can i declare a 2D array as following ?

For a 2D array just have one range in the brackets, instead of two.

type dataout is array (6 downto 0) of std_logic_vector(7 downto 0);

This will result in an array of 7 std_logic_vectors of range 7 downto 0.

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For a 2D array just have one range in the brackets, instead of two.

type dataout is array (6 downto 0) of std_logic_vector(7 downto 0);

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Nope, thats not a 2d array, thats a 1d array of 1d array. So you access it like this:

output <= dout(x).

A 2d array is declared like the OP posted. He made a 2d array of 1d arrays. You would access it like this:

output <= dout(x,y); --and then add 2nd set of brackets to get individual bits

thanks a lot my friend!

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Nope, thats not a 2d array, thats a 1d array of 1d array. So you access it like this:

output <= dout(x).

A 2d array is declared like the OP posted. He made a 2d array of 1d arrays. You would access it like this:

output <= dout(x,y); --and then add 2nd set of brackets to get individual bits

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i wanna initialize this vector, i have a 2d matrix of std_logic_vector

can i write like this:

constant d : dataout :=(
                -- Content           -- Address(i,j)
                "00111100",        -- 0,0               "11110000"    --0,1               "01100010"   --0,2                 "01100010"   --0,3           ...
                         
                "01100010",        -- 1,0               "10011000",   --1,1              "10111000"   --1,2               "00111000"    --1,3           ...
                      
                "00111100",        -- 2,0               "11110000"   --2,1                "01100010"  --2,2                 "01100010"   --2,3            ...
               
                   ........