Proxies allow to extend the functionality of a smart contract by making it essentialy modifiable. One of the key properties of smart contracts is their immutability, however there have been many examples where this has been a problem in the past. 

For instance, a protocol identifying a serious bug while deployed on mainnet, and not being able to fix it. Or a hacker exploting a bug and no way to remediate the issue (btw, [pausing](https://github.com/OpenZeppelin/cairo-contracts/blob/main/src/openzeppelin/security/pausable/library.cairo) functionality would be really useful here too). Sure, one could re-deploy the contract, but what about the contract storage? 

Due to this, knowing how to safely implement the proxy pattern can be a great tool in a developer's arsenal. 

Yet again, OpenZeppelin made everyone's life easier and implemented a standard [Proxy pattern](https://docs.openzeppelin.com/contracts-cairo/0.4.0b/proxies) which anyone can use to make their contracts upgreadable. 

Here is the default Proxy preset provided by [OZ](https://github.com/OpenZeppelin/cairo-contracts/blob/main/src/openzeppelin/upgrades/presets/Proxy.cairo). This is very simple, has the `__default__` function (which is like our fallback in Solidity), a `L1 handler`, and a constructor. Before deploying this contract, developers should declare the contract class of their implementation contract. After, they can deploy the Proxy contract and pass the implementation_hash as parameter to the constructor.

The actual Proxy library contract contains a couple of `storage_var` as seen below:

```javascript 
@storage_var
func Proxy_implementation_hash() -> (class_hash: felt){
}

@storage_var
func Proxy_admin() -> (proxy_admin: felt){
}

@storage_var
func Proxy_initialized() -> (initialized: felt){
}
```

Always remember to not deploy your implementation contract (the one that contains the actual logic of your protocol). Instead this should be `declared` as a `contract_class` with:

`starknet declare --contract contract_compiled.json`

One other important piece of advice to keep in mind is to use the [initializer](https://github.com/OpenZeppelin/cairo-contracts/blob/main/src/openzeppelin/security/initializable/library.cairo) "modifier" for the initialization function of an implementation contract. This will prevent your contract from being initialized twice (which could be a very big problem).

ctrlc03

What is the proxy pattern and how it can help make my smart contracts upgradable in Cairo Language? Is there any standard lib for that?

Newton

What is the proxy pattern and how it can help make my smart contracts upgradable in Cairo Language?

Hello. Does starknet have name service? what is the difference between starknet id and starknet name service? Are they same projects?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Does starknet have name service? 

Hey folks, do you know how I could read a felt* from ap?

I want to do something like this:

```rs
    let (hello : felt*) = alloc();
    tempvar world : felt* = hello;
    let world_like : felt* = [ap-1];
```

when I do tempvar world = hello, I ensure I add hello (the pointer) to [ap] and then I increase ap so world_like = [ap-1] should be the same thing than world

But I get this error:

```
Cannot assign an expression of type 'felt' to a reference of type 'felt*'.
    let world_like : felt* = [ap-1];
```

When doing a similar code:

```rust
        let (func_pc: felt*) = get_label_location(storage_var);
        _prepare_call(pedersen_ptr, range_check_ptr, len_inputs, inputs + len_inputs);
        call abs ap[11 - 6 * 2];
```

I get ```
Cannot apply subscript-operator to non-pointer, non-tuple type 'felt'.
        call abs ap[11 - 6 * 2];  // removing alloc_locals by calling ap[11-6*2] ?
```

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

how I could read a felt* from ap in Cairo lang?

In the following function I am presented with the following error, even though my code does not make it to this function. What am I doing wrong?


`func fill_in_row{output_ptr: felt*}(row_idx : felt, row_size : felt, curr_row : felt*) {` 


 `if (row_idx == row_size-2) {`


 `return ();`


 `}`


 `let val : felt = curr_row[row_idx-1] + curr_row[row_idx];`


 `serialize_word(val);`


 `curr_row[row_idx+1] = val;`


 `return fill_in_row(row_idx+1, row_size, curr_row);`


`}`


Error: code:80:5: While expanding the reference 'curr_row' in:


 curr_row[row_idx+1] = val;


 ^******^


code:72:73: Expected a register. Found: [fp + (-4)].


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/1156)_

Cairo Error: Getting Error while setting value of item in array

Hello! This is day 14 of the 17 days of the Cairo Challenge. And I have no idea how to solve the [playground](https://www.cairo-lang.org/playground/) exercise “Hints”.  Perhaps you can help me?

```
// The following function uses hints to efficiently compute the square root `res`
// of the argument `n`.
// The idea is that the verifier only needs to be convinced that res * res = n,
// it doesn't really care how res was computed.
// In such cases, we don't have to compute res in pure Cairo - we can write a
// piece of Python code inside the Cairo program, which is called a "hint".
// A hint is a piece of code that the prover runs to initialize
// some memory cells. Note that it is completely transparent from the verifier's
// point of view, so the result of the hint *must* be verified using pure Cairo
// instructions (e.g., the "assert n = res * res" instruction below).
//
// 1. Comment out the line "assert n = res * res;" and run the code. Does it still output
//    the expected value? Can you explain why the assert is nonetheless essential?
//    Hint: Recall that from the verifier's point of view, the hint does not exist.
// 2. Uncomment the assert line and change the function to compute the fourth
//    root.

%builtins output

from starkware.cairo.common.serialize import serialize_word

// Computes the square root (over the integers) of `n`.
// Prover assumption: The square root exists.
func sqrt(n) -> (res: felt) {
    alloc_locals;
    local res;

    // Set the value of res using a Python hint.
    %{
        import math

        # Use the ids variable to access the value of a Cairo variable.
        ids.res = int(math.sqrt(ids.n))
    %}

    // From the verifier's point of view, the hint is completely transparent.
    // The following line guarantees that `res` is the square root
    // (either the positive or negative) of `n`.
    assert n = res * res;
    return (res=res);
}

func main{output_ptr: felt*}() {
    let (res) = sqrt(256);
    serialize_word(res);
    return ();
}
```

---

> Answers to this question are a part of the ✨ 17 days of Cairo Lang with Playground & Newton. ✨
> 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Newton-550dfcda6a2d449ca468be293c776578)*

How to use Hints in Cairo Lang?

Can I transpile any solidity version?

Does AA support "Session Keys" so I can avoid signing too many transactions when they are redundant?

---

This question is participating in [Newton Bounties | Incentivized Test Program](https://www.notion.so/Newton-Bounties-Incentivized-Test-Program-94322ad48e7d40a3adcf36f6b26174cd) Every verified valid question gets $10 USDC (mainnet)

Does Account Abstraction support "Session Keys"?

How can I send a Uint256 amount of ERC20 tokens from L1 to starknet?
I have "amount" as a uint256 on L1 but how should I build my payload for "`sendMessageToL2`" to match the Uint256 format of Cairo?

---

This question is participating in [Newton Bounties | Incentivized Test Program](https://www.notion.so/Newton-Bounties-Incentivized-Test-Program-94322ad48e7d40a3adcf36f6b26174cd) Every verified valid question gets $10 USDC (mainnet)

How can I send a Uint256 amount of ERC20 tokens from L1 to starknet? And how should I build my payload for "sendMessageToL2" to match the Uint256 format of Cairo?

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What is the proxy pattern and how it can help make my smart contracts upgradable in Cairo Language?

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