[1] The reason for the name "adaptable function object" is that adaptable function objects may be used by function object adaptors.

[2] The unary_function and binary_function bases are similar to the input_iterator, output_iterator, forward_iterator, bidirectional_iterator, and random_access_iterator bases: they are completely empty, and serve only to provide type information.

[3] This is an example of how to use function objects; it is not the recommended way of calculating the sum of elements in a vector. The accumulate algorithm is a better way of calculating a sum.