C++核心准则ES.44: 不要让执行结果受函数参数的求值次序影响​

ES.44: Don't depend on order of evaluation of function arguments

ES.44: 不要让执行结果受函数参数的求值次序影响

Reason（原因）

Because that order is unspecified.

因为函数参数的求值次序是无定义的。

Note（注意）

C++17 tightens up the rules for the order of evaluation, but the order of evaluation of function arguments is still unspecified.

C++17收紧了运算次序规则，但是函数参数的求值次序依然是无定义的。

Example（示例）

int i = 0;
f(++i, ++i);

The call will most likely be f(0, 1) or f(1, 0), but you don't know which. Technically, the behavior is undefined. In C++17, this code does not have undefined behavior, but it is still not specified which argument is evaluated first.

调用的结果很可能是f(0，1)或者f(1，0)，但是不知道会是哪一个。技术上，这个行为是无定义的。在C++17中，这段代码不会是一个无定义的行为，但是依然没有明确那个参数先求值。

Example（示例）

Overloaded operators can lead to order of evaluation problems:

重载的运算符可能引入计算次序的问题。

f1()->m(f2());          // m(f1(), f2())
cout << f1() << f2();   // operator<<(operator<<(cout, f1()), f2())

In C++17, these examples work as expected (left to right) and assignments are evaluated right to left (just as ='s binding is right-to-left)

在C++17中，这些代码示例可以像期待的那样执行（从左向右），同时赋值会从右向左计算（就像赋值运算符=绑定了从右向左计算次序）

f1() = f2();    // undefined behavior in C++14; in C++17, f2() is evaluated before f1()

Enforcement（实施建议）

Can be detected by a good analyzer.

这个问题可以被实现良好的代码解析器检出。

链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es44-dont-depend-on-order-of-evaluation-of-function-arguments