C++核心準(zhǔn)則C.33:如果類包含擁有所有權(quán)的指針成員,定義析構(gòu)函數(shù)

C.33: If a class has an owning pointer member, define a destructor

C.33:如果類包含擁有所有權(quán)的指針成員，定義析構(gòu)函數(shù)

Reason（原因）

An owned object must be deleted upon destruction of the object that owns it.

從屬對(duì)象必須通過擁有該對(duì)象的所有者類的析構(gòu)函數(shù)銷毀。

Example（示例）

A pointer member may represent a resource.A T* should not do so, but in older code, that's common. Consider a T* a possible owner and therefore suspect.

指針成員可能用于表達(dá)某個(gè)資源。T*不應(yīng)該這么做，但是在舊一些的代碼中，這種做法很常見?？紤]到T*作為所有者使用的可能性，并確認(rèn)。

template
class Smart_ptr {
 ? ?T* p; ? // BAD: vague about ownership of *p
 ? ?// ...
public:
 ? ?// ... no user-defined default operations ...
};

void use(Smart_ptr p1)
{
 ? ?// error: p2.p leaked (if not nullptr and not owned by some other code)
 ? ?auto p2 = p1;
}

Note that if you define a destructor, you must define or delete all default operations:

注意：一旦定義了析構(gòu)函數(shù)，就必須定義或者禁止所有的默認(rèn)操作。

譯者注：這里的默認(rèn)操作指的是默認(rèn)構(gòu)造函數(shù)，拷貝/移動(dòng)構(gòu)造函數(shù)，拷貝/移動(dòng)運(yùn)算符和析構(gòu)函數(shù)。

template
class Smart_ptr2 {
 ? ?T* p; ? // BAD: vague about ownership of *p
 ? ?// ...
public:
 ? ?// ... no user-defined copy operations ...
 ? ?~Smart_ptr2() { delete p; } ?// p is an owner!
};

void use(Smart_ptr2 p1)
{
 ? ?auto p2 = p1; ? // error: double deletion
}

The default copy operation will just copy the p1.p into p2.p leading to a double destruction of p1.p. Be explicit about ownership:

默認(rèn)拷貝操作只是將p1.p的值賦給p2.p（不包含其指向?qū)ο蟮目截悾@會(huì)導(dǎo)致p1.p的雙重析構(gòu)。明確所有權(quán)：

template
class Smart_ptr3 {
 ? ?owner p; ? // OK: explicit about ownership of *p
 ? ?// ...
public:
 ? ?// ...
 ? ?// ... copy and move operations ...
 ? ?~Smart_ptr3() { delete p; }
};

void use(Smart_ptr3 p1)
{
 ? ?auto p2 = p1; ? // OK: no double deletion
}

譯者注：實(shí)際上并不是改變p的類型為owner就可以解決問題的。注意這段代碼通過注釋實(shí)現(xiàn)了拷貝和移動(dòng)操作，而前一段代碼沒有。*>
Note（注意）

Often the simplest way to get a destructor is to replace the pointer with a smart pointer (e.g., std::unique_ptr) and let the compiler arrange for proper destruction to be done implicitly.

一般來說，得到析構(gòu)函數(shù)最簡(jiǎn)單的方式是將指針換成智能指針（例如std::unique_ptr）并且讓編譯器提供適當(dāng)?shù)碾[式執(zhí)行的析構(gòu)動(dòng)作。

Note（注意）

Why not just require all owning pointers to be "smart pointers"? That would sometimes require non-trivial code changes and may affect ABIs.

為什么不簡(jiǎn)單地要求所有的所有者指針都變成“智能指針”？因?yàn)槟菢幼鲇袝r(shí)會(huì)引起重大的代碼變更并且影響二進(jìn)制接口。

Enforcement（實(shí)施建議）

• A class with a pointer data member is suspect.

  帶有指針類型數(shù)據(jù)成員的類都是可疑的。

• A class with an owner should define its default operations.

  擁有owner成員的類應(yīng)該定義默認(rèn)操作。

譯者注：owner的定義就是T，只是在源代碼層次上增加了信息量，方便讀者理解和工具檢查。編譯器看起來和之前的T沒有任何區(qū)別，因此在二進(jìn)制生成物層面上沒有變化，這樣就保證了既有代碼可以安全地引入這種做法。
原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c33-if-a-class-has-an-owning-pointer-member-define-a-destructor

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