C++核心准则C.66:保证移动操作不会抛出异常

C.66: Make move operations noexcept

C.66:保证移动操作不会抛出异常

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.

抛出异常的移动操作会破坏大多数人的符合逻辑的推测。不会抛出异常的移动可以被标准库和C++语言更加高效地使用。

Example（示例）

template<typename T>
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.

这些操作都不会抛出异常。

Example, bad（反面示例）

template<typename T>
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的问题不止是效率低，由于拷贝时需要分配内存，它可能还会抛出异常。

Enforcement（实施建议）

(Simple) A move operation should be marked noexcept.

(简单)移动操作应该被声明为noexcept。

原文链接

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

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