cppreference Expressions # Discarded-value expressions

NOTE:

1、discarded-value expressions即“弃值表达式”。

2、需要注意的是：discarded-value expressions，不是unevaluated expressions，discarded-value expressions会被evaluated，但是它的value会被discard。

A discarded-value expression is an expression that is used for its side-effects only. The value calculated from such expression is discarded. Such expressions include

1、the full expression of any expression statement,

2、the left-hand argument of the built-in comma operator, or

NOTE:

1、后面有专门关于built-in comma operator的描述；

3、the argument of a cast-expression that casts to the type void.

NOTE:

1、参见 Cast-to-type-void 章节

Array-to-pointer and function-to-pointer conversions are never applied to the value calculated by a discarded-value expression. The lvalue-to-rvalue conversion is applied if and only if the expression is a volatile-qualified glvalue and has one of the following forms (built-in meaning required, possibly parenthesized)

• id-expression
• array subscript expression
• class member access expression
• indirection
• pointer-to-member operation
• conditional expression where both the second and the third operands are one of these expressions,
• comma expression where the right operand is one of these expressions.

In addition, if the lvalue is of volatile-qualified class type, a volatile copy-constructor is required to initialize the resulting rvalue temporary.

NOTE:

1、没有读懂