A pointer to a function, a pointer to member function, a pointer to member
data, or a class type whose objects can appear immediately to the left
of a function call operator.
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function pointer types
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member (function or data) pointer types
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all kinds of function objects
& a_free_function
& a_class::a_static_member_function
& a_class::a_nonstatic_data_member
& a_class::a_nonstatic_member_function
std::less<int>()
bind(std::less<int>(), _1, 5)
lambda::_1 += lambda::_2;
fusion::make_fused_function_object(std::less<int>())