Ethereum: Can we encode a structure in calldata when calling another function?
In Ethereum, when calling another function within a contract, you can pass arguments in a variety of ways. However, a common limitation is that the data type we can pass to functions cannot be directly encoded in calldata. This is because calldata stores values in memory and requires them to be of a specific type, which may not match the data types passed to the function.
In this article, we will explore whether it is possible to encode a structure (i.e. an object) in calldata when we call another function within an Ethereum contract.
The problem: encodings
Let’s consider the example provided at the beginning. We have a “structure Price” and a function “isPriceOkay” that takes a “Calldata Price” argument:
“solidity”
structure Price {
uint value;
}
function isPriceOkay(Price calldata price) public pure returns (bool) {
return …
}
Notice how we are trying to pass the entire struct instance ("price") as an argument. However, in Solidity, if you want to pass an object of a specific type (like "struct Price"), you can use a "value" field to store it in memory.
Unfortunately, when calling another function within a contract using calldata, we cannot simply assign the entire struct instance to the calldata value. Instead, we must use a combination of indexing and assignments to achieve this.
Workarounds: Indexing and Assignments
Here are two common workarounds that allow us to encode a structure in calldata when we call another function:
Indexing: We can define an index on the "Price" structure inside the calldata value, like this:
solidity
struct Price {
uint value;
}
function isPriceOkay(Price[] memory price) public pure returns (bool) {
return …
}
By defining an array of price with a size equal to the number of Price structures we want to encode, we can access each element by its index (0, 1, etc.). However, this approach still requires manual indexing and assignment inside the function call.
Using data keyword
: We can also use data keyword in Solidity 0.6.0 and later versions to store values directly in calldata. This allows us to define a struct-like data structure where each element is stored as an integer rather than as separate variables:
solidity
struct Price {
uint value;
}
function isPriceOkay(Price[] memory price) public pure returns (bool) {
return …
}
In this case, we can just pass the price argument directly to calldata, without any indexing or assignment:
solidity
contract MyContract {
function myFunction() public pure returns () {
// Pass price as argument here…
isPriceOkay(price);
}
}
“
Keep in mind, however, that usingdataintroduces additional complexity and may not be the best approach for all use cases.
Conclusion
In summary, while it is technically possible to encode a structure in calldata when calling another function within an Ethereum contract, doing so requires careful consideration of indexing, assignment, or using workarounds such as thedata` keyword. These approaches can be useful in certain scenarios where you need to store complex data structures directly in calldata.
When deciding whether to use these workarounds, consider factors such as:
- Size and structure of the data you want to encode
- Your contract’s performance and gas efficiency requirements
- Complexity and readability of your codebase
Ultimately, it is important to weigh the pros and cons of each approach before choosing the best solution for your specific use case.
