三元和逗号运算符

Question

int a,b;
a = 1 ? 1,2 : 3,4; // a = 2
b = 0 ? 1,2 : 3,4; // b = 3

逗号运算符 returns 始终在逗号的右侧，但是如果我们对变量赋值 returns 左侧，除非我们使用 (). 那么第一个表达式到底是怎么给出 2.

我认为它是 a = 1,2 所以它应该是 1 但实际上是 a=2。

为什么？

Answer 1

由于operator precedence（逗号运算符的优先级最低），您的代码实际上看起来像

int a,b;
(a = 1 ? (1,2) : 3),4; // a = 2
(b = 0 ? (1,2) : 3),4; // b = 3

因此，根据三元条件规则，引用 C11，章节 §6.5.15

The first operand is evaluated; there is a sequence point between its evaluation and the evaluation of the second or third operand (whichever is evaluated). The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the result is the value of the second or third operand (whichever is evaluated), converted to the type described below. ¹¹⁰⁾

[...]

^{110) A conditional expression does not yield an lvalue.}

对于第一种情况，计算并返回第二个操作数。
对于第二种情况，计算并返回第三个操作数。

三元和逗号运算符

Ternary and comma operator

c++

c

ternary-operator

comma-operator