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POWhat is the definition of bitwise operators in C++?
 

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  1. POWhat is the definition of bitwise operators in C++?
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    <p>According to the standard, operator &lt;&lt; yields an Undefined Behavior for a negative signed first operand.</p> <p><strong>C++11 5.8.2</strong></p> <pre><code>The value of E1 &lt;&lt; E2 is E1 left-shifted E2 bit positions; vacated bits are zero- filled. If E1 has an unsigned type, the value of the result is E1 × 2 pow E2, reduced modulo one more than the maximum value representable in the result type. Otherwise, if E1 has a signed type and non-negative value, and E1 × 2 pow E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined </code></pre> <p>This is understandable as the layout of integers in memory is implementation defined.</p> <p><strong>C++11 3.9.1.7</strong></p> <pre><code>this International Standard permits 2’s complement, 1’s complement and signed magnitude representations for integral types. </code></pre> <p>On the other hand, the standard do not seem to define exactly what bitwise &amp; | and ^ should do.</p> <p><strong>C++11 5.11 Bitwise AND operator</strong></p> <pre><code>and-expression: equality-expression and-expression &amp; equality-expression 1 The usual arithmetic conversions are performed; the result is the bitwise AND function of the operands. The operator applies only to integral or unscoped enumeration operands. </code></pre> <p><strong>C++11 5.12 Bitwise exclusive OR operator</strong></p> <pre><code>exclusive-or-expression: and-expression exclusive-or-expression ˆ and-expression 1 The usual arithmetic conversions are performed; the result is the bitwise exclusive OR function of the operands. The operator applies only to integral or unscoped enumeration operands. </code></pre> <p><strong>C++11 5.13 Bitwise inclusive OR operator</strong></p> <pre><code>inclusive-or-expression: exclusive-or-expression inclusive-or-expression | exclusive-or-expression 1 The usual arithmetic conversions are performed; the result is the bitwise inclusive OR function of its operands. The operator applies only to integral or unscoped enumeration operands. </code></pre> <p>The definition of those operators completely eludes me. Is it somewhere else in the standard ? Is the result implementation defined for signed integer ?</p> <p>As an example let's look at this code:</p> <pre><code>signed char a=-1; signed char b=3; signed char c=a&amp;b; </code></pre> <p>With 2's complement, a is 1111 1111, and b is 0000 0011. Finally c equals 0000 0011 (+3).</p> <p>With 1's complement, a is 1111 1110, and b is 0000 0011. Does c equals 0000 0010 (+2) ?</p> <p>With sign-magnitude, a is 1000 0001, and b is 0000 0011. Does c equals 0000 0001 (+1) ?</p> <p>If you have access to platforms using 1's complement or sign-magnitude, what is the result on those platforms ?</p>
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    1. COI'm sure bitwise boolean operators only care about bitwise operations and don't concern themselves with signedness.
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    2. CO"... standard do not seem to define exactly what bitwise & | and ^ should do" - yes it does and you quoted that part. What's missing from the definition?
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    3. CO@ Tony: Then why do we have an UB in operator << ? The bits of the signed integer could be move as if the operand were an unsigned integer.
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