C.182: Use anonymous unions to implement tagged unions
C.182:使用匿名聯合體實現附帶標籤的聯合體
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.
良好設計的命名聯合體是類型安全的。無名聯合體簡化了包含(標籤，聯合體)對的類的設計。
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頁到218頁（中文版：C++程序設計語言(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.
這段代碼有些複雜。使用用戶定義的賦值和析構函數處理一種類型不是那麼容易。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準庫中增加variant的原因之一。
<code>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
}/<code>使用匿名聯合體的好處就是在使用其成員時，不需要A.B的形式。--譯者注
Enforcement(實施建議)
???
原文鏈接：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c181-avoid-naked-unions
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