What does {...} as a part of a function call mean?

Question

In the function swapExactETHForTokens of Uniswap is called like this:

 uniswap.swapExactETHForTokens{value: msg.value}(
            amountOut,
            path,
            msg.sender,
            deadline
        );

What does the part {value: msg.value} mean? Is this really correct syntasis at all? Meaning, the {...} itself in a function call.

And does the function not accept the value argument in the normal way, inside (...)?

Answer 1

That's how you can call a function and provide value to it.

The concept of value refers to Ethers sent along the call. The value is taken from the contract balance. You can't provide the value as a regular parameter inside () in Solidity if you want to send Ether, and not just some uint which happens to be called value.

Here's an example. If you call demo (remember to include value when calling the function, otherwise the contract doesn't have any value to forward), the some function call doesn't forward any value. But other call forwards 6 weis. Note also that functions need to be declared payable in order to be able to receive value.

contract A {   
    function demo() public payable {
        B inst = new B();
        inst.some(5);
        inst.other{value: 6}();
    }
}
contract B {
    uint private _value;
    function some(uint value) public {
        _value = value;
    }
    function other() payable public {
    }
}

You can see the syntax for example here: https://docs.soliditylang.org/en/latest/control-structures.html#external-function-calls