c++ - 级联流插入运算符不起作用

Question

我正在阅读一篇关于 Overloading the Stream Insertion Operator (<<) 的文章.它强调应该返回输出流对象以确保运算符正确级联。但是好像没有返回，输出还是正确的，这里有什么问题吗？

#include<iostream>

class Rational
{
    friend std::ostream& operator<<(std::ostream&, const Rational&);

    private:
        int numerator;
        int denominator;
    public:
        Rational(int num, int den): numerator{num}, denominator{den} {}
};

std::ostream& operator<<(std::ostream& lhs, const Rational& rhs)
{
    lhs << rhs.numerator << "/" << rhs.denominator;
    //return lhs;
}

int main()
{
    Rational r1(3, 5);
    std::cout << "The value of r1 is " << r1 << std::endl; // After commenting return lhs; still works fine
}

Answer 1

这是 UB，因为应该返回对象的函数在没有 return 语句的情况下结束。它可能会很好地工作，但不能保证。

来自标准，$6.6.3/2 The return statement[stmt.return] :

(强调我的)

Flowing off the end of a constructor, a destructor, or a function with a cv void return type is equivalent to a return with no operand. Otherwise, flowing off the end of a function other than main (basic.start.main) results in undefined behavior.

---

您可能想查看 clang 的结果;给出警告

warning: control reaches end of non-void function [-Wreturn-type]

并导致无限递归。