I'm having trouble understanding how Post Increment (++), Pre Increment work together in an example [duplicate]

问题

I'm having trouble understanding how Post Increment (++), Pre Increment work together in an example.

x++ means add 1 to the variable But I am confused with this example:

using namespace std;
/ run this program using the console pauser or add your own getch, system("pause") or input loop */
int main() {
    int a;
    a=8;
    cout<<++a<<a++<<endl;
    cout<<a<<endl;
    return 0;
}

I assume this means in first increased by 1 and in second it will first assign and then increment Which means the result should be 9 8 and 9 But when I compile it, I get 10 8 and 10. I don't understand.

回答1:

Your confusion has nothing to do with the pre- and post-increment, but with the evaluation order of the operator <<. There are plenty of threads on this, here is a good one imho: SO discussion about evaluation order

The summary is :

Before C++17, if you have an expression such as std::cout << f(a) << g(a) << std::endl;, the order of evaluation (f first or g first) is not specified.

This becomes clearer when we look at what the expression above means. For the overloaded operator<<, it effectively becomes

operator<<(operator<<(std::cout, f(a)), g(a));
so: 
function  (<--------- arg 1 --------->,<arg2>)

In this case, the evaluation is unsequenced, and it is not defined whether arg1 or arg2 will be evaluated first.

With C++17, the order is specified from left to right.

From [n4659] §8.2.2 : 5

If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator.

I interpret this as follows: even if the operator is overloaded, if it is called as an operator (i.e. std::cout << f(a) << g(a) << std::endl;), it will effectively be evaluated as

std::cout.operator<<(f(a)).operator<<(g(a)).operator<<(std::endl);

However, if the call is made explicitely as

operator<<(operator<<(std::cout, f(a)), g(a));

it will be treated as a function call and the order is still not specified.

To be safe, you are better off splitting up your prints/evaluations in separate statements (i.e. separated by ;) unless you have a good reason not to (and know the details well), especially since different operators behave differently (e.g. + remains unsequenced after C++17).

来源：https://stackoverflow.com/questions/65213547/im-having-trouble-understanding-how-post-increment-pre-increment-work-tog

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c++

operators

post-increment

pre-increment