C.84: A swap function may not fail
C.84:swap函數不應該失敗
Reason（原因）
swap is widely used in ways that are assumed never to fail and programs cannot easily be written to work correctly in the presence of a failing swap. The standard-library containers and algorithms will not work correctly if a swap of an element type fails.
swap函數被廣泛地使用的方式就是假設它永遠不會失敗，而且也很難寫出即使swap出錯也能正常動作的程序。標準庫容器和算法在元素交換失敗時也無法正常工作。
Example, bad（反面示例）
void swap(My_vector& x, My_vector& y)
{
auto tmp = x; // copy elements
x = y;
y = tmp;
}
This is not just slow, but if a memory allocation occurs for the elements in tmp, this swap may throw and would make STL algorithms fail if used with them.
這段代碼的問題不僅是慢，而且如果因爲tmp的元素髮生了內存申請，如果使用它的話，這個swap可能拋出異常並令STL算法失敗。
Enforcement（實施建議）
(Simple) When a class has a swap member function, it should be declared noexcept.
(簡單)如果類包含swap成員函數，它應該被聲明爲noexcept。
原文鏈接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c84-a-swap-function-may-not-fail
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