solidity/docs/control-structures.rst

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Expressions and Control Structures
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.. index:: ! parameter, parameter;input, parameter;output

Input Parameters and Output Parameters
======================================

As in Javascript, functions may take parameters as input;
unlike in Javascript and C, they may also return arbitrary number of
parameters as output.

Input Parameters
----------------

The input parameters are declared the same way as variables are. As an
exception, unused parameters can omit the variable name.
For example, suppose we want our contract to
accept one kind of external calls with two integers, we would write
something like::

    contract Simple {
        function taker(uint _a, uint _b) {
            // do something with _a and _b.
        }
    }

Output Parameters
-----------------

The output parameters can be declared with the same syntax after the
``returns`` keyword. For example, suppose we wished to return two results:
the sum and the product of the two given integers, then we would
write::

    contract Simple {
        function arithmetics(uint _a, uint _b) returns (uint o_sum, uint o_product) {
            o_sum = _a + _b;
            o_product = _a * _b;
        }
    }

The names of output parameters can be omitted.
The output values can also be specified using ``return`` statements.
The ``return`` statements are also capable of returning multiple
values, see :ref:`multi-return`.
Return parameters are initialized to zero; if they are not explicitly
set, they stay to be zero.

Input parameters and output parameters can be used as expressions in
the function body.  There, they are also usable in the left-hand side
of assignment.

.. index:: if, else, while, do/while, for, break, continue, return, switch, goto

Control Structures
===================

Most of the control structures from JavaScript are available in Solidity
except for ``switch`` and ``goto``. So
there is: ``if``, ``else``, ``while``, ``do``, ``for``, ``break``, ``continue``, ``return``, ``? :``, with
the usual semantics known from C or JavaScript.

Parentheses can *not* be omitted for conditionals, but curly brances can be omitted
around single-statement bodies.

Note that there is no type conversion from non-boolean to boolean types as
there is in C and JavaScript, so ``if (1) { ... }`` is *not* valid
Solidity.

.. _multi-return:

Returning Multiple Values
-------------------------

When a function has multiple output parameters, ``return (v0, v1, ...,
vn)`` can return multiple values.  The number of components must be
the same as the number of output parameters.

.. index:: ! function;call, function;internal, function;external

.. _function-calls:

Function Calls
==============

Internal Function Calls
-----------------------

Functions of the current contract can be called directly ("internally"), also recursively, as seen in
this nonsensical example::

    contract C {
        function g(uint a) returns (uint ret) { return f(); }
        function f() returns (uint ret) { return g(7) + f(); }
    }

These function calls are translated into simple jumps inside the EVM. This has
the effect that the current memory is not cleared, i.e. passing memory references
to internally-called functions is very efficient. Only functions of the same
contract can be called internally.

External Function Calls
-----------------------

The expressions ``this.g(8);`` and ``c.g(2);`` (where ``c`` is a contract
instance) are also valid function calls, but this time, the function
will be called "externally", via a message call and not directly via jumps.
Please note that function calls on ``this`` cannot be used in the constructor, as the
actual contract has not been created yet.

Functions of other contracts have to be called externally. For an external call,
all function arguments have to be copied to memory.

When calling functions of other contracts, the amount of Wei sent with the call and
the gas can be specified with special options ``.value()`` and ``.gas()``, respectively::

    contract InfoFeed {
        function info() payable returns (uint ret) { return 42; }
    }


    contract Consumer {
        InfoFeed feed;
        function setFeed(address addr) { feed = InfoFeed(addr); }
        function callFeed() { feed.info.value(10).gas(800)(); }
    }

The modifier ``payable`` has to be used for ``info``, because otherwise, the `.value()`
option would not be available.

Note that the expression ``InfoFeed(addr)`` performs an explicit type conversion stating
that "we know that the type of the contract at the given address is ``InfoFeed``" and
this does not execute a constructor. Explicit type conversions have to be
handled with extreme caution. Never call a function on a contract where you
are not sure about its type.

We could also have used ``function setFeed(InfoFeed _feed) { feed = _feed; }`` directly.
Be careful about the fact that ``feed.info.value(10).gas(800)``
only (locally) sets the value and amount of gas sent with the function call and only the
parentheses at the end perform the actual call.

Function calls cause exceptions if the called contract does not exist (in the
sense that the account does not contain code) or if the called contract itself
throws an exception or goes out of gas.

.. warning::
    Any interaction with another contract imposes a potential danger, especially
    if the source code of the contract is not known in advance. The current
    contract hands over control to the called contract and that may potentially
    do just about anything. Even if the called contract inherits from a known parent contract,
    the inheriting contract is only required to have a correct interface. The
    implementation of the contract, however, can be completely arbitrary and thus,
    pose a danger. In addition, be prepared in case it calls into other contracts of
    your system or even back into the calling contract before the first
    call returns. This means
    that the called contract can change state variables of the calling contract
    via its functions. Write your functions in a way that, for example, calls to
    external functions happen after any changes to state variables in your contract
    so your contract is not vulnerable to a reentrancy exploit.

