C++核心准则C.182:使用匿名联合体实现附带标签的联合体

C.182: Use anonymous unions to implement tagged unions

C.182:使用匿名联合体实现附带标签的联合体

Reason(原因)

A well-designed tagged union is type safe. An anonymous union simplifies the definition of a class with a (tag, union) pair.

良好设计的命名联合体是类型安全的。无名联合体简化了包含(标签，联合体)对的类的设计。

Example(示例)

This example is mostly borrowed from TC++PL4 pp216-218. You can look there for an explanation.

这段示例代码主要借用自TC++PL4的216页到218页（中文版：C++程序设计语言(原书第四版)p186-p188）。你可以查看该书中的解释。

The code is somewhat elaborate. Handling a type with user-defined assignment and destructor is tricky. Saving programmers from having to write such code is one reason for including variant in the standard.

这段代码有些复杂。使用用户定义的赋值和析构函数处理一种类型不是那么容易。把程序员从必须编写这样的代码的情况中解救出来是在标准库中增加variant的原因之一。

class Value { // two alternative representations represented as a union
private:
    enum class Tag { number, text };
    Tag type; // discriminant

    union { // representation (note: anonymous union)
        int i;
        string s; // string has default constructor, copy operations, and destructor
    };
public:
    struct Bad_entry { }; // used for exceptions

    ~Value();
    Value& operator=(const Value&);   // necessary because of the string variant
    Value(const Value&);
    // ...
    int number() const;
    string text() const;

    void set_number(int n);
    void set_text(const string&);
    // ...
};

int Value::number() const
{
    if (type != Tag::number) throw Bad_entry{};
    return i;
}

string Value::text() const
{
    if (type != Tag::text) throw Bad_entry{};
    return s;
}

void Value::set_number(int n)
{
    if (type == Tag::text) {
        s.~string();      // explicitly destroy string
        type = Tag::number;
    }
    i = n;
}

void Value::set_text(const string& ss)
{
    if (type == Tag::text)
        s = ss;
    else {
        new(&s) string{ss};   // placement new: explicitly construct string
        type = Tag::text;
    }
}

Value& Value::operator=(const Value& e)   // necessary because of the string variant
{
    if (type == Tag::text && e.type == Tag::text) {
        s = e.s;    // usual string assignment
        return *this;
    }

    if (type == Tag::text) s.~string(); // explicit destroy

    switch (e.type) {
    case Tag::number:
        i = e.i;
        break;
    case Tag::text:
        new(&s) string(e.s);   // placement new: explicit construct
    }

    type = e.type;
    return *this;
}

Value::~Value()
{
    if (type == Tag::text) s.~string(); // explicit destroy
}

使用匿名联合体的好处就是在使用其成员时，不需要A.B的形式。

--译者注

Enforcement(实施建议)

???

原文链接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c181-avoid-naked-unions

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