The operators `||` and `&&` apply the common short-circuiting rules. This means that in the expression `f(x) || g(y)`, if `f(x)` evaluates to `true`, `g(y)` will not be evaluated even if it may have side-effects.
`int` / `uint`: Signed and unsigned integers of various sizes. Keywords `uint8` to `uint256` in steps of `8` (unsigned of 8 up to 256 bits) and `int8` to `int256`. `uint` and `int` are aliases for `uint256` and `int256`, respectively.
It is possible to query the balance of an address using the property `balance`
and to send Ether (in units of wei) to an address using the `send` function:
::
address x = 0x123;
address myAddress = this;
if (x.balance < 10 && myAddress.balance >= 10) x.send(10);
..note::
If `x` is a contract address, its code (more specifically: its fallback function, if present) will be executed together with the `send` call (this is a limitation of the EVM and cannot be prevented). If that execution runs out of gas or fails in any way, the Ether transfer will be reverted. In this case, `send` returns `false`.
Furthermore, to interface with contracts that do not adhere to the ABI,
the function `call` is provided which takes an arbitrary number of arguments of any type. These arguments are padded to 32 bytes and concatenated. One exception is the case where the first argument is encoded to exactly four bytes. In this case, it is not padded to allow the use of function signatures here.
`call` returns a boolean indicating whether the invoked function terminated (`true`) or caused an EVM exception (`false`). It is not possible to access the actual data returned (for this we would need to know the encoding and size in advance).
In a similar way, the function `delegatecall` can be used: The difference is that only the code of the given address is used, all other aspects (storage, balance, ...) are taken from the current contract. The purpose of `delegatecall` is to use library code which is stored in another contract. The user has to ensure that the layout of storage in both contracts is suitable for delegatecall to be used. Prior to homestead, only a limited variant called `callcode` was available that did not provide access to the original `msg.sender` and `msg.value` values.
All three functions `call`, `delegatecall` and `callcode` are very low-level functions and should only be used as a *last resort* as they break the type-safety of Solidity.
Division on integer literals used to truncate in earlier versions, but it will now convert into a rational number, i.e. `5 / 2` is not equal to `2`, but to `2.5`.
String Literals are written with double quotes (`"abc"`). As with integer literals, their type can vary, but they are implicitly convertible to `bytes` if they fit, to `bytes` and to `string`.
Complex types, i.e. types which do not always fit into 256 bits have to be handled
more carefully than the value-types we have already seen. Since copying
them can be quite expensive, we have to think about whether we want them to be
stored in **memory** (which is not persisting) or **storage** (where the state
variables are held).
Data location
-------------
Every complex type, i.e. *arrays* and *structs*, has an additional
annotation, the "data location", about whether it is stored in memory or in storage. Depending on the
context, there is always a default, but it can be overridden by appending
either `storage` or `memory` to the type. The default for function parameters (including return parameters) is `memory`, the default for local variables is `storage` and the location is forced
to `storage` for state variables (obviously).
There is also a third data location, "calldata", which is a non-modifyable
non-persistent area where function arguments are stored. Function parameters
(not return parameters) of external functions are forced to "calldata" and
it behaves mostly like memory.
Data locations are important because they change how assignments behave:
Assignments between storage and memory and also to a state variable (even from other state variables)
always create an independent copy.
Assignments to local storage variables only assign a reference though, and
this reference always points to the state variable even if the latter is changed
in the meantime.
On the other hand, assignments from a memory stored reference type to another
memory-stored reference type does not create a copy.
Arrays have a `length` member to hold their number of elements.
Dynamic arrays can be resized in storage (not in memory) by changing the
`.length` member. This does not happen automatically when attempting to access elements outside the current length. The size of memory arrays is fixed (but dynamic, i.e. it can depend on runtime parameters) once they are created.
**push**:
Dynamic storage arrays and `bytes` (not `string`) have a member function called `push` that can be used to append an element at the end of the array. The function returns the new length.
..warning::
It is not yet possible to use arrays of arrays in external functions.
..warning::
Due to limitations of the EVM, it is not possible to return
dynamic content from external function calls. The function `f` in
`contract C { function f() returns (uint[]) { ... } }` will return
something if called from web3.js, but not if called from Solidity.
The only workaround for now is to use large statically-sized arrays.
The contract does not provide the full functionality of a crowdfunding
contract, but it contains the basic concepts necessary to understand structs.
Struct types can be used inside mappings and arrays and they can itself
contain mappings and arrays.
It is not possible for a struct to contain a member of its own type,
although the struct itself can be the value type of a mapping member.
This restriction is necessary, as the size of the struct has to be finite.
Note how in all the functions, a struct type is assigned to a local variable
(of the default storage data location).
This does not copy the struct but only stores a reference so that assignments to
members of the local variable actually write to the state.
Of course, you can also directly access the members of the struct without
assigning it to a local variable, as in
`campaigns[campaignID].amount = 0`.
..index:: !mapping
Mappings
========
Mapping types are declared as `mapping _KeyType => _ValueType`, where
`_KeyType` can be almost any type except for a mapping and `_ValueType`
can actually be any type, including mappings.
Mappings can be seen as hashtables which are virtually initialized such that
every possible key exists and is mapped to a value whose byte-representation is
all zeros. The similarity ends here, though: The key data is not actually stored
in a mapping, only its `sha3` hash used to look up the value.
Because of this, mappings do not have a length or a concept of a key or value being "set".
Mappings are only allowed for state variables (or as storage reference types
in internal functions).
..index:: assignment, ! delete, lvalue
Operators Involving LValues
===========================
If `a` is an LValue (i.e. a variable or something that can be assigned to), the following operators are available as shorthands:
`a += e` is equivalent to `a = a + e`. The operators `-=`, `*=`, `/=`, `%=`, `a |=`, `&=` and `^=` are defined accordingly. `a++` and `a--` are equivalent to `a += 1` / `a -= 1` but the expression itself still has the previous value of `a`. In contrast, `--a` and `++a` have the same effect on `a` but return the value after the change.
delete
------
`delete a` assigns the initial value for the type to `a`. I.e. for integers it is equivalent to `a = 0`, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements reset. For structs, it assigns a struct with all members reset.
`delete` has no effect on whole mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted.
It is important to note that `delete a` really behaves like an assignment to `a`, i.e. it stores a new object in `a`.
