我们在条件三元运算符中使用逗号时发现的东西？

Question

好吧,我在三元运算符中有一个关于逗号的问题.剪掉垃圾,代码如下:

void test_comma_in_condition(void)
{
    int ia, ib, ic;

    ia = ib = ic = 0;
    bool condition=true;

    cout<<"Original:"<<endl;
    cout<<"ia: "<<ia<<endl;
    cout<<"ib: "<<ib<<endl;
    condition?(ia=1, ib=2):(ia=11, ib=12);
    cout<<"After:"<<endl;
    cout<<"ia: "<<ia<<endl;
    cout<<"ib: "<<ib<<endl;

    ia = ib = ic = 0;
    condition?ia=1, ib=2, ic=3:ib=22,ia=21, ic=23;
    cout<<"The operation must be bracketed, or you'll see..."<<endl;
    cout<<"ia: "<<ia<<endl;
    cout<<"ib: "<<ib<<endl;
    cout<<"ic: "<<ic<<endl;

    condition?ia=1, ib=2, ic=3:ia=21, ib=22, ic=23;
    cout<<"The operation must be bracketed, or you'll see..."<<endl;
    cout<<"ia: "<<ia<<endl;
    cout<<"ib: "<<ib<<endl;
    cout<<"ic: "<<ic<<endl; 

    return;
}

输出将如下:

Original:
ia: 0
ib: 0
After:
ia: 1
ib: 2
The operation must be bracketed, or you'll see...
ia: 21
ib: 2
ic: 23
The operation must be bracketed, or you'll see...
ia: 1
ib: 22
ic: 23

这合法吗？

Answer 1

这是运营商优先权的问题.你的表达:

condition?ia=1, ib=2, ic=3:ib=22,ia=21, ic=23;

编译器将其理解为:

(condition?(ia=1, ib=2, ic=3):(ib=22)),ia=21, ic=23;

此时,您应该能够看到为什么获得程序输出.

Answer 2

是的,条件表达式的相关语法是:

logical-or-expression ? expression : assignment-expression

对于赋值表达式(也可以是条件表达式或throw-expression):

logical-or-expression assignment-operator assignment-expression

对于带逗号运算符的表达式(赋值表达式也可以是表达式):

expression , assignment-expression

这意味着构造a ? b : c, d不能被解析为等价,a ? b : (c, d)因为c, d它不是赋值表达式,但必须被解析为等价(a ? b : c), d.

没有未定义的行为,condition ? ia=1,ib=2,ic=3 : ia=21, ib=22, ic=23;因为condition在评估?:包含逗号运算符的每个子表达式的第二个或第三个操作数之前对序列进行排序,在评估第二个操作数之前对逗号运算符的第一个操作数进行排序操作数.