this is from UniswapV2Pair.sol

token0 and token1 are declared:

address public token0;
address public token1; 

Then in swap function

 function swap( uint amount0Out,uint amount1Out,address to,bytes calldata data) external lock {
            amount0Out > 0 || amount1Out > 0,
        (uint112 _reserve0, uint112 _reserve1, ) = getReserves(); // gas savings
            amount0Out < _reserve0 && amount1Out < _reserve1,
            "UniswapV2: INSUFFICIENT_LIQUIDITY"

        uint balance0;
        uint balance1;
            // scope for _token{0,1}, avoids stack too deep errors
            address _token0 = token0;
            address _token1 = token1;
            require(to != _token0 && to != _token1, "UniswapV2: INVALID_TO");
            if (amount0Out > 0) _safeTransfer(_token0, to, amount0Out); // optimistically transfer tokens
            if (amount1Out > 0) _safeTransfer(_token1, to, amount1Out); // optimistically transfer tokens
            if (data.length > 0)
            balance0 = IERC20(_token0).balanceOf(address(this));
            balance1 = IERC20(_token1).balanceOf(address(this));

Inside function a new scope is opened and comment says: "// scope for _token{0,1}, avoids stack too deep errors". How does it avoid stack-too-deep errors and why Uniswap needs only for "_token{0,1}"? I searched and found out that

Scoping a piece of code with { ... } has the same impact as placing that piece of code in a separate function.

Why did

1 Answer 1


Every function provides a little stack for each function (Stack has a maximum depth of 1024 elements and supports the word size of 256 bits). So, that means that increase in local variables will ultimately fill up the stack fast.

Providing a separate scope for _token0, and _token1 helps in that issue. Since both these variables are just being used in these few lines, they can be put in a different scope, thus getting saved from the stack too deep errors.

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