result_of 用于推导可调用对象的返回类型。
大致定义：
template
<class Ty
> struct result_of {
typedef T0 type;
};
Ty: 可调用类型，
The template class defines its member type as a synonym for the return type of a function call described by its template argument Ty. The template argument must be of the form Fty(T1, T2, ..., TN), where Fty is a callable type. The template determines the return type according to the first of the following rules that applies:
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if Fty is a pointer to function type R(*)(U1, U2, ..., UN) the return type is R;
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如果Fty是个R(*)(U1, U2, ..., UN) 函数指针，那么返回类型是R
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if Fty is a reference to function type R(&)(U1, U2, ..., UN) the return type is R;
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如果Fty是个R(&)(U1, U2, ..., UN) 函数指针，那么返回类型是R
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if Fty is a pointer to member function type R(U1::*)(U2, ..., UN) the return type is R;
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如果Fty是个R(U1::*)(U2, ..., UN)成员函数指针，那么返回类型是R
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if Fty is a pointer to data member type R U1::* the return type is R;
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如果Fty是个R U1::* 成员变量指针，那么返回类型是R
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if Fty is a class with a member typedef result_type the return type is Fty::result_type;
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如果Fty是一个类并且其中定义了result_type ，那么返回类型是Fty::result_type
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if N is 0 (that is, Ty is of the form Fty()) the return type is void;
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如果Fty函数不带有任何的参数，那么返回类型是void
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if Fty is a class with a member template named result the return type is Fty::result<T1, T2, ..., TN>::type;
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如果Fty是个类并且有个成员模板是result,那么返回类型是Fty::result<T1, T2, ..., TN>::type
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in all other cases it is an error.
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其它情况就会出错