C++核心準(zhǔn)則C.44:默認(rèn)構(gòu)造函數(shù)最好簡單而且不會(huì)拋出異常

C.44: Prefer default constructors to be simple and non-throwing

C.44:默認(rèn)構(gòu)造函數(shù)最好簡單而且不會(huì)拋出異常

Reason（原因）

Being able to set a value to "the default" without operations that might fail simplifies error handling and reasoning about move operations.

默認(rèn)構(gòu)造函數(shù)可以將內(nèi)容設(shè)置到默認(rèn)狀態(tài)而不需要可能引起失敗的操作，簡化了錯(cuò)誤處理和針對移動(dòng)操作的推測。

Example, problematic（問題示例）

template
// elem points to space-elem element allocated using new
class Vector0 {
public:
 ? ?Vector0() :Vector0{0} {}
 ? ?Vector0(int n) :elem{new T[n]}, space{elem + n}, last{elem} {}
 ? ?// ...
private:
 ? ?own elem;
 ? ?T* space;
 ? ?T* last;
};

This is nice and general, but setting a Vector0 to empty after an error involves an allocation, which may fail. Also, having a default Vector represented as {new T[0], 0, 0} seems wasteful. For example, Vector0 v[100] costs 100 allocations.

這段代碼整潔且普通，但是如果過在涉及到內(nèi)存分配的錯(cuò)誤之后生成一個(gè)空的Vector0對象時(shí)，可能會(huì)失敗。同時(shí)，讓默認(rèn)的Vector表現(xiàn)為{new T[0], 0, 0}有些浪費(fèi)。例如Vector0v[100]需要100次內(nèi)存分配。

100次內(nèi)存分配似乎有些問題。譯者的理解是只要一次分配100個(gè)整數(shù)的空間就好。

另外new T[0]有點(diǎn)奇怪。

Example（示例）

template
// elem is nullptr or elem points to space-elem element allocated using new
class Vector1 {
public:
 ? ?// sets the representation to {nullptr, nullptr, nullptr}; doesn't throw
 ? ?Vector1() noexcept {}
 ? ?Vector1(int n) :elem{new T[n]}, space{elem + n}, last{elem} {}
 ? ?// ...
private:
 ? ?own elem = nullptr;
 ? ?T* space = nullptr;
 ? ?T* last = nullptr;
};

Using {nullptr, nullptr, nullptr} makes Vector1{} cheap, but a special case and implies run-time checks. Setting a Vector1 to empty after detecting an error is trivial.

使用{nullptr, nullptr, nullptr}讓Vector1{}的代價(jià)更小，但是特殊的情況，（由于產(chǎn)生了沒有數(shù)據(jù)的情況，譯者注）需要運(yùn)行時(shí)檢查。在檢測到錯(cuò)誤之后將Vector1設(shè)為空的處理是小事情。

Enforcement（實(shí)施建議）

• Flag throwing default constructors.

• 提示拋出異常的構(gòu)造函數(shù)。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c44-prefer-default-constructors-to-be-simple-and-non-throwing

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