C++核心準則C.151:使用make_shared建構shared_ptr管理的對象

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

C.151:使用make_shared建構shared_ptr管理的對象

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為構造動作提供了更加簡明的表達。由于它将shared_ptr的計數置于對象之後，使用它還可以提供減少另外一次的增加計數的機會。

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(實施建議)

• Flag the repetitive usage of template specialization list<Bar>
• 提示重複使用模闆特化參數清單的情況
• Flag variables declared to be shared_ptr<Bar>
• 提示使用shared_ptr直接定義變量的情況。