c - How values are evaluated in this expression in GCC

Question

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

I have an expression in a program, initial value of i = 10

int j = i++ + i++;

it sets j as 20 but

int j = i++ + ++i;

it sets j as 22
Why is there a difference of two between statements? I think, difference should be of 1.
I know this is undefined in C, but why GCC is doing such things?

Answer 1

对于 i++，它就像：首先做方程，然后增加 i。

对于 ++ i 就像：首先增加 i 然后做方程。

所以我猜 i++ + ++i 被解释为 i + (++(++i)) 所以结果是 22。

有线的事情发生在有线的语法上，只是不要做这样的事情；）。

如果我是对的 ++i + ++i 也将是 22。