C++核心準(zhǔn)則C.127:包含虛函數(shù)的類應(yīng)該有虛析構(gòu)函數(shù)或保護(hù)析構(gòu)函...

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C.127: A class with a virtual function should have a virtual or protected destructor

C.127:包含虛函數(shù)的類應(yīng)該有虛析構(gòu)函數(shù)或保護(hù)析構(gòu)函數(shù)?

Reason(原因)

A class with a virtual function is usually (and in general) used via a pointer to base. Usually, the last user has to call delete on a pointer to base, often via a smart pointer to base, so the destructor should be public and virtual. Less commonly, if deletion through a pointer to base is not intended to be supported, the destructor should be protected and nonvirtual; see?C.35.

包含虛函數(shù)的類通常(大多數(shù)情況下)通過指向基類的指針使用。通常，最后一個(gè)使用者必須通過指向基類的指針調(diào)用delete操作，通常是指向基類的智能指針，因此析構(gòu)函數(shù)應(yīng)該是公開的虛函數(shù)。稍微特殊一些的情況是：如果不希望支持通過指向基類的指針銷毀對象，析構(gòu)函數(shù)應(yīng)該是保護(hù)的非虛函數(shù)。參見C.35。

Example, bad(反面示例)

struct?B?{
????virtual?int?f()?=?0;
????//?...?no?user-written?destructor,?defaults?to?public?nonvirtual?...
};

//?bad:?derived?from?a?class?without?a?virtual?destructor
struct?D?:?B?{
????string?s?{"default"};
};

void?use()
{
????unique_ptr?p?=?make_unique();
????//?...
}?//?undefined?behavior.?May?call?B::~B?only?and?leak?the?string

Note(注意)

There are people who don't follow this rule because they plan to use a class only through a shared_ptr: std::shared_ptr p = std::make_shared(args); Here, the shared pointer will take care of deletion, so no leak will occur from an inappropriate delete of the base. People who do this consistently can get a false positive, but the rule is important -- what if one was allocated using make_unique? It's not safe unless the author of B ensures that it can never be misused, such as by making all constructors private and providing a factory function to enforce the allocation with make_shared.

也有一些人計(jì)劃只通過shared_ptr使用類：std::shared_ptr p=

std::make_shared(args);這段代碼中共享指針會負(fù)責(zé)對象的銷毀，因此不會因?yàn)椴贿m當(dāng)?shù)匿N毀操作而引起內(nèi)存泄露。一直這么做的人會產(chǎn)生一個(gè)錯(cuò)誤的判斷。但是準(zhǔn)則還是重要的--如果某人使用make_unique申請內(nèi)存會怎么樣？這種做法不夠安全，除非B的生成者可以確保它永遠(yuǎn)不會被誤用，例如通過讓所有的構(gòu)造函數(shù)都私有而且提供一個(gè)工廠方法保證所有的內(nèi)存分配都通過make_shared進(jìn)行。

Enforcement(實(shí)施建議)

• A class with any virtual functions should have a destructor that is either public and virtual or else protected and nonvirtual.

  包含虛函數(shù)的類的析構(gòu)函數(shù)要么是公開的虛函數(shù)，要么是保護(hù)的非虛函數(shù)。

• Flag delete of a class with a virtual function but no virtual destructor.

  提示針對包含虛函數(shù)卻沒有虛析構(gòu)函數(shù)的類的銷毀操作。

原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#enforcement-123

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