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I am just starting in blockchain development and learning . I have created some tokens and deployed on Ethereum blockchain.

I can easily transfer my token by ETH in my wallets. But now my requirement is after every successful transaction 0.5% token will be burn. I have written some code for this but nothing happened. Or if burn process is working then how can i check burning token?

I am using this code.

pragma solidity ^0.4.4;

contract Token {
    function totalSupply() constant returns (uint256 supply) {}

    /// @param _owner The address from which the balance will be retrieved
    /// @return The balance
    function balanceOf(address _owner) constant returns (uint256 balance) {}

    /// @notice send `_value` token to `_to` from `msg.sender`
    /// @param _to The address of the recipient
    /// @param _value The amount of token to be transferred
    /// @return Whether the transfer was successful or not
    function transfer(address _to, uint256 _value) returns (bool success) {}

    /// @notice send `_value` token to `_to` from `_from` on the condition it is approved by `_from`
    /// @param _from The address of the sender
    /// @param _to The address of the recipient
    /// @param _value The amount of token to be transferred
    /// @return Whether the transfer was successful or not
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {}

    /// @notice `msg.sender` approves `_addr` to spend `_value` tokens
    /// @param _spender The address of the account able to transfer the tokens
    /// @param _value The amount of wei to be approved for transfer
    /// @return Whether the approval was successful or not
    function approve(address _spender, uint256 _value) returns (bool success) {}

    /// @param _owner The address of the account owning tokens
    /// @param _spender The address of the account able to transfer the tokens
    /// @return Amount of remaining tokens allowed to spent
    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {}

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

}

contract StandardToken is Token {

    function transfer(address _to, uint256 _value) returns (bool success) {
        //Default assumes totalSupply can't be over max (2^256 - 1).
        //If your token leaves out totalSupply and can issue more tokens as time goes on, you need to check if it doesn't wrap.
        //Replace the if with this one instead.
        //if (balances[msg.sender] >= _value && balances[_to] + _value > balances[_to]) {
        if (balances[msg.sender] >= _value && _value > 0) {
            balances[msg.sender] -= _value;
            balances[_to] += _value;
            Transfer(msg.sender, _to, _value);
            return true;
        } else { return false; }
    }

    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
        //same as above. Replace this line with the following if you want to protect against wrapping uints.
        //if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && balances[_to] + _value > balances[_to]) {
        if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && _value > 0) {
            balances[_to] += _value;
            balances[_from] -= _value;
            allowed[_from][msg.sender] -= _value;
            Transfer(_from, _to, _value);
            return true;
        } else { return false; }
    }

    function balanceOf(address _owner) constant returns (uint256 balance) {
        return balances[_owner];
    }

    function approve(address _spender, uint256 _value) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
      return allowed[_owner][_spender];
    }

    mapping (address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;
    uint256 public totalSupply;
}

contract NSDCCoin is StandardToken { // CHANGE THIS. Update the contract name.

    /* Public variables of the token */

    /*
    NOTE:
    The following variables are OPTIONAL vanities. One does not have to include them.
    They allow one to customise the token contract & in no way influences the core functionality.
    Some wallets/interfaces might not even bother to look at this information.
    */
    string public name;                   // Token Name
    uint8 public decimals;                // How many decimals to show. To be standard complicant keep it 18
    string public symbol;                 // An identifier: eg SBX, XPR etc..
    string public version = 'H1.0'; 
    uint256 public unitsOneEthCanBuy;     // How many units of your coin can be bought by 1 ETH?
    uint256 public totalEthInWei;         // WEI is the smallest unit of ETH (the equivalent of cent in USD or satoshi in BTC). We'll store the total ETH raised via our ICO here.  
    address public fundsWallet;           // Where should the raised ETH go?

    // This notifies clients about the amount burnt
    event Burn(address indexed from, uint256 value);
    // This is a constructor function 
    // which means the following function name has to match the contract name declared above
    function NSDCCoin() {
        uint256 total_coins = 10000 * 10 ** uint256(18);
        balances[msg.sender] = total_coins;               // Give the creator all initial tokens. This is set to 1000 for example. If you want your initial tokens to be X and your decimal is 5, set this value to X * 100000. (CHANGE THIS)
        totalSupply = total_coins;                        // Update total supply (1000 for example) (CHANGE THIS)
        name = "NSDC";                                   // Set the name for display purposes (CHANGE THIS)
        decimals = 18;                                               // Amount of decimals for display purposes (CHANGE THIS)
        symbol = "NS$";                                             // Set the symbol for display purposes (CHANGE THIS)
        unitsOneEthCanBuy = 300;                                      // Set the price of your token for the ICO (CHANGE THIS)
        fundsWallet = msg.sender;                                    // The owner of the contract gets ETH
    }

    function() payable{
        totalEthInWei = totalEthInWei + msg.value;
        uint256 amount = msg.value * unitsOneEthCanBuy;
        if (balances[fundsWallet] < amount) {
            return;
        }

        balances[fundsWallet] = balances[fundsWallet] - amount;
        balances[msg.sender] = balances[msg.sender] + amount;

