make_shared vs shared_ptr ctor:
See debates here:
c++11 – Difference in make_shared and normal shared_ptr in C++ – Stack Overflow
总结：
- make_shared 只涉及一次堆内存分配，shared_ptr ctor 涉及两次（new the Control Block. new an Object to be managed.）;
- 详见图（不支持 non-public 构造函数），~~虽然我觉得这个纯属是抬杠~~；
- 从 Efficiency 的角度来说，make_shared 不一定更优，从异常安全（Exception Safety）的角度来说，make_shared 会更好.
What is Linkage?
c++ – What is external linkage and internal linkage? – Stack Overflow
