Return private value from contract

I need help with returning private string value in smart contract. This value has to be available only to paid account.

I need help with returning private string value in smart contract. This value has to be available only to paid account.

I am preloading it inside:

static async getInitialProps(props) {

const privateInfo = await itemContract.methods.getPrivateInfo().call()

}

Smart contract:

string private privateInfo;
mapping(address => bool) public paidAccounts;

...

function getPrivateInfo() public view returns(string) {
require(paidAccounts[msg.sender] == true);
return(privateInfo);
}

I keep getting an error "ERROR: The returned value is not a convertible string:" It seems to be working without require(). Is it because I am calling it without paying for a gas, as it has to make calculation "==" ?

Is there any other option to hide private value and show only for paid accounts?

I appreciate for any kind of suggestions or ideas.

Learning Solidity

Solidity is a high-level language used to implement smart contracts. This is an object oriented language designed to target the Ethereum Virtual Machine. Let's explore it!

Introduction

Solidity is a high-level language used to implement smart contracts. This is an object oriented language designed to target the Ethereum Virtual Machine. Let's explore it!

Let's go!!!

Let's create a file called Contract.sol

First, you must define the version you are using. This is an information the compiler needs.

pragma solidity ^0.4.22;

All code in Ethereum belongs to a Contract. Let's create a contract and define a few variables inside it.

pragma solidity ^0.4.22;

contract DeveloperFactory {
// Let's create a Developer!
uint dnaDigits = 16;
uint ageDigits = 2;
}

Solidity is a typed language. uint stand for Unsigned Integer ( non negative integers ). These variables are state variables. They will be permanently stored in the contract storage ( in the Ethereum Blockchain ). Our Developer has a dna of 16 digits and an age of 2 digits.

Let's keep going!

Struct and arrays

pragma solidity ^0.4.22;

contract DeveloperFactory {
// Let's create a Developer!
uint dnaDigits = 16;
uint ageDigits = 2;

struct Developer {
    string name;
    uint dna;
    uint age;
}

The struct variable allows us to define more complex data structures. Here the Developer struc takes a string called name, a uint called dna and a uint called age.

Solidity also has arrays. You can create dynamic or fixed arrays. Here, our Developer array is dynamic because we do not specify a length. So we can keep adding Developers to our army without any limitations.

Developer[5] public developers is a fixed array that can contain 5 Developer struct.

Functions

A function would look something like this:

pragma solidity ^0.4.22;

contract DeveloperFactory {
// Let's create a Developer!
uint maxAge = 100;
uint minAge = 5;

struct Developer {
    string name;
    uint id;
    uint age;
}

Developer[] public developers;

function _createDeveloper( string _name, uint _id, uint _age ) private{
    developers.push( Developer( _name, _id, _age ) );
}

function _generateRandomId( string _str ) private pure returns (uint){
    uint rand = uint(keccak256(_str));
    return rand;
}

function createRandomDeveloper( string _name, uint _age ) public view {
    require(_age > minAge);
    require(_age < maxAge);
    uint randId = _generateRandomId( _name );
    _createDeveloper(_name, randId, _age );
}

}

We create functions with the function keyword. Functions can take parameters. By default, functions are public. I added the private keyword to make this function private. I also chose to add an underscore before a private function or variable to distinguish them from public variables. You don't have to do this, I just find it easier to read.

Ethereum has the hash function keccak256 built in. This is a version of SHA3. Pass it any string and you get a 256-bit hexadecimal number.

As you can see, we are typecasting the keccak256 value into a uint value and we return it.

Aside from the private keyword, there are several things you can add to a function:

  • The function returns something: Use the returns keyword and specify the type the function returns. Here, it will return a uint type.
  • The view keyword means that our function needs to look at some of our contract's variables, but not modify them. Our function createRandomDeveloper needs to look at minAge and maxAge variables.
  • The pure keyword means that the function is not accessing any data in the app. It only returns something depending on its parameters. Our _generateRandomId is pure.

We have three functions. _generateRandomId generates a random Id for our developer by using the keccak256 built in function. _createDeveloper creates and pushes a new Developer struct into our developers array. createRandomDeveloper is the only public function. It checks if the age provided is correct. The require statements will throw errors if it is not the case ( age greater than 100 and lower than 5 in our case ). So, this last function is the one that can be called from outside our contract.

Events

You can also create events so you can communicate what happens on the blockchain to your front-end app. Your app would then listen to those events and react accordingly.

pragma solidity ^0.4.22;

contract DeveloperFactory {
// Let's create a Developer!

event NewDeveloper(uint devId, string name, uint age);

uint maxAge = 100;
uint minAge = 5;

struct Developer {
    string name;
    uint id;
    uint age;
}

Developer[] public developers;

function _createDeveloper( string _name, uint _id, uint _age ) private{
    uint id = developers.push( Developer( _name, _id, _age ) ) - 1;
    newDeveloper(id, _name, _age);
}

function _generateRandomId( string _str ) private pure returns (uint){
    uint rand = uint(keccak256(_str));
    return rand;
}

function createRandomDeveloper( string _name, uint _age ) public view {
    require(_age > minAge);
    require(_age < maxAge);
    uint randId = _generateRandomId( _name );
    _createDeveloper(_name, randId, _age );
}

}

Mappings

After struct and arrays, we can also store data in mappings. Mappings are a key-value store. For example:

mapping(uint => string) public keyUintStringValue;
mapping(address => uint) public addressToUint;

Here, we have two public mappings. The first one has a uint key and a stringvalue. The second has an address key and a uint value.