Named Calls and Anonymous Function Parameters
---------------------------------------------

Function call arguments can also be given by name, in any order,
if they are enclosed in ``{ }`` as can be seen in the following
example. The argument list has to coincide by name with the list of
parameters from the function declaration, but can be in arbitrary order.

::

    pragma solidity ^0.4.0;

    contract C {
        function f(uint key, uint value) { ... }

        function g() {
            // named arguments
            f({value: 2, key: 3});
        }
    }

Omitted Function Parameter Names
--------------------------------

The names of unused parameters (especially return parameters) can be omitted.
Those names will still be present on the stack, but they are inaccessible.

::

    pragma solidity ^0.4.0;

    contract C {
        // omitted name for parameter
        function func(uint k, uint) returns(uint) {
            return k;
        }
    }
    

.. index:: ! new, contracts;creating

.. _creating-contracts:

Creating Contracts via ``new``
==============================

A contract can create a new contract using the ``new`` keyword. The full
code of the contract being created has to be known in advance, so recursive
creation-dependencies are not possible.

::

    pragma solidity ^0.4.0;

    contract D {
        uint x;
        function D(uint a) payable {
            x = a;
        }
    }


    contract C {
        D d = new D(4); // will be executed as part of C's constructor

        function createD(uint arg) {
            D newD = new D(arg);
        }

        function createAndEndowD(uint arg, uint amount) {
            // Send ether along with the creation
            D newD = (new D).value(amount)(arg);
        }
    }

As seen in the example, it is possible to forward Ether to the creation using the ``.value()`` option,
but it is not possible to limit the amount of gas. If the creation fails
(due to out-of-stack, not enough balance or other problems), an exception
is thrown.

Order of Evaluation of Expressions
==================================

The evaluation order of expressions is not specified (more formally, the order
in which the children of one node in the expression tree are evaluated is not
specified, but they are of course evaluated before the node itself). It is only
guaranteed that statements are executed in order and short-circuiting for
boolean expressions is done. See :ref:`order` for more information.

.. index:: ! assignment

Assignment
==========

.. index:: ! assignment;destructuring

Destructuring Assignments and Returning Multiple Values
-------------------------------------------------------

Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose size is a constant at compile-time. Those tuples can be used to return multiple values at the same time and also assign them to multiple variables (or LValues in general) at the same time::

    contract C {
        uint[] data;

        function f() returns (uint, bool, uint) {
            return (7, true, 2);
        }

        function g() {
            // Declares and assigns the variables. Specifying the type explicitly is not possible.
            var (x, b, y) = f();
            // Assigns to a pre-existing variable.
            (x, y) = (2, 7);
            // Common trick to swap values -- does not work for non-value storage types.
            (x, y) = (y, x);
            // Components can be left out (also for variable declarations).
            // If the tuple ends in an empty component,
            // the rest of the values are discarded.
            (data.length,) = f(); // Sets the length to 7
            // The same can be done on the left side.
            (,data[3]) = f(); // Sets data[3] to 2
            // Components can only be left out at the left-hand-side of assignments, with
            // one exception:
            (x,) = (1,);
            // (1,) is the only way to specify a 1-component tuple, because (1) is
            // equivalent to 1.
        }
    }

Complications for Arrays and Structs
------------------------------------

The semantics of assignment are a bit more complicated for non-value types like arrays and structs.
Assigning *to* a state variable always creates an independent copy. On the other hand, assigning to a local variable creates an independent copy only for elementary types, i.e. static types that fit into 32 bytes. If structs or arrays (including ``bytes`` and ``string``) are assigned from a state variable to a local variable, the local variable holds a reference to the original state variable. A second assignment to the local variable does not modify the state but only changes the reference. Assignments to members (or elements) of the local variable *do* change the state.

.. index:: ! scoping, declarations, default value

.. _default-value:

Scoping and Declarations
========================

A variable which is declared will have an initial default value whose byte-representation is all zeros.
The "default values" of variables are the typical "zero-state" of whatever the type is. For example, the default value for a ``bool``
is ``false``. The default value for the ``uint`` or ``int`` types is ``0``. For statically-sized arrays and ``bytes1`` to ``bytes32``, each individual
element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, ``bytes``
and ``string``, the default value is an empty array or string.

