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Modified contracts.rst

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Changed inline code syntax
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Denton Liu 2016-05-24 11:29:00 -04:00
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@ -20,7 +20,7 @@ Contracts can be created "from outside" or from Solidity contracts.
When a contract is created, its constructor (a function with the same When a contract is created, its constructor (a function with the same
name as the contract) is executed once. name as the contract) is executed once.
From `web3.js`, i.e. the JavaScript From :code:`web3.js`, i.e. the JavaScript
API, this is done as follows:: API, this is done as follows::
// The json abi array generated by the compiler // The json abi array generated by the compiler
@ -53,7 +53,7 @@ API, this is done as follows::
Internally, constructor arguments are passed after the code of Internally, constructor arguments are passed after the code of
the contract itself, but you do not have to care about this the contract itself, but you do not have to care about this
if you use `web3.js`. if you use :code:`web3.js`.
If a contract wants to create another contract, the source code If a contract wants to create another contract, the source code
(and the binary) of the created contract has to be known to the creator. (and the binary) of the created contract has to be known to the creator.
@ -143,38 +143,38 @@ a "message call") and external
ones that do), there are four types of visibilities for ones that do), there are four types of visibilities for
functions and state variables. functions and state variables.
Functions can be specified as being `external`, Functions can be specified as being :code:`external`,
`public`, `internal` or `private`, where the default is :code:`public`, :code:`internal` or :code:`private`, where the default is
`public`. For state variables, `external` is not possible :code:`public`. For state variables, :code:`external` is not possible
and the default is `internal`. and the default is :code:`internal`.
`external`: :code:`external`:
External functions are part of the contract External functions are part of the contract
interface, which means they can be called from other contracts and interface, which means they can be called from other contracts and
via transactions. An external function `f` cannot be called via transactions. An external function :code:`f` cannot be called
internally (i.e. `f()` does not work, but `this.f()` works). internally (i.e. :code:`f()` does not work, but :code:`this.f()` works).
External functions are sometimes more efficient when External functions are sometimes more efficient when
they receive large arrays of data. they receive large arrays of data.
`public`: :code:`public`:
Public functions are part of the contract Public functions are part of the contract
interface and can be either called internally or via interface and can be either called internally or via
messages. For public state variables, an automatic accessor messages. For public state variables, an automatic accessor
function (see below) is generated. function (see below) is generated.
`internal`: :code:`internal`:
Those functions and state variables can only be Those functions and state variables can only be
accessed internally (i.e. from within the current contract accessed internally (i.e. from within the current contract
or contracts deriving from it), without using `this`. or contracts deriving from it), without using :code:`this`.
`private`: :code:`private`:
Private functions and state variables are only Private functions and state variables are only
visible for the contract they are defined in and not in visible for the contract they are defined in and not in
derived contracts. derived contracts.
.. note:: .. note::
Everything that is inside a contract is visible to Everything that is inside a contract is visible to
all external observers. Making something `private` all external observers. Making something :code:`private`
only prevents other contract from accessing and modifying only prevents other contract from accessing and modifying
the information, but it will still be visible to the the information, but it will still be visible to the
whole world outside of the blockchain. whole world outside of the blockchain.
@ -191,9 +191,9 @@ return parameter list for functions.
uint public data; uint public data;
} }
Other contracts can call `c.data()` to retrieve the value of data in state Other contracts can call :code:`c.data()` to retrieve the value of data in state
storage, but are not able to call `f`. Contracts derived from `c` can call storage, but are not able to call :code:`f`. Contracts derived from :code:`c` can call
`setData` to alter the value of `data` (but only in their own state). :code:`setData` to alter the value of :code:`data` (but only in their own state).
.. index:: ! accessor;function, ! function;accessor .. index:: ! accessor;function, ! function;accessor
@ -202,15 +202,15 @@ Accessor Functions
The compiler automatically creates accessor functions for The compiler automatically creates accessor functions for
all public state variables. The contract given below will all public state variables. The contract given below will
have a function called `data` that does not take any have a function called :code:`data` that does not take any
arguments and returns a uint, the value of the state arguments and returns a uint, the value of the state
variable `data`. The initialization of state variables can variable :code:`data`. The initialization of state variables can
be done at declaration. be done at declaration.
