C++核心准则C.151:使用make_shared构建shared_ptr管理的对象

C.151: Use make_shared() to construct objects owned by shared_ptrs

C.151:使用make_shared构建shared_ptr管理的对象

Reason(原因)

make_shared gives a more concise statement of the construction. It also gives an opportunity to eliminate a separate allocation for the reference counts, by placing the shared_ptr's use counts next to its object.

make_shared为构造动作提供了更加简明的表达。由于它将shared_ptr的计数置于对象之后，使用它还可以提供减少另外一次的增加计数的机会。

Example(示例)

void test() {
    // OK: but repetitive; and separate allocations for the Bar and shared_ptr's use count
    shared_ptr<Bar> p {new Bar{7}};

    auto q = make_shared<Bar>(7);   // Better: no repetition of Bar; one object
}      

Enforcement(实施建议)

• Flag the repetitive usage of template specialization list<Bar>
• 提示重复使用模板特化参数列表的情况
• Flag variables declared to be shared_ptr<Bar>
• 提示使用shared_ptr直接定义变量的情况。