C++核心準(zhǔn)則C.182:使用匿名聯(lián)合體實(shí)現(xiàn)附帶標(biāo)簽的聯(lián)合體

C.182: Use anonymous?unions to implement tagged unions

C.182:使用匿名聯(lián)合體實(shí)現(xiàn)附帶標(biāo)簽的聯(lián)合體

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.

良好設(shè)計(jì)的命名聯(lián)合體是類型安全的。無名聯(lián)合體簡(jiǎn)化了包含(標(biāo)簽，聯(lián)合體)對(duì)的類的設(shè)計(jì)。

Example(示例)

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

這段示例代碼主要借用自TC++PL4的216頁(yè)到218頁(yè)（中文版：C++程序設(shè)計(jì)語(yǔ)言(原書第四版)p186-p188）。你可以查看該書中的解釋。

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.

這段代碼有些復(fù)雜。使用用戶定義的賦值和析構(gòu)函數(shù)處理一種類型不是那么容易。把程序員從必須編寫這樣的代碼的情況中解救出來是在標(biāo)準(zhǔn)庫(kù)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
}

使用匿名聯(lián)合體的好處就是在使用其成員時(shí)，不需要A.B的形式。

--譯者注

Enforcement(實(shí)施建議)

???

原文鏈接：

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

覺得本文有幫助？請(qǐng)分享給更多人。

關(guān)注【面向?qū)ο笏伎肌枯p松學(xué)習(xí)每一天！

面向?qū)ο箝_發(fā)，面向?qū)ο笏伎迹?/span>