Most synthesizers will instantiate adder structures if you write:

library ieee;
use ieee.numeric_std.all;

signal a: unsigned(8 downto 0); 0 to 2**9-1
signal b: unsigned(7 downto 0); 0 to 2**8-1
signal b: unsigned(7 downto 0); 0 to 2**8-1

a <= b + c;

Note, that the signal a, holding the result, must be one bit wider than 
the signals holding operands.

Consider - which means read about - using the packages numeric_std (in 
contrast to std_logic_vector).

In case you want to see explicit code of an adder, search the internet. 
I am not sure, but I assume that somebody posted such code just for 
educational purposes.