🍣 add some boilerplates

master_solidity/boilerplates/learning/token.sol

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+pragma solidity ^0.5.10;
+
+contract Token {
+    // An `address` is comparable to an email address - it's used to identify an account on Ethereum.
+    // Addresses can represent a smart contract or an external (user) accounts.
+    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#address
+    address public owner;
+
+    // A `mapping` is essentially a hash table data structure.
+    // This `mapping` assigns an unsigned integer (the token balance) to an address (the token holder).
+    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#mapping-types
+    mapping (address => uint) public balances;
+
+    // Events allow for logging of activity on the blockchain.
+    // Ethereum clients can listen for events in order to react to contract state changes.
+    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#events
+    event Transfer(address from, address to, uint amount);
+
+    // Initializes the contract's data, setting the `owner`
+    // to the address of the contract creator.
+    constructor() public {
+        // All smart contracts rely on external transactions to trigger its functions.
+        // `msg` is a global variable that includes relevant data on the given transaction,
+        // such as the address of the sender and the ETH value included in the transaction.
+        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
+        owner = msg.sender;
+    }
+
+    // Creates an amount of new tokens and sends them to an address.
+    function mint(address receiver, uint amount) public {
+        // `require` is a control structure used to enforce certain conditions.
+        // If a `require` statement evaluates to `false`, an exception is triggered,
+        // which reverts all changes made to the state during the current call.
+        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions
+
+        // Only the contract owner can call this function
+        require(msg.sender == owner, "You are not the owner.");
+
+        // Enforces a maximum amount of tokens
+        require(amount < 1e60, "Maximum issuance exceeded");
+
+        // Increases the balance of `receiver` by `amount`
+        balances[receiver] += amount;
+    }
+
+    // Sends an amount of existing tokens from any caller to an address.
+    function transfer(address receiver, uint amount) public {
+        // The sender must have enough tokens to send
+        require(amount <= balances[msg.sender], "Insufficient balance.");
+
+        // Adjusts token balances of the two addresses
+        balances[msg.sender] -= amount;
+        balances[receiver] += amount;
+
+        // Emits the event defined earlier
+        emit Transfer(msg.sender, receiver, amount);
+    }
+}