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    <p><strong>Edit: This answer is incorrect for the updated question, it applies to the question as originally stated.</strong></p> <p><code>(i++)++</code> shouldn't work with either gcc or g++, whether or not they are parsing the file as C or C++ in both languages postfix increment requires an <em>lvalue</em> as an operand and the result is an <em>rvalue</em>. </p> <p>(Note that <em>rvalue</em> is only used formally in C++. In C the results of expressions are either explicitly <em>lvalues</em> or they're just not <em>lvalues</em>. The term <em>rvalue</em> isn't used in the normative text of the C standard at all.)</p> <p>Simplisticly, an <em>lvalue</em> is an expression that refers to an object, i.e. conceptually a region of storage somewhere. A modifiable <em>lvalue</em> is something that it is valid to assign to so can appear on the <em>left</em> hand side of an assignment expression.</p> <p>An <em>rvalue</em> is just a value, something that you can assign a modifiable <em>lvalue</em> from, but not something that you can assign to. It can only appear on the <em>right</em> side of an assignment expression.</p> <p><code>g++</code> gives me the error: <code>lvalue required as increment operand</code>.</p> <p>Prefix increment (and decrement) are slightly different. In C++ the result of a prefix increment is explicitly an <em>lvalue</em> (5.3.2/1 [expr.pre.incr]); in C it is explicitly not an <em>lvalue</em> (6.5.3.1 states that ++E is equivalent to (E+=1); 6.5.16 says that the result of any assignment is not an lvalue).</p> <p><code>(++i)++</code> is, therefore, semantically correct only in C++, however it has <em>undefined behaviour</em> because you are storing a value to an object twice without an intervening <em>sequence point</em>.</p>
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    1. CO@Charles Bailey:My guess is c-expression returns r-value and c++-expression returns l-value.
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    2. CO@Jagan: Why do you think you should guess? It's just not true; in both C and C++ the result of postfix increment expression is an _rvalue_.
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    3. CO@Charles Bailey: What about preincrement expression ?
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