The accessor functions have external visibility. If the The accessor functions have external visibility. If the
symbol is accessed internally (i.e. without `this.`), symbol is accessed internally (i.e. without :code:`this.`),
it is a state variable and if it is accessed externally it is a state variable and if it is accessed externally
(i.e. with `this.`), it is a function. (i.e. with :code:`this.`), it is a function.
:: ::
@ -414,15 +414,15 @@ ultimately, also the block headers have to be supplied because
the contract can only see the last 256 block hashes). the contract can only see the last 256 block hashes).
Up to three parameters can Up to three parameters can
receive the attribute `indexed` which will cause the respective arguments receive the attribute :code:`indexed` which will cause the respective arguments
to be searched for: It is possible to filter for specific values of to be searched for: It is possible to filter for specific values of
indexed arguments in the user interface. indexed arguments in the user interface.
If arrays (including `string` and `bytes`) are used as indexed arguments, the If arrays (including :code:`string` and :code:`bytes`) are used as indexed arguments, the
sha3-hash of it is stored as topic instead. sha3-hash of it is stored as topic instead.
The hash of the signature of the event is one of the topics except if you The hash of the signature of the event is one of the topics except if you
declared the event with `anonymous` specifier. This means that it is declared the event with :code:`anonymous` specifier. This means that it is
not possible to filter for specific anonymous events by name. not possible to filter for specific anonymous events by name.
All non-indexed arguments will be stored in the data part of the log. All non-indexed arguments will be stored in the data part of the log.
@ -475,8 +475,8 @@ Low-Level Interface to Logs
=========================== ===========================
It is also possible to access the low-level interface to the logging It is also possible to access the low-level interface to the logging
mechanism via the functions `log0`, `log1`, `log2`, `log3` and `log4`. mechanism via the functions :code:`log0`, :code:`log1`, :code:`log2`, :code:`log3` and :code:`log4`.
`logi` takes `i + 1` parameter of type `bytes32`, where the first :code:`logi` takes :code:`i + 1` parameter of type :code:`bytes32`, where the first
argument will be used for the data part of the log and the others argument will be used for the data part of the log and the others
as topics. The event call above can be performed in the same way as as topics. The event call above can be performed in the same way as
@ -490,7 +490,7 @@ as topics. The event call above can be performed in the same way as
); );
where the long hexadecimal number is equal to where the long hexadecimal number is equal to
`sha3("Deposit(address,hash256,uint256)")`, the signature of the event. :code:`sha3("Deposit(address,hash256,uint256)")`, the signature of the event.
Additional Resources for Understanding Events Additional Resources for Understanding Events
============================================== ==============================================
@ -591,7 +591,7 @@ Details are given in the following example.
uint info; uint info;
} }
Note that above, we call `mortal.kill()` to "forward" the Note that above, we call :code:`mortal.kill()` to "forward" the
destruction request. The way this is done is problematic, as destruction request. The way this is done is problematic, as
seen in the following example:: seen in the following example::
@ -615,10 +615,10 @@ seen in the following example::
contract Final is Base1, Base2 { contract Final is Base1, Base2 {
} }
A call to `Final.kill()` will call `Base2.kill` as the most A call to :code:`Final.kill()` will call :code:`Base2.kill` as the most
derived override, but this function will bypass derived override, but this function will bypass
`Base1.kill`, basically because it does not even know about :code:`Base1.kill`, basically because it does not even know about
`Base1`. The way around this is to use `super`:: :code:`Base1`. The way around this is to use :code:`super`::
contract mortal is owned { contract mortal is owned {
function kill() { function kill() {
@ -640,10 +640,10 @@ derived override, but this function will bypass
contract Final is Base2, Base1 { contract Final is Base2, Base1 {
} }
If `Base1` calls a function of `super`, it does not simply If :code:`Base1` calls a function of :code:`super`, it does not simply
call this function on one of its base contracts, it rather call this function on one of its base contracts, it rather
calls this function on the next base contract in the final calls this function on the next base contract in the final
inheritance graph, so it will call `Base2.kill()` (note that inheritance graph, so it will call :code:`Base2.kill()` (note that
the final inheritance sequence is -- starting with the most the final inheritance sequence is -- starting with the most
derived contract: Final, Base1, Base2, mortal, owned). derived contract: Final, Base1, Base2, mortal, owned).