Addresses

The Ethereum blockchain is made up of addresses. Each account has an unique address. It takes the following form: 0x0cE440255306E921F41612C46F1v6df9Cc969183. Each address has a certain amount of Ether, which is the cryptocurrency used on the blockchain, and can receive or send Ether to other addresses.

With that in mind, let's create a new mapping for our DeveloperFactory:

pragma solidity ^0.4.22;

contract DeveloperFactory {

event NewDeveloper(uint devId, string name, uint age);

uint maxAge = 100;
uint minAge = 5;

struct Developer {
    string name;
    uint id;
    uint age;
}

Developer[] public developers;

mapping (address => uint) public ownerDevCount;
mapping (uint => address) public devToOwner;

function _createDeveloper( string _name, uint _id, uint _age ) private{
    uint id = developers.push( Developer( _name, _id, _age ) ) - 1;
    NewDeveloper(id, _name, _age);
}

function _generateRandomId( string _str ) private pure returns (uint){
    uint rand = uint(keccak256(_str));
    return rand;
}

function createRandomDeveloper( string _name, uint _age ) public view {
    require(_age > minAge);
    require(_age < maxAge);
    uint randId = _generateRandomId( _name );
    _createDeveloper(_name, randId, _age );
}

}

In the first mapping, we will keep track of the number of devs each account ( address ) created. In the second, we will keep track of the owners for each dev.

msg.sender

Each contract is passive, they don't do anything until someone triggers them. msg.sender is a global variable that allows us to know which address is responsible for the triggering. It could be a account or another smart contract.

With that information, we can update our mappings appropriately. In the _createDeveloper function, we will increase the ownerDevCount for this particular address. In the devToOwner mapping, we will indicate that the newly created developer is owned by the msg.sender address.

pragma solidity ^0.4.22;

contract DeveloperFactory {

event NewDeveloper(uint devId, string name, uint age);

uint maxAge = 100;
uint minAge = 5;

struct Developer {
    string name;
    uint id;
    uint age;
}

Developer[] public developers;

mapping (address => uint) public ownerDevCount;
mapping (uint => address) public devToOwner;

function _createDeveloper( string _name, uint _id, uint _age ) private{
    uint id = developers.push( Developer( _name, _id, _age ) ) - 1;
    ownerDevCount[msg.sender]++;
    devToOwner[id] = msg.sender;
    NewDeveloper(id, _name, _age);
}

function _generateRandomId( string _str ) private pure returns (uint){
    uint rand = uint(keccak256(_str));
    return rand;
}

function createRandomDeveloper( string _name, uint _age ) public view {
    require(_age > minAge);
    require(_age < maxAge);
    uint randId = _generateRandomId( _name );
    _createDeveloper(_name, randId, _age );
}

}

Notice the ++ notation to increase the ownerDevCount[msg.sender], just live Javascript!

Inheritance and import

Contracts can inherit from other contracts. If you open a new file call DeveloperLearning.sol, you can make it inherit from DeveloperFactory.sol:

pragma solidity ^0.4.22;

import "./DeveloperFactory.sol";

contract DeveloperLearning is DeveloperFactory {
// I have now access to DeveloperFactory functions
}

Notice how we imported the DeveloperFactory contract ( assuming it was in the same folder ). To declare inheritance, we use the keyword is.

payable

In order for a contract to receive ether, we need to have the payable keyword to a function. The amount sent will be accessible in the global variable msg.value. So we could make sure that a certain amount of ether is sent to the contract before the creation of a developer:

pragma solidity ^0.4.22;

contract DeveloperFactory {

event NewDeveloper(uint devId, string name, uint age);

uint maxAge = 100;
uint minAge = 5;

struct Developer {
    string name;
    uint id;
    uint age;
}

Developer[] public developers;

mapping (address => uint) public ownerDevCount;
mapping (uint => address) public devToOwner;

function _createDeveloper( string _name, uint _id, uint _age ) private{
    uint id = developers.push( Developer( _name, _id, _age ) ) - 1;
    ownerDevCount[msg.sender]++;
    devToOwner[id] = msg.sender;
    NewDeveloper(id, _name, _age);
}

function _generateRandomId( string _str ) private pure returns (uint){
    uint rand = uint(keccak256(_str));
    return rand;
}

function createRandomDeveloper( string _name, uint _age ) public payable {
    require(_age > minAge);
    require(_age < maxAge);
    require(msg.value == 5);
    uint randId = _generateRandomId( _name );
    _createDeveloper(_name, randId, _age );
}

}

Memory and Storage

In Solidity, there are two places where variables are stored: in storage or in memory. A variable stored in memory is temporary, it exists while the function is used, then it is discarded. A variable stored in storage exists permanently on the blockchain. Most of the time, you don't have to worry about where to store your variables, as Solidity handles it for you.

For example, state variables ( maxAge, minAge, Developer ), declared outside of functions, are stored in storage. Variables like randId, id, rand are stored in memory.

But, in some cases, you want to explicitly declare where to store certain variables. Solidity allows you to do that with memory and storagekeywords.




how to initialize Empty array in stuct [Solidity]

I am suffering to initialize an empty array for struct when a struct is made.

I am suffering to initialize an empty array for struct when a struct is made.

pragma solidity ^0.5.1;

contract Board {

//storage
Post[] posts;

//struct
struct Post {
    address author;
    string title;
    string content;
    Comment[] comments;
}

struct Comment {
    address author;
    string comment;
}

//add-post
function addPost(address _author, string memory _title, string memory _content) public {
    posts.push(Post(_author, _title, _content, /* HERE IS THE PROBLEM POINT */));
}

}

I want to initialize the comments(struct member) with Empty Array(Type: Comment). Which code should I use for The Problem Point??