C++核心准则R.22: 使用make_shared()构建共享shared_ptr
R.22: Use make_shared() to make shared_ptrs
R.22: 使用make_shared()构建共享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.
如果你首先构建一个对象然后将它交给shared_ptr的构造函数,和使用make_shared的情况相比,你(很有可能)多执行了一次分配动作(和将要发生的一次释放动作)。因为参照计数(此处应该是shared_ptr对象,译者注)的分配必须和对象的分配分别进行。
Example(示例)
Consider(考虑下面的代码):
shared_ptr p1 { new X{2} }; // bad
auto p = make_shared(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对象,因此它通常比显式调用new的代码更简短(执行也更高效)。
Enforcement(实施建议)
(Simple) Warn if a shared_ptr is constructed from the result of new rather than make_shared.
(简单)如果使用new的结果构造shared_ptr而不是make_shared,报警。
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#r22-use-make_shared-to-make-shared_ptrs
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