The actual function that is called when using super is The actual function that is called when using super is
@ -670,9 +670,9 @@ the base constructors. This can be done at two places::
} }
} }
Either directly in the inheritance list (`is Base(7)`) or in Either directly in the inheritance list (:code:`is Base(7)`) or in
the way a modifier would be invoked as part of the header of the way a modifier would be invoked as part of the header of
the derived constructor (`Base(_y * _y)`). The first way to the derived constructor (:code:`Base(_y * _y)`). The first way to
do it is more convenient if the constructor argument is a do it is more convenient if the constructor argument is a
constant and defines the behaviour of the contract or constant and defines the behaviour of the contract or
describes it. The second way has to be used if the describes it. The second way has to be used if the
@ -691,7 +691,7 @@ Solidity follows the path of Python and uses "`C3 Linearization <https://en.wiki
to force a specific order in the DAG of base classes. This to force a specific order in the DAG of base classes. This
results in the desirable property of monotonicity but results in the desirable property of monotonicity but
disallows some inheritance graphs. Especially, the order in disallows some inheritance graphs. Especially, the order in
which the base classes are given in the `is` directive is which the base classes are given in the :code:`is` directive is
important. In the following code, Solidity will give the important. In the following code, Solidity will give the
error "Linearization of inheritance graph impossible". error "Linearization of inheritance graph impossible".
@ -701,9 +701,9 @@ error "Linearization of inheritance graph impossible".
contract A is X {} contract A is X {}
contract C is A, X {} contract C is A, X {}
The reason for this is that `C` requests `X` to override `A` The reason for this is that :code:`C` requests :code:`X` to override :code:`A`
(by specifying `A, X` in this order), but `A` itself (by specifying :code:`A, X` in this order), but :code:`A` itself
requests to override `X`, which is a contradiction that requests to override :code:`X`, which is a contradiction that
cannot be resolved. cannot be resolved.
A simple rule to remember is to specify the base classes in A simple rule to remember is to specify the base classes in
@ -715,7 +715,7 @@ the order from "most base-like" to "most derived".
Abstract Contracts Abstract Contracts
****************** ******************
Contract functions can lack an implementation as in the following example (note that the function declaration header is terminated by `;`):: Contract functions can lack an implementation as in the following example (note that the function declaration header is terminated by :code:`;`)::
contract Feline { contract Feline {
function utterance() returns (bytes32); function utterance() returns (bytes32);
@ -738,10 +738,10 @@ Libraries
************ ************
Libraries are similar to contracts, but their purpose is that they are deployed Libraries are similar to contracts, but their purpose is that they are deployed
only once at a specific address and their code is reused using the `DELEGATECALL` only once at a specific address and their code is reused using the :code:`DELEGATECALL`
(`CALLCODE` until homestead) (:code:`CALLCODE` until Homestead)
feature of the EVM. This means that if library functions are called, their code feature of the EVM. This means that if library functions are called, their code
is executed in the context of the calling contract, i.e. `this` points to the is executed in the context of the calling contract, i.e. :code:`this` points to the
calling contract and especially the storage from the calling contract can be calling contract and especially the storage from the calling contract can be
accessed. As a library is an isolated piece of source code, it can only access accessed. As a library is an isolated piece of source code, it can only access
state variables of the calling contract if they are explicitly supplied (it state variables of the calling contract if they are explicitly supplied (it
@ -750,14 +750,14 @@ would have to way to name them, otherwise).
Libraries can be seen as implicit base contracts of the contracts that use them. Libraries can be seen as implicit base contracts of the contracts that use them.
They will not be explicitly visible in the inheritance hierarchy, but calls They will not be explicitly visible in the inheritance hierarchy, but calls
to library functions look just like calls to functions of explicit base to library functions look just like calls to functions of explicit base
contracts (`L.f()` if `L` is the name of the library). Furthermore, contracts (:code:`L.f()` if :code:`L` is the name of the library). Furthermore,
`internal` functions of libraries are visible in all contracts, just as :code:`internal` functions of libraries are visible in all contracts, just as
if the library were a base contract. Of course, calls to internal functions if the library were a base contract. Of course, calls to internal functions
use the internal calling convention, which means that all internal types use the internal calling convention, which means that all internal types
can be passed and memory types will be passed by reference and not copied. can be passed and memory types will be passed by reference and not copied.
