练习12.5：我们未编写接受一个initializer_list explicit（参见7.5.4节，第264页）参数的构造函数。讨论这个设计策略的优点和缺点。
My: 优点是可以直接使用花括号列表来隐式转换成StrBlob（隐式转换机制）。
Pros：
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The compiler will not use this constructor in an automatic conversion.
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We can realize clearly which class we have used.
Cons:
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We always uses the constructor to construct a temporary StrBlob object.
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cannot use the copy form of initialization with an explicit constructor. not easy to use.