Title: Ethereum: Calls to self failing when passing structs

Introduction

Ethereum is a decentralized platform that allows developers to build and deploy smart contracts, which are self-executing contracts with the terms of the agreement written directly into lines of code. However, one common issue that can arise in Ethereum is when a contract calls itself using the .call method and attempts to pass a struct (a data structure) as an argument.

The Problem

When a contract calls itself using the .call method, it typically expects a specific data type, such as an integer or a string. However, when passing a struct as an argument, Ethereum’s Solidity compiler throws an error due to its 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 issues when trying to call a function on a contract, as the contract may attempt to access the struct’s members or try to assign a value to one of its members.

The Issue

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

To fix 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");

}

The Solution

To resolve this issue, developers can use the following approaches:

  • Create a new function that returns an array: As shown above, create a new function that takes in data and returns an array of values.

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

pragma solidity ^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 for working with structs in Solidity.

Conclusion

In conclusion, passing a struct to a contract using the .call method can result in an error due to Ethereum’s lack of support for structs. To solve this issue, developers need to create new functions that return arrays instead of structs or use libraries that provide functions for working 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

Ethereum: Calls to self failing when passing structs

  • Solidity documentation: [

  • OpenZeppelin documentation: [

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

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دسته‌ها: CRYPTOCURRENCY