A variable declared anywhere within a function will be in scope for the *entire function*, regardless of where it is declared.
This happens because Solidity inherits its scoping rules from JavaScript.
This is in contrast to many languages where variables are only scoped where they are declared until the end of the semantic block.
As a result, the following code is illegal and cause the compiler to throw an error, ``Identifier already declared``::

    pragma solidity ^0.4.0;

    contract ScopingErrors {
        function scoping() {
            uint i = 0;

            while (i++ < 1) {
                uint same1 = 0;
            }

            while (i++ < 2) {
                uint same1 = 0;// Illegal, second declaration of same1
            }
        }

        function minimalScoping() {
            {
                uint same2 = 0;
            }

            {
                uint same2 = 0;// Illegal, second declaration of same2
            }
        }

        function forLoopScoping() {
            for (uint same3 = 0; same3 < 1; same3++) {
            }

            for (uint same3 = 0; same3 < 1; same3++) {// Illegal, second declaration of same3
            }
        }
    }

In addition to this, if a variable is declared, it will be initialized at the beginning of the function to its default value.
As a result, the following code is legal, despite being poorly written::

    function foo() returns (uint) {
        // baz is implicitly initialized as 0
        uint bar = 5;
        if (true) {
            bar += baz;
        } else {
            uint baz = 10;// never executes
        }
        return bar;// returns 5
    }

.. index:: ! exception, ! throw, ! assert, ! require, ! revert

Error handling: Assert, Require, Revert and Exceptions
======================================================

Solidity uses state-reverting exceptions to handle errors. Such an exception will undo all changes made to the
state in the current call (and all its sub-calls) and also flag an error to the caller.
The convenience functions ``assert`` and ``require`` can be used to check for conditions and throw an exception
if the condition is not met. The difference between the two is that ``assert`` should only be used for internal errors
and ``require`` should be used to check external conditions (invalid inputs or errors in external components).
The idea behind that is that analysis tools can check your contract and try to come up with situations and
series of function calls that will reach a failing assertion. If this is possible, this means there is a bug
in your contract you should fix.

There are two other ways to trigger execptions: The ``revert`` function can be used to flag an error and
revert the current call. In the future it might be possible to also include details about the error
in a call to ``revert``. The ``throw`` keyword can also be used as an alternative to ``revert()``.

When exceptions happen in a sub-call, they "bubble up" (i.e. exceptions are rethrown) automatically. Exceptions to this rule are ``send`` 
and the low-level functions ``call``, ``delegatecall`` and ``callcode`` -- those return ``false`` in case
of an exception instead of "bubbling up".

Catching exceptions is not yet possible.

In the following example, you can see how ``require`` can be used to easily check conditions on inputs
and how ``assert`` can be used for internal error checking::

    pragma solidity ^0.4.0;

    contract Sharer {
        function sendHalf(address addr) payable returns (uint balance) {
            require(msg.value % 2 == 0); // Only allow even numbers
            uint balanceBeforeTransfer = this.balance;
            addr.transfer(msg.value / 2);
            // Since transfer throws an exception on failure and
            // cannot call back here, there should be no way for us to
            // still have half of the money.
            assert(this.balance == balanceBeforeTransfer - msg.value / 2);
            return this.balance;
        }
    }

An ``assert``-style exception is generated in the following situations:

#. If you access an array at a too large or negative index (i.e. ``x[i]`` where ``i >= x.length`` or ``i < 0``).
#. If you access a fixed-length ``bytesN`` at a too large or negative index.
#. If you divide or modulo by zero (e.g. ``5 / 0`` or ``23 % 0``).
#. If you shift by a negative amount.
#. If you convert a value too big or negative into an enum type.
#. If you call a zero-initialized variable of internal function type.
#. If you call ``assert`` with an argument that evaluates to false.

A ``require``-style exception is generated in the following situations:

#. Calling ``throw``.
#. Calling ``require`` with an argument that evaluates to ``false``.
#. If you call a function via a message call but it does not finish properly (i.e. it runs out of gas, has no matching function, or throws an exception itself), except when a low level operation ``call``, ``send``, ``delegatecall`` or ``callcode`` is used.  The low level operations never throw exceptions but indicate failures by returning ``false``.
#. If you create a contract using the ``new`` keyword but the contract creation does not finish properly (see above for the definition of "not finish properly").
#. If you perform an external function call targeting a contract that contains no code.
#. If your contract receives Ether via a public function without ``payable`` modifier (including the constructor and the fallback function).
#. If your contract receives Ether via a public getter function.
#. If a ``.transfer()`` fails.

Internally, Solidity performs a revert operation (instruction ``0xfd``) for a ``require``-style exception and executes an invalid operation
(instruction ``0xfe``) to throw an ``assert``-style exception. In both cases, this causes
the EVM to revert all changes made to the state. The reason for reverting is that there is no safe way to continue execution, because an expected effect
did not occur. Because we want to retain the atomicity of transactions, the safest thing to do is to revert all changes and make the whole transaction
(or at least call) without effect. Note that ``assert``-style exceptions consume all gas available to the call, while
``revert``-style exceptions will not consume any gas starting from the Metropolis release.