hyperledger-solang_solang/docs/language/managing_values.rst

Managing values
===============

Sending and receiving value
___________________________

Value in Solidity is represented by ``uint128``.

.. note::

    On Polkadot, contracts can be compiled with a different type for ``T::Balance``. If you
    need support for a different type, please raise an
    `issue <https://github.com/hyperledger-solang/solang/issues>`_.

.. _balance:

Checking your balance
_____________________

.. tabs::

    .. group-tab:: Polkadot

        Polkadot cannot check the balance for contracts other than the current
        one. If you need to check the balance of another contract, then add a balance
        function to that contract like the one below, and call that function instead.

        .. code-block:: solidity

            function balance() public returns (uint128) {
                return address(this).balance;
            }

    .. group-tab:: Solana

        On Solana, the balance of an account can be accessed using the ``lamports`` member of the ``AccountInfo``
        struct. Every account whose value we want to check must be declared with an account annotation.

        .. code-block::

            @account(my_acc)
            function balance() external returns (uint64) {
                return tx.accounts.my_acc.lamports;
            }

Creating contracts with an initial value
________________________________________

You can specify the value you want to be deposited in the new contract by
specifying ``{value: 100 ether}`` before the constructor arguments. This is
explained in :ref:`sending value to the new contract <sending_values>`.

Sending value with an external call
___________________________________

You can specify the value you want to be sent along with the function call by
specifying ``{value: 100 ether}`` before the function arguments. This is
explained in :ref:`passing value and gas with external calls <passing_value_gas>`.

.. _send_transfer:

Sending value using ``send()`` and ``transfer()``
_________________________________________________

.. tabs::

    .. group-tab:: Polkadot

        The ``send()`` and ``transfer()`` functions are available as method on a
        ``address payable`` variable. The single arguments is the amount of value you
        would like to send. The difference between the two functions is what happens
        in the failure case: ``transfer()`` will revert the current call, ``send()``
        returns a ``bool`` which will be ``false``.

        In order for the receiving contract to receive the value, it needs a ``receive()``
        function, see :ref:`fallback() and receive() function <fallback_receive>`.

        Here is an example:

        .. code-block:: solidity

            contract A {
                B other;

                constructor() {
                    other = new B();

                    bool complete = payable(other).transfer(100);

                    if (!complete) {
                        // oops
                    }

                    // if the following fails, our transaction will fail
                    other.send(100);
                }
            }

            contract B {
                receive() payable external {
                    // ..
                }
            }

        .. note::
            On Subtrate, this uses the ``seal_transfer()`` mechanism rather than ``seal_call()``, since this
            does not come with gas overhead. This means the ``receive()`` function is not required in the
            receiving contract, and it will not be called if it is present. If you want the ``receive()``
            function to be called, use ``address.call{value: 100}("")`` instead.

    .. group-tab:: Solana

        On Solana, there are no ``transfer`` and ``send`` functions. In order to alter the balance of accounts,
        one might increment or decrement the ``lamports`` field from the ``AccountInfo`` struct directly. This
        is only possible if the accounts whose balance is being changed are owned by the program.

        .. code-block::

            @mutableAccount(acc1)
            @mutableAccount(acc2)
            function transfer(uint64 amount) external {
                tx.accounts.acc1.lamports += amount;
                tx.accounts.acc2.lamports -= amount;
            }