C++核心準則C.22:保持默認操作的一貫性

C.22: Make default operations consistent

C.22:?保持默認操作的一貫性

Reason（原因）

The default operations are conceptually a matched set. Their semantics are interrelated. Users will be surprised if copy/move construction and copy/move assignment do logically different things. Users will be surprised if constructors and destructors do not provide a consistent view of resource management. Users will be surprised if copy and move don't reflect the way constructors and destructors work.

默認操作從概念上講是配合嚴密的一整套處理。它們的語義是相互關(guān)聯(lián)的。如果拷貝/移動構(gòu)造和拷貝/移動賦值做的是邏輯上不同的事情，用戶會感到詫異；如果構(gòu)造函數(shù)和析構(gòu)函數(shù)沒有為資源管理提供一致的想法，用戶會感到詫異；如果拷貝和移動操作沒有對應(yīng)構(gòu)造函數(shù)和析構(gòu)函數(shù)的動作，用戶會感到詫異。

Example, bad（反面示例）

class Silly {   // BAD: Inconsistent copy operations
    class Impl {
        // ...
    };
    shared_ptr p;
public:
    Silly(const Silly& a) : p{a.p} { *p = *a.p; }   // deep copy
    Silly& operator=(const Silly& a) { p = a.p; }   // shallow copy
    // ...
};

These operations disagree about copy semantics. This will lead to confusion and bugs.

這些操作（拷貝構(gòu)造和賦值）關(guān)于拷貝的語義不同（風(fēng)別是深拷貝和淺拷貝）。這會導(dǎo)致困惑和錯誤。

Enforcement（實施建議）

• (Complex) A copy/move constructor and the corresponding copy/move assignment operator should write to the same member variables at the same level of dereference.

  (復(fù)雜) 拷貝/移動構(gòu)造函數(shù)和對應(yīng)的拷貝/移動賦值運算符應(yīng)該以同樣的的解引用級別寫入同樣的成員變量。

• (Complex) Any member variables written in a copy/move constructor should also be initialized by all other constructors.

  (復(fù)雜）在拷貝/移動構(gòu)造函數(shù)中寫入的任何成員變量也應(yīng)該被其他的構(gòu)造函數(shù)初始化。

• (Complex) If a copy/move constructor performs a deep copy of a member variable, then the destructor should modify the member variable.

  (復(fù)雜)如果拷貝/移動構(gòu)造函數(shù)對成員變量進行深拷貝，那么析構(gòu)函數(shù)應(yīng)該修改該成員變量。

• (Complex) If a destructor is modifying a member variable, that member variable should be written in any copy/move constructors or assignment operators.

• (復(fù)雜)如果析構(gòu)函數(shù)修改某個成員變量，那么這個成員變量應(yīng)該在拷貝/移動構(gòu)造函數(shù)或者賦值運算符中被寫入。

原文鏈接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c22-make-default-operations-consistent

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