mirror of
https://github.com/ethereum/solidity
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399 lines
14 KiB
ReStructuredText
399 lines
14 KiB
ReStructuredText
.. index:: ! functions
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.. _functions:
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*********
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Functions
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*********
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.. _function-parameters-return-variables:
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Function Parameters and Return Variables
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========================================
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As in JavaScript, functions may take parameters as input. Unlike in JavaScript
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and C, functions may also return an arbitrary number of values as output.
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Function Parameters
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-------------------
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Function parameters are declared the same way as variables, and the name of
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unused parameters can be omitted.
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For example, if you want your contract to accept one kind of external call
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with two integers, you would use something like::
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pragma solidity >=0.4.16 <0.6.0;
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contract Simple {
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uint sum;
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function taker(uint _a, uint _b) public {
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sum = _a + _b;
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}
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}
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Function parameters can be used as any other local variable and they can also be assigned to.
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.. note::
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An :ref:`external function<external-function-calls>` cannot accept a
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multi-dimensional array as an input
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parameter. This functionality is possible if you enable the new
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experimental ``ABIEncoderV2`` feature by adding ``pragma experimental ABIEncoderV2;`` to your source file.
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An :ref:`internal function<external-function-calls>` can accept a
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multi-dimensional array without enabling the feature.
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.. index:: return array, return string, array, string, array of strings, dynamic array, variably sized array, return struct, struct
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Return Variables
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----------------
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Function return variables are declared with the same syntax after the
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``returns`` keyword.
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For example, suppose you want to return two results: the sum and the product of
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two integers passed as function parameters, then you use something like::
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pragma solidity >=0.4.16 <0.6.0;
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contract Simple {
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function arithmetic(uint _a, uint _b)
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public
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pure
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returns (uint o_sum, uint o_product)
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{
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o_sum = _a + _b;
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o_product = _a * _b;
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}
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}
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The names of return variables can be omitted.
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Return variables can be used as any other local variable and they
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are initialized with their :ref:`default value <default-value>` and have that value unless explicitly set.
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You can either explicitly assign to return variables and
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then leave the function using ``return;``,
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or you can provide return values
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(either a single or :ref:`multiple ones<multi-return>`) directly with the ``return``
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statement::
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pragma solidity >=0.4.16 <0.6.0;
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contract Simple {
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function arithmetic(uint _a, uint _b)
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public
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pure
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returns (uint o_sum, uint o_product)
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{
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return (_a + _b, _a * _b);
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}
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}
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This form is equivalent to first assigning values to the
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return variables and then using ``return;`` to leave the function.
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.. note::
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You cannot return some types from non-internal functions, notably
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multi-dimensional dynamic arrays and structs. If you enable the
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new experimental ``ABIEncoderV2`` feature by adding ``pragma experimental
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ABIEncoderV2;`` to your source file then more types are available, but
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``mapping`` types are still limited to inside a single contract and you
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cannot transfer them.
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.. _multi-return:
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Returning Multiple Values
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-------------------------
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When a function has multiple return types, the statement ``return (v0, v1, ..., vn)`` can be used to return multiple values.
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The number of components must be the same as the number of return types.
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.. index:: ! view function, function;view
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.. _view-functions:
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View Functions
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==============
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Functions can be declared ``view`` in which case they promise not to modify the state.
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.. note::
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If the compiler's EVM target is Byzantium or newer (default) the opcode
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``STATICCALL`` is used for ``view`` functions which enforces the state
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to stay unmodified as part of the EVM execution. For library ``view`` functions
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``DELEGATECALL`` is used, because there is no combined ``DELEGATECALL`` and ``STATICCALL``.
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This means library ``view`` functions do not have run-time checks that prevent state
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modifications. This should not impact security negatively because library code is
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usually known at compile-time and the static checker performs compile-time checks.
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The following statements are considered modifying the state:
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#. Writing to state variables.
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#. :ref:`Emitting events <events>`.
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#. :ref:`Creating other contracts <creating-contracts>`.
