重载的按位或运算符（'|'）是否具有明确定义的评估顺序？

Question

如果我的 C++ class 重载了按位或运算符 (|)，C++ 语言规范是否保证传递给对该运算符的一系列调用的参数将被计算为左-向右？还是评估顺序实现已定义（或未定义）？

（IIRC C++ 的内置 | 运算符具有实现定义的求值顺序；但当运算符为 class 重载时可能会有所不同？）

下面是一个程序，它举例说明了我要问的问题：这个程序是否保证打印出 0 1 2 3 4（就像它在我目前所在的 Mac 上所做的那样），或者可能它在某些环境中合法地打印出 4 3 2 1 0（或其他一些顺序）？

#include <iostream>

class mytype_t
{
public:
   mytype_t(int v) : _val(v) {/* empty */}

   mytype_t operator | (const mytype_t & rhs) const {return (_val | rhs._val);}

private:
   int _val;
};

mytype_t func(int v)
{
   std::cout << v << std::endl;
   return mytype_t(v);
}

int main(int, char **)
{
   mytype_t x = func(0) | func(1) | func(2) | func(3) | func(4);
   return 0;
}

Answer 1

如果内置运算符规定了特定的顺序，则参数的计算顺序也与重载的顺序相同。这是相关段落（来自 n4659，C++17 草案），强调我的：

[over.match.oper]

2 If either operand has a type that is a class or an enumeration, a user-defined operator function might be declared that implements this operator or a user-defined conversion can be necessary to convert the operand to a type that is appropriate for a built-in operator. In this case, overload resolution is used to determine which operator function or built-in operator is to be invoked to implement the operator. Therefore, the operator notation is first transformed to the equivalent function-call notation as summarized in Table 12 (where @ denotes one of the operators covered in the specified subclause). However, the operands are sequenced in the order prescribed for the built-in operator (Clause [expr]).

所以不，重载的 operator| 将没有明确定义的评估顺序，因为内置没有。

重载的按位或运算符（'|'）是否具有明确定义的评估顺序？

Does an overloaded bitwise-or operator ('|') have a well-defined evaluation-order?

c++

operator-overloading

language-lawyer

bitwise-or