R.22: Use make_shared() to make shared_ptrs
R.22: 使用make_shared()構建共享shared_ptr
Reason(原因)
If you first make an object and then give it to a shared_ptr constructor, you (most likely) do one more allocation (and later deallocation) than if you use make_shared() because the reference counts must be allocated separately from the object.
如果你首先構建一個對象然後將它交給shared_ptr的構造函數,和使用make_shared的情況相比,你(很有可能)多執行了一次分配動作(和將要發生的一次釋放動作)。因爲參照計數(此處應該是shared_ptr對象,譯者注)的分配必須和對象的分配分別進行。
Example(示例)
Consider(考慮下面的代碼):
shared_ptr<X> p1 { new X{2} }; // bad
auto p = make_shared<X>(2); // good
The make_shared() version mentions X only once, so it is usually shorter (as well as faster) than the version with the explicit new.
make_shared方式只是用一次X對象,因此它通常比顯式調用new的代碼更簡短(執行也更高效)。
Enforcement(實施建議)
(Simple) Warn if a shared_ptr is constructed from the result of new rather than make_shared.
(簡單)如果使用new的結果構造shared_ptr而不是make_shared,報警。
原文鏈接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#r22-use-make_shared-to-make-shared_ptrs
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