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#. Using ``selfdestruct``.
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#. Sending Ether via calls.
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#. Calling any function not marked ``view`` or ``pure``.
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#. Using low-level calls.
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#. Using inline assembly that contains certain opcodes.
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::
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pragma solidity ^0.5.0;
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contract C {
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function f(uint a, uint b) public view returns (uint) {
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return a * (b + 42) + now;
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}
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}
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.. note::
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``constant`` on functions used to be an alias to ``view``, but this was dropped in version 0.5.0.
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.. note::
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Getter methods are automatically marked ``view``.
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.. note::
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Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
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for ``view`` functions.
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This enabled state modifications in ``view`` functions through the use of
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invalid explicit type conversions.
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By using ``STATICCALL`` for ``view`` functions, modifications to the
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state are prevented on the level of the EVM.
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.. index:: ! pure function, function;pure
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.. _pure-functions:
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Pure Functions
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==============
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Functions can be declared ``pure`` in which case they promise not to read from or modify the state.
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.. note::
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If the compiler's EVM target is Byzantium or newer (default) the opcode ``STATICCALL`` is used,
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which does not guarantee that the state is not read, but at least that it is not modified.
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In addition to the list of state modifying statements explained above, the following are considered reading from the state:
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#. Reading from state variables.
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#. Accessing ``address(this).balance`` or ``<address>.balance``.
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#. Accessing any of the members of ``block``, ``tx``, ``msg`` (with the exception of ``msg.sig`` and ``msg.data``).
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#. Calling any function not marked ``pure``.
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#. Using inline assembly that contains certain opcodes.
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::
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pragma solidity ^0.5.0;
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contract C {
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function f(uint a, uint b) public pure returns (uint) {
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return a * (b + 42);
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}
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}
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Pure functions are able to use the `revert()` and `require()` functions to revert
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potential state changes when an :ref:`error occurs <assert-and-require>`.
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Reverting a state change is not considered a "state modification", as only changes to the
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state made previously in code that did not have the ``view`` or ``pure`` restriction
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are reverted and that code has the option to catch the ``revert`` and not pass it on.
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This behaviour is also in line with the ``STATICCALL`` opcode.
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.. warning::
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It is not possible to prevent functions from reading the state at the level
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of the EVM, it is only possible to prevent them from writing to the state
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(i.e. only ``view`` can be enforced at the EVM level, ``pure`` can not).
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.. note::
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Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
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for ``pure`` functions.
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This enabled state modifications in ``pure`` functions through the use of
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invalid explicit type conversions.
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By using ``STATICCALL`` for ``pure`` functions, modifications to the
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state are prevented on the level of the EVM.
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.. note::
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Prior to version 0.4.17 the compiler did not enforce that ``pure`` is not reading the state.
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It is a compile-time type check, which can be circumvented doing invalid explicit conversions
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between contract types, because the compiler can verify that the type of the contract does
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not do state-changing operations, but it cannot check that the contract that will be called
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at runtime is actually of that type.
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.. index:: ! fallback function, function;fallback
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.. _fallback-function:
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Fallback Function
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=================
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A contract can have exactly one unnamed function. This function cannot have
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arguments, cannot return anything and has to have ``external`` visibility.
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It is executed on a call to the contract if none of the other
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functions match the given function identifier (or if no data was supplied at
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all).
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Furthermore, this function is executed whenever the contract receives plain
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Ether (without data). Additionally, in order to receive Ether, the fallback function
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must be marked ``payable``. If no such function exists, the contract cannot receive
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Ether through regular transactions.
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In the worst case, the fallback function can only rely on 2300 gas being
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available (for example when `send` or `transfer` is used), leaving little
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room to perform other operations except basic logging. The following operations
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will consume more gas than the 2300 gas stipend:
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- Writing to storage
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- Creating a contract
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- Calling an external function which consumes a large amount of gas
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- Sending Ether
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Like any function, the fallback function can execute complex operations as long as there is enough gas passed on to it.
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.. note::
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Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
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any payload supplied with the call.
