C++核心準(zhǔn)則C.66:保證移動(dòng)操作不會(huì)拋出異常

斜方砷鐵礦與方解石

C.66: Make move operations noexcept

C.66:保證移動(dòng)操作不會(huì)拋出異常

Reason（原因）

A throwing move violates most people's reasonably assumptions. A non-throwing move will be used more efficiently by standard-library and language facilities.

拋出異常的移動(dòng)操作會(huì)破壞大多數(shù)人的符合邏輯的推測。不會(huì)拋出異常的移動(dòng)可以被標(biāo)準(zhǔn)庫和C++語言更加高效地使用。

Example（示例）

template
class Vector {
public:
 ? ?Vector(Vector&& a) noexcept :elem{a.elem}, sz{a.sz} { a.sz = 0; a.elem = nullptr; }
 ? ?Vector& operator=(Vector&& a) noexcept { elem = a.elem; sz = a.sz; a.sz = 0; a.elem = nullptr; }
 ? ?// ...
private:
 ? ?T* elem;
 ? ?int sz;
};

These operations do not throw.

這些操作都不會(huì)拋出異常。

Example, bad（反面示例）

template
class Vector2 {
public:
 ? ?Vector2(Vector2&& a) { *this = a; } ? ? ? ? ? ? // just use the copy
 ? ?Vector2& operator=(Vector2&& a) { *this = a; } ?// just use the copy
 ? ?// ...
private:
 ? ?T* elem;
 ? ?int sz;
};

This Vector2 is not just inefficient, but since a vector copy requires allocation, it can throw.

Vector2的問題不止是效率低，由于拷貝時(shí)需要分配內(nèi)存，它可能還會(huì)拋出異常。

Enforcement（實(shí)施建議）

(Simple) A move operation should be marked noexcept.

(簡單)移動(dòng)操作應(yīng)該被聲明為noexcept。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c66-make-move-operations-noexcept

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