In order to realise this in the EVM, code of internal library functions In order to realise this in the EVM, code of internal library functions
(and all functions called from therein) will be pulled into the calling (and all functions called from therein) will be pulled into the calling
contract and a regular `JUMP` call will be used instead of a `DELEGATECALL`. contract and a regular :code:`JUMP` call will be used instead of a :code:`DELEGATECALL`.
.. index:: using for, set .. index:: using for, set
@ -823,13 +823,13 @@ data types, functions also work without any storage
reference parameters, can have multiple storage reference reference parameters, can have multiple storage reference
parameters and in any position. parameters and in any position.
The calls to `Set.contains`, `Set.insert` and `Set.remove` The calls to :code:`Set.contains`, :code:`Set.insert` and :code:`Set.remove`
are all compiled as calls (`DELEGATECALL`s) to an external are all compiled as calls (:code:`DELEGATECALL`) to an external
contract/library. If you use libraries, take care that an contract/library. If you use libraries, take care that an
actual external function call is performed. actual external function call is performed.
`msg.sender`, `msg.value` and `this` will retain their values :code:`msg.sender`, :code:`msg.value` and :code:`this` will retain their values
in this call, though (prior to Homestead, `msg.sender` and in this call, though (prior to Homestead, :code:`msg.sender` and
`msg.value` changed, though). :code:`msg.value` changed, though).
The following example shows how to use memory types and The following example shows how to use memory types and
internal functions in libraries in order to implement internal functions in libraries in order to implement
@ -895,8 +895,8 @@ final bytecode by a linker
(see :ref:`commandline-compiler`) on how to use the (see :ref:`commandline-compiler`) on how to use the
commandline compiler for linking). If the addresses are not commandline compiler for linking). If the addresses are not
given as arguments to the compiler, the compiled hex code given as arguments to the compiler, the compiled hex code
will contain placeholders of the form `__Set______` (where will contain placeholders of the form :code:`__Set______` (where
`Set` is the name of the library). The address can be filled :code:`Set` is the name of the library). The address can be filled
manually by replacing all those 40 symbols by the hex manually by replacing all those 40 symbols by the hex
encoding of the address of the library contract. encoding of the address of the library contract.
@ -915,14 +915,14 @@ Restrictions for libraries in comparison to contracts:
Using For Using For
********* *********
The directive `using A for B;` can be used to attach library The directive :code:`using A for B;` can be used to attach library
functions (from the library `A`) to any type (`B`). functions (from the library :code:`A`) to any type (:code:`B`).
These functions will receive the object they are called on These functions will receive the object they are called on
as their first parameter (like the `self` variable in as their first parameter (like the :code:`self` variable in
Python). Python).
The effect of `using A for *;` is that the functions from The effect of :code:`using A for *;` is that the functions from
the library `A` are attached to any type. the library :code:`A` are attached to any type.
In both situations, all functions, even those where the In both situations, all functions, even those where the
type of the first parameter does not match the type of type of the first parameter does not match the type of
@ -930,7 +930,7 @@ the object, are attached. The type is checked at the
point the function is called and function overload point the function is called and function overload
resolution is performed. resolution is performed.
The `using A for B;` directive is active for the current The :code:`using A for B;` directive is active for the current
scope, which is limited to a contract for now but will scope, which is limited to a contract for now but will
be lifted to the global scope later, so that by including be lifted to the global scope later, so that by including
a module, its data types including library functions are a module, its data types including library functions are
@ -1014,5 +1014,5 @@ It is also possible to extend elementary types in that way::
Note that all library calls are actual EVM function calls. This means that Note that all library calls are actual EVM function calls. This means that
if you pass memory or value types, a copy will be performed, even of the if you pass memory or value types, a copy will be performed, even of the
`self` variable. The only situation where no copy will be performed :code:`self` variable. The only situation where no copy will be performed
is when storage reference variables are used. is when storage reference variables are used.