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.. warning::
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The fallback function is also executed if the caller meant to call
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a function that is not available. If you want to implement the fallback
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function only to receive ether, you should add a check
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like ``require(msg.data.length == 0)`` to prevent invalid calls.
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.. warning::
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Contracts that receive Ether directly (without a function call, i.e. using ``send`` or ``transfer``)
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but do not define a fallback function
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throw an exception, sending back the Ether (this was different
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before Solidity v0.4.0). So if you want your contract to receive Ether,
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you have to implement a payable fallback function.
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.. warning::
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A contract without a payable fallback function can receive Ether as a recipient of a `coinbase transaction` (aka `miner block reward`)
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or as a destination of a ``selfdestruct``.
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A contract cannot react to such Ether transfers and thus also cannot reject them. This is a design choice of the EVM and Solidity cannot work around it.
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It also means that ``address(this).balance`` can be higher than the sum of some manual accounting implemented in a contract (i.e. having a counter updated in the fallback function).
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::
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pragma solidity ^0.5.0;
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contract Test {
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// This function is called for all messages sent to
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// this contract (there is no other function).
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// Sending Ether to this contract will cause an exception,
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// because the fallback function does not have the `payable`
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// modifier.
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function() external { x = 1; }
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uint x;
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}
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// This contract keeps all Ether sent to it with no way
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// to get it back.
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contract Sink {
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function() external payable { }
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}
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contract Caller {
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function callTest(Test test) public returns (bool) {
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(bool success,) = address(test).call(abi.encodeWithSignature("nonExistingFunction()"));
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require(success);
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// results in test.x becoming == 1.
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// address(test) will not allow to call ``send`` directly, since ``test`` has no payable
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// fallback function. It has to be converted to the ``address payable`` type via an
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// intermediate conversion to ``uint160`` to even allow calling ``send`` on it.
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address payable testPayable = address(uint160(address(test)));
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// If someone sends ether to that contract,
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// the transfer will fail, i.e. this returns false here.
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return testPayable.send(2 ether);
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}
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}
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.. index:: ! overload
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.. _overload-function:
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Function Overloading
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====================
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A contract can have multiple functions of the same name but with different parameter
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types.
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This process is called "overloading" and also applies to inherited functions.
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The following example shows overloading of the function
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``f`` in the scope of contract ``A``.
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::
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pragma solidity >=0.4.16 <0.6.0;
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contract A {
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function f(uint _in) public pure returns (uint out) {
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out = _in;
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}
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function f(uint _in, bool _really) public pure returns (uint out) {
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if (_really)
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out = _in;
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}
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}
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Overloaded functions are also present in the external interface. It is an error if two
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externally visible functions differ by their Solidity types but not by their external types.
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::
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pragma solidity >=0.4.16 <0.6.0;
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// This will not compile
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contract A {
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function f(B _in) public pure returns (B out) {
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out = _in;
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}
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function f(address _in) public pure returns (address out) {
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out = _in;
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}
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}
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contract B {
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}
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Both ``f`` function overloads above end up accepting the address type for the ABI although
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they are considered different inside Solidity.
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Overload resolution and Argument matching
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-----------------------------------------
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Overloaded functions are selected by matching the function declarations in the current scope
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to the arguments supplied in the function call. Functions are selected as overload candidates
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if all arguments can be implicitly converted to the expected types. If there is not exactly one
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candidate, resolution fails.
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.. note::
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Return parameters are not taken into account for overload resolution.
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::
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pragma solidity >=0.4.16 <0.6.0;
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contract A {
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function f(uint8 _in) public pure returns (uint8 out) {
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out = _in;
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}
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function f(uint256 _in) public pure returns (uint256 out) {
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out = _in;
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}
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}
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Calling ``f(50)`` would create a type error since ``50`` can be implicitly converted both to ``uint8``
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and ``uint256`` types. On another hand ``f(256)`` would resolve to ``f(uint256)`` overload as ``256`` cannot be implicitly
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converted to ``uint8``.
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