问题
I have no idea why the result of the two sub programs below are different:
int a , b;
a = 13, b=12;
(a > b)? (a++,b--):(a--,b++); // Now a is 14 and b is 11
a = 13, b=12;
(a > b)? a++,b-- : a--,b++; // Now a is 14 but b is 12
However for these cases, the results are identical:
a = 13, b=12;
(a < b) ? a++,b-- : a--,b++; // Now a is 12 and b is 13
a = 13, b=12;
(a < b) ? (a++,b--) : (a--,b++); // Again a is 12 and b is 13
Why parentheses make difference for the statement after "?", but make no difference for the statement after ":"? Do you have any idea?
回答1:
This one:
(a > b)? a++,b-- : a--,b++;
is equivalent to:
((a > b) ? (a++, b--) : a--), b++;
so b is always incremented and only sometimes decremented. There is no way to parse the comma operator between ? and : other than as parenthesized in the 'equivalent to' expression. But after the :, the unparenthesized comma terminates the ternary ?: operator and leaves the increment as unconditionally executed. The precedence of the comma operator is very, very low.
回答2:
The relevant parts of the C++ grammar are:
conditional-expression:
logical-or-expression
logical-or-expression ? expression : assignment-expression
assignment-expression:
conditional-expression
logical-or-expression assignment-operator assignment-expression
throw-expression
expression:
assignment-expression
expression, assignment-expression
In summary, while the 'middle' of a conditional expression can be a full expression extending up to the :, the last sub-expression can only be an assignment-expression which excludes expressions using the comma operator (other than where the comma operator appears as part of valid sub-expression of an assignment-expression such as a parenthesized primary-expression or as the second operand of another conditional-expression).
In C, the last sub-expression is more restricted, it cannot even be an assignment-expression although this is not a concern in your example.
conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
回答3:
In this case
(a > b)? a++,b-- : a--,b++;
It is equivalent to
((a > b)? a++,b-- : a--),b++;
回答4:
I guess it's because x ? y cannot be considered a valid expression, therefore the comma can't split the operator there. x ? y : z is a valid expression, the the comma after the colon can split into two expressions.
来源：https://stackoverflow.com/questions/16854007/comma-operator-precedence-while-used-with-operator