Ethereum: Is it possible to encode a struct to calldata when calling another function?

Ethereum: Can we encode a struct to 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 encoded directly into 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 struct (i.e., an object) to calldata when calling another function within an Ethereum contract.

The Problem: Encodings

Let’s consider the example provided at the beginning. We have a struct Price and a function isPriceOkay that takes a calldata price argument:

structure Price {
unit value;
}
function isPriceOkay(Price calldata price) public pure returns (bool) {
return ...
}

Notice how we are trying to pass the entire instance of the struct (price) as an argument. However, in Solidity, when 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 inside a contract using calldata, we cannot simply assign the entire instance of the struct to the calldata value. Instead, we need to use a combination of indexing and assignments to achieve this.

Workarounds: Indexing and Assignments

Here are two common workarounds that allow us to hardcode a struct to calldata when calling another function:

• Indexing: We can define an index for the Price struct inside the calldata value, like this:

struct Price {
unit 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 structs we want to encode, we can access each element using its index (0, 1, etc.). However, this approach still requires manual indexing and assignment within the function call.

• Using the data keyword: We can also use the data keyword in Solidity 0.6.0 and later 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:

struct Price {
unitvalue;
}
function isPriceOkay(Price[] memory price) pure public returns (bool) {
return ...
}

In this case, we can simply pass the price argument directly to calldata without any indexing or assignment:

contract MyContract {
function myFunction() pure public returns () {
// Pass the price as an argument here...
isPriceOkay(price);
}
}

However, keep in mind that using data introduces additional complexity and may not be the best approach for all use cases.

Conclusion

In conclusion, while it is technically possible to hardcode a struct to calldata when calling another function within an Ethereum contract, this requires careful consideration of indexing, assignment, or using workarounds like the data keyword. These approaches can be useful in specific scenarios where you need to store complex data structures directly in calldata.

When deciding whether to use these alternative solutions, consider factors such as:

• The size and structure of the data you are trying to encode

• The performance and gas efficiency needs of your contract

• The complexity and readability of your codebase

Ultimately, it is essential to weigh the pros and cons of each approach before choosing the best solution for your specific use case.

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