定制new和delete
# 定制new和delete
# Item 49：了解new-handler的行为 Understand the behavior of the new-handler.
- new handler的行为（处理new失败时候throw的异常）， 底层是怎样构成的？
# Item 50：了解new和delete的合理替换时机 Understand when it makes sense to replace new and delete.
- operator new和operator delete的合理替换时机；
# Item 51：编写new和delete时需固守常规 Adhere to convention when writing new and delete.
- operator new和operator delete需要遵循的一些规则
# Item 52：写了placement new也要写placement delete Write placement delete if you write placement new.
- placement new和placement delete