result_of 用于推導可調用對象的返回類型。
大致定義：
template
<class Ty
> struct result_of {
typedef T0 type;
};
Ty: 可調用類型，
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是個R(*)(U1, U2, ..., UN) 函數指針，那么返回類型是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是個R(&)(U1, U2, ..., UN) 函數指針，那么返回類型是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是個R(U1::*)(U2, ..., UN)成員函數指針，那么返回類型是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是個R U1::* 成員變量指針，那么返回類型是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是一個類并且其中定義了result_type ，那么返回類型是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函數不帶有任何的參數，那么返回類型是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是個類并且有個成員模板是result,那么返回類型是Fty::result<T1, T2, ..., TN>::type
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in all other cases it is an error.
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其它情況就會出錯