Ethereum: Calls to self-fail when passing structures

Title: Ethereum: Calls to themselves fail when passing structs

Introduction

Ethereum is a decentralized platform that allows developers to create and deploy smart contracts, which are self-executing contracts with the terms of the agreement written directly into lines of code. One common problem that can occur in Ethereum is when a contract is called using the “.call” method and tries to pass a struct (data structure) as an argument.

Problem

When a contract is called using the “.call” method, it typically expects a specific data type, such as an integer or string. However, when passing a struct as an argument, the Ethereum Solidity compiler throws an error due to the lack of support for structs in the same way that arrays do.

The resulting error output is:

Error: Failed to decode output: Error: data type not allowed: 'struct'

This can cause significant problems when trying to call a function on a contract, as the contract may try to access members of the struct or attempt to assign a value to one of its members.

Problem

In Solidity 0.6.0 and later, the data keyword has been removed from structs. This means that you can no longer pass structs directly as arguments to functions when using .call. Instead, you must use arrays or other data structures to represent the fields of a struct.

To resolve this issue, developers need to create a new function that returns an array instead of a struct:

pragma solidity ^ 0,6,0;
struct MyStruct {
uint256 x;
string y;
}
function myFunction() public pure returns (MyStruct) {
return MyStruct(x: 1, y: "Hello");
}

Solution

Developers can use the following approaches to resolve this issue:

• Create a new function that returns an array

  

  : As mentioned above, create a new function that takes the data and returns an array of values.

• Use arrays instead of structs: Replace the “MyStruct” struct with an array:

pragma strength ^ 0,6,0;
struct MyArray {
uint256 x;
string y;
}
function myFunction() public pure returns (MyArray) {
return MyArray(x: 1, y: "Hello");
}

• Use libraries that support structs: Some libraries, such as OpenZeppelin, provide functions to work with structs in Solidity.

Conclusion

Finally, passing a struct to a contract using the .call method can lead to an error due to the lack of support for structs in Ethereum. To resolve this issue, developers must create new functions that return arrays instead of structs, or use libraries that provide functions to work with structs. By following these steps, developers can ensure that their contracts work correctly and do not encounter errors when passing structs as arguments.

Additional Resources

• Solidity Documentation: [

• OpenZeppelin Documentation: [

Note: This article is intended to provide a general overview of the problem and possible solutions, but may not cover all possible scenarios or edge cases.