tmp/tmp4y_k9y90/{from.md → to.md}
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| 1 |
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### Assignment and compound assignment operators <a id="expr.assign">[[expr.assign]]</a>
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| 3 |
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The assignment operator (`=`) and the compound assignment operators all
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group right-to-left. All require a modifiable lvalue as their left
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operand; their result is an lvalue of the type of the left operand,
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referring to the left operand. The result in all cases is a bit-field if
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the left operand is a bit-field. In all cases, the assignment is
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sequenced after the value computation of the right and left operands,
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and before the value computation of the assignment expression. The right
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operand is sequenced before the left operand. With respect to an
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indeterminately-sequenced function call, the operation of a compound
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assignment is a single evaluation.
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[*Note 1*: Therefore, a function call cannot intervene between the
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lvalue-to-rvalue conversion and the side effect associated with any
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single compound assignment operator. — *end note*]
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``` bnf
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assignment-expression:
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conditional-expression
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yield-expression
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throw-expression
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logical-or-expression assignment-operator initializer-clause
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```
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``` bnf
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assignment-operator: one of
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'= *= /= %= += -= >>= <<= &= ^= |='
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```
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In simple assignment (`=`), let `V` be the result of the right operand;
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the object referred to by the left operand is modified [[defns.access]]
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by replacing its value with `V` or, if the object is of integer type,
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with the value congruent [[basic.fundamental]] to `V`.
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If the right operand is an expression, it is implicitly converted
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[[conv]] to the cv-unqualified type of the left operand.
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When the left operand of an assignment operator is a bit-field that
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cannot represent the value of the expression, the resulting value of the
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bit-field is *implementation-defined*.
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An assignment whose left operand is of a volatile-qualified type is
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deprecated [[depr.volatile.type]] unless the (possibly parenthesized)
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assignment is a discarded-value expression or an unevaluated operand
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[[term.unevaluated.operand]].
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The behavior of an expression of the form `E1 op= E2` is equivalent to
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`E1 = E1 op E2` except that `E1` is evaluated only once.
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[*Note 2*: The object designated by `E1` is accessed
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twice. — *end note*]
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For `+=` and `-=`, `E1` shall either have arithmetic type or be a
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pointer to a possibly cv-qualified completely-defined object type. In
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all other cases, `E1` shall have arithmetic type.
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If the value being stored in an object is read via another object that
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overlaps in any way the storage of the first object, then the overlap
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shall be exact and the two objects shall have the same type, otherwise
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the behavior is undefined.
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[*Note 3*: This restriction applies to the relationship between the
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left and right sides of the assignment operation; it is not a statement
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about how the target of the assignment can be aliased in general. See
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[[basic.lval]]. — *end note*]
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A *braced-init-list* B may appear on the right-hand side of
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- an assignment to a scalar of type `T`, in which case B shall have at
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most a single element. The meaning of `x = B` is `x = t`, where `t` is
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an invented temporary variable declared and initialized as `T t = B`.
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- an assignment to an object of class type, in which case B is passed as
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the argument to the assignment operator function selected by overload
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resolution [[over.assign]], [[over.match]].
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[*Example 1*:
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``` cpp
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complex<double> z;
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z = { 1,2 }; // meaning z.operator=({1,2\)}
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z += { 1, 2 }; // meaning z.operator+=({1,2\)}
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int a, b;
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a = b = { 1 }; // meaning a=b=1;
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a = { 1 } = b; // syntax error
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```
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— *end example*]
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