programming - why set values in this way doesn't work?

question is as follows

define a list

ttt={1,2};

and if I set values in this way

{ttt[[1]],ttt[[2]]}={3,4}

then the value of list ttt now becomes {3,4}

Now I try this

f:={ttt[[1]],ttt[[2]]}

then ?f will show that f:={tmp[[1]],tmp[[2]]}

then if I just write

f={3,4}

this will not affect the values of list ttt .

the question is that I want the values of list ttt changes as well when I set values to f. How to do this?

---

More Question!!

why the following code didn't change the value of ttt??

Table[With[{i = i}, Defer@ttt[[i]]], {i, 1, 2}] = {111, 222}

while

Table[With[{i = i}, Defer@ttt[[i]]], {i, 1, 2}]

really gives

{ttt[[1]],ttt[[2]]}

Answer

I would approach this by combining the functionality of bump given in:
Elegant manipulation of the variables list
with my step evaluation function described here:
How do I evaluate only one step of an expression?

The step function is needed to (easily) recover the expression {ttt[[1]], ttt[[2]]} from the definition of f without it fully evaluating. It can be used like this, with the injector pattern:

ttt = {1, 2};
f := {ttt[[1]], ttt[[2]]}


step[f] /. _[x_] :> (x = {3, 4})

ttt

{3, 4}

To make this more convenient, and to work with other functions (see the first question linked above for examples) I would then write (after loading step):

func_[boost[expr_], arg___] ^:= step[expr] /. _[x_] :> func[x, arg]

Example of use:

boost[f] = {5, 6};

ttt

{5, 6}