7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.20 Comma operator [expr.comma]

1
#
The comma operator groups left-to-right.
A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression.
The left expression is sequenced before the right expression ([intro.execution]).
The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a bit-field.
2
#
In contexts where comma is given a special meaning,
[Example
: in lists of arguments to functions ([expr.call]) and lists of initializers ([dcl.init]) — end example
]
the comma operator as described in this subclause can appear only in parentheses.
[Example
: 
f(a, (t=3, t+2), c);
has three arguments, the second of which has the value 5.
— end example
]
3
#
[Note
:
A comma expression appearing as the expr-or-braced-init-list of a subscripting expression is deprecated; see [depr.comma.subscript].
— end note
]