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POOverflowing signed/unsigned assignments and its results
 

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  1. POOverflowing signed/unsigned assignments and its results
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    <p>I'm reading a Stroustrup's book "The C++ Programming Language 4th edition" and have three questions regarding overflowing assignments (particularily to signed/unsigned chars, as the book exemplifies). Firstly, according to standard 5/4 paragraph:</p> <blockquote> <p>If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, <strong>the behavior is undefined</strong>.</p> </blockquote> <p>(Except when a destination variable is unsigned - the result is clearly defined in such case). But does this definition also pertain to assignments?</p> <p>Because in my opinion there are many contrary statements in the book, all in chapter 6. The first one corresponds to the aforementioned paragraph, however the following comments doesn't:</p> <blockquote> <p>Variables of the three <code>char</code> types can be freely assigned to each other. However, assigning a too large value to a <code>signed char</code> is still undefined. For example:</p> <pre><code>void g(char c, signed char sc, unsigned char uc) { c = 255; //implementation-defined if plain chars are signed and have 8 bits c = sc; //OK c = uc; //implementation-defined if plain chars are signed and if uc's value is too large sc = uc; //implementation-defined if uc's value is too large uc = sc; //OK: conversion to unsigned sc = c; //implementation-defined if plain chars are unsigned and if c's value is too large uc = c; //OK: conversion to unsigned } </code></pre> </blockquote> <p><strong>First question</strong>: since assigning a too large value is UB, then why comments say it's implementation-defined?</p> <p>Next we have the following example:</p> <blockquote> <p>To be concrete, assume that a <code>char</code> is 8 bits:</p> <pre><code>signed char sc = -160; unsigned char uc = sc; //uc == 116 (because 256-160==116) cout &lt;&lt; uc; //print 't' </code></pre> </blockquote> <p><strong>Second question</strong>: apart from the fact that the first assignment is supposedly UB, what formula exactly has the author used in order to come up with 116? In my test, <code>uc</code> got the value of 96.</p> <p>And the last quote:</p> <blockquote> <p>An integer can be converted to another integer type. If the destination is <code>signed</code>, the value is unchanged if it can be represented in the destination type; otherwise, the value is implementation-defined:</p> <p><code>signed char sc = 1023; //implementation-defined</code></p> <p>Plausible results are 127 and -1.</p> </blockquote> <p><strong>Third question</strong>: again, apart from the fact that this contrary to what have been said earlier about UB, why possible results are 127 and -1? I guess it has something to do with one's and two's complement, but what are the precise formulas used?</p>
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    1. COCould you, please, provide the chapter with the above mentioned text for one to check this in PDF version.
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    2. CO@SChepurin it's chapter 6.
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    3. COAnd section number? I don't believe the book changed dramatically from third edition, since there ch. 6 is *Expressions and Statements*.
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