C++核心準則C.30:如果一個類需要明確的銷毀動作,定義析構函數(shù)

C.30: Define a destructor if a class needs an explicit action at object destruction

如果一個類需要明確的銷毀動作，定義析構函數(shù)

Reason（原因）

A destructor is implicitly invoked at the end of an object's lifetime. If the default destructor is sufficient, use it. Only define a non-default destructor if a class needs to execute code that is not already part of its members' destructors.

析構函數(shù)在對象的生命周期結束時被隱式調用。如果默認的析構函數(shù)已經(jīng)足夠，沒有必要另外定義。只有在一個類需要其成員析構函數(shù)處理之外的動作時定義非默認的析構函數(shù)。

Example（示例）

template
struct final_action {   // slightly simplified
    A act;
    final_action(A a) :act{a} {}
    ~final_action() { act(); }
};

template
final_action finally(A act)   // deduce action type
{
    return final_action{act};
}

void test()
{
    auto act = finally([]{ cout << "Exit test\n"; });  // establish exit action
    // ...
    if (something) return;   // act done here
    // ...
} // act done here

The whole purpose of final_action is to get a piece of code (usually a lambda) executed upon destruction.

final_action唯一的目的就是讓一段代碼（通常是lambda表達式）在final_action被銷毀時執(zhí)行。

Note（注意）

There are two general categories of classes that need a user-defined destructor:

通常有兩種情況類需要用戶定義析構函數(shù)。

• A class with a resource that is not already represented as a class with a destructor, e.g., a vector or a transaction class.

  類管理的資源沒有表現(xiàn)為包含析構函數(shù)的類。例如vector或者事務類。

• A class that exists primarily to execute an action upon destruction, such as a tracer or final_action.

  類存在的主要目的就是在析構時執(zhí)行某個動作。例如tracer和final_action。
  

Example, bad（反面示例）

class Foo {   // bad; use the default destructor
public:
    // ...
    ~Foo() { s = ""; i = 0; vi.clear(); }  // clean up
private:
    string s;
    int i;
    vector vi;
};

The default destructor does it better, more efficiently, and can't get it wrong.

默認的析構函數(shù)可以做得更好，更有效，還不會有錯。

Note（注意）

If the default destructor is needed, but its generation has been suppressed (e.g., by defining a move constructor), use =default.

如果需要默認析構函數(shù)，但是其產生已經(jīng)被抑制（例如由于定義了移動構造函數(shù)），使用=default（明確要求生成，譯者注）。

Enforcement（實施建議）

Look for likely "implicit resources", such as pointers and references. Look for classes with destructors even though all their data members have destructors.

尋找可能的“隱式資源”，例如指針和引用。尋找有析構函數(shù)的類，即使它們所有的數(shù)據(jù)成員都有析構函數(shù)。

原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c30-define-a-destructor-if-a-class-needs-an-explicit-action-at-object-destruction

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