The formatter need not be an ordinary function. 格式化器不必是一个普通函数。
The most common usage of these templates is in function object adapters, thus allowing them to adapt plain functions as well as function objects. 这些模板最常见的用法是用在函数对象适配器中，以使得适配器可以象适配函数对象一样适配普通函数。