        Transfer(fundsWallet, msg.sender, amount); // Broadcast a message to the blockchain

        //Transfer ether to fundsWallet
        fundsWallet.transfer(msg.value);                               
    }

    /* Approves and then calls the receiving contract */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);

        //call the receiveApproval function on the contract you want to be notified. This crafts the function signature manually so one doesn't have to include a contract in here just for this.
        //receiveApproval(address _from, uint256 _value, address _tokenContract, bytes _extraData)
        //it is assumed that when does this that the call *should* succeed, otherwise one would use vanilla approve instead.
        if(!_spender.call(bytes4(bytes32(sha3("receiveApproval(address,uint256,address,bytes)"))), msg.sender, _value, this, _extraData)) { throw; }
        return true;
    }
    
    /**
     * Destroy tokens
     *
     * Remove `_value` tokens from the system irreversibly
     *
     * @param _value the amount of money to burn
     */
    function burn(uint256 _value) public returns (bool success) {
        require(balances[msg.sender] >= _value);   // Check if the sender has enough
        balances[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        Burn(msg.sender, _value);
        return true;
    }

    /**
     * Destroy tokens from other account
     *
     * Remove `_value` tokens from the system irreversibly on behalf of `_from`.
     *
     * @param _from the address of the sender
     * @param _value the amount of money to burn
     */
    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balances[_from] >= _value);                // Check if the targeted balance is enough
        require(_value <= allowed[_from][msg.sender]);    // Check allowance
        balances[_from] -= _value;                         // Subtract from the targeted balance
        allowed[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
        totalSupply -= _value;                              // Update totalSupply
        Burn(_from, _value);
        return true;
    }
}
2
  • Where your burning? Feb 14 '18 at 12:55
  • Actually i don't know how can i achieve burn process. I just found burn code in ethereum site and use it. Can you please explain about burn process.
    – Jitendra
    Feb 14 '18 at 13:03
2

You need to override functions transfer and transferFrom in your NSDCCoin smart contract like this:

function transfer(address _to, uint256 _value) returns (bool success) {
  uint256 toBurn = _value / 200;

  if (StandardToken.transfer (_to, _value - toBurn)) {
    require (burn (toBurn));
    return true;
  } else return false;
}

function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
  uint256 toBurn = _value / 200;

  if (StandardToken.transferFrom (_from, _to, _value - toBurn)) {
    require (burnFrom (_from, toBurn));
    return true;
  } else return false;
}

As you may see, overridden functions calculate number of tokens to be burned and reduce _value by this calculated number before passing to to original functions. Then, in case of successful transfer, they burn tokens that ought to be burned.

1

It depends on what you need. If you're planning for some sort of proof-of-burn, I know only one coin that has it: Slimcoin. There's lots of theory (and some practical stuff also) at https://en.bitcoin.it/wiki/Proof_of_burn

But if you just want to burn some coins, then you really don't need any Burn method. You don't implicitly need to "burn" anything - just lower the balances and that's it.

2
  • Thanks for this clarification. can you please told me one thing, currently i am working on test mode. So for deploy my token i placed my source code on ethereumscan. and after deploy if i want to do some changes in it then how can i do it?
    – Jitendra
    Feb 14 '18 at 13:23
  • 1
    After you deploy your contract, it's immutable. You can't change it. If you have further questions, please open a new question. Feb 14 '18 at 13:27
1

I am not sure why you need such functionality but If you want to burn 0.5% of amount on every transfer, you can modify your transfer function to be:

 function transferAndBurn(address _to, uint256 _value) public returns (bool){
            // calculate percentage
            uint burn_token = (_value*50)/100;
            require(_value+burn_token > _value);
            require( balances[msg.sender] >= _value+burn_token);
            balances[msg.sender] = balances[msg.sender] - _value  ;
            balances[_to] += _value;
            require( burn(burn_token));
            return true;
        }

Or implement same logic to whenever you want to burn some tokens. Just note that some of the checks are omitted for simpliciy.

2
  • Thanks @prashant. i am working on testing mode yet. so can you please guide me, after deploy my token's sourcecode on ethereumscan, if i want to do some change in my token then how can i edit on ethereumscan or there are any other way to do this?
    – Jitendra
    Feb 14 '18 at 13:19
  • 1
    Value*50/100 seems to be the 50% of burned...
    – Rick Park
    Dec 16 '18 at 1:30

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