modified your edit to add info for 64-bit system, which seem to be the case for the OP (according to the printed value of 1.84467e+22).
2025-11-26T12:29:22.327Z+00:00
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