c++ - 如何在过载时强制发出 "statement has no effect"警告==

Question

在下面的例子中:

class Test

{
public:
    Test(int _value) { value = _value; };
    const bool operator==(int _value) const { return value == _value; };
private:
    int value;
};

int main(void)
{
    int a;
    a == 1;

    Test b(1);
    b == 1;

    return 0;
}

编译给出以下内容:

$ g++ -Wall -pedantic -o test test.cc
a.cc: In function ‘int main()’:
a.cc:13:7: warning: statement has no effect [-Wunused-value]
     a == 1;
     ^

这很好，因为它警告我输入错误并错误输入 == for =

但是我的Test类也是如此。我如何标记类或 operator== 的定义以使编译器用另一个“语句无效”警告我“b == 1”行？

Answer 1

在 C++17 中，您可以将运算符重载标记为 [[nodiscard]] :

[[nodiscard]] bool operator==(int _value) const { return value == _value; }

如果返回值未被使用，这将鼓励编译器产生警告。

live example on wandbox.org

---

来自cppreference:

[[nodiscard]]

Appears in a function declaration, enumeration declaration, or class declaration. If a function declared nodiscard or a function returning an enumeration or class declared nodiscard by value is called from a discarded-value expression other than a cast to void, the compiler is encouraged to issue a warning.