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chriseth
2020-02-27 15:06:36 +01:00
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docs/control-structures.rst
+90 -20
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@@ -75,9 +75,10 @@ all function arguments have to be copied to memory.
it is a message call as part of the overall transaction.
When calling functions of other contracts, you can specify the amount of Wei or
gas sent with the call with the special options ``.value()`` and ``.gas()``,
respectively. Any Wei you send to the contract is added to the total balance
of the contract:
gas sent with the call with the special options ``{value: 10, gas: 10000}``.
Note that it is discouraged to specify gas values explicitly, since the gas costs
of opcodes can change in the future. Any Wei you send to the contract is added
to the total balance of that contract:
::
@@ -90,14 +91,14 @@ of the contract:
contract Consumer {
InfoFeed feed;
function setFeed(InfoFeed addr) public { feed = addr; }
function callFeed() public { feed.info.value(10).gas(800)(); }
function callFeed() public { feed.info{value: 10, gas: 800}(); }
}
You need to use the modifier ``payable`` with the ``info`` function because
otherwise, the ``.value()`` option would not be available.
otherwise, the ``value`` option would not be available.
.. warning::
Be careful that ``feed.info.value(10).gas(800)`` only locally sets the
Be careful that ``feed.info{value: 10, gas: 800}`` only locally sets the
``value`` and amount of ``gas`` sent with the function call, and the
parentheses at the end perform the actual call. So in this case, the
function is not called and the ``value`` and ``gas`` settings are lost.
@@ -121,6 +122,11 @@ throws an exception or goes out of gas.
external functions happen after any changes to state variables in your contract
so your contract is not vulnerable to a reentrancy exploit.
.. note::
Before Solidity 0.6.2, the recommended way to specify the value and gas
was to use ``f.value(x).gas(g)()``. This is still possible but deprecated
and will be removed with Solidity 0.7.0.
Named Calls and Anonymous Function Parameters
---------------------------------------------
@@ -196,17 +202,81 @@ is compiled so recursive creation-dependencies are not possible.
function createAndEndowD(uint arg, uint amount) public payable {
// Send ether along with the creation
D newD = (new D).value(amount)(arg);
D newD = new D{value: amount}(arg);
newD.x();
}
}
As seen in the example, it is possible to send Ether while creating
an instance of ``D`` using the ``.value()`` option, but it is not possible
an instance of ``D`` 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.
Salted contract creations / create2
-----------------------------------
When creating a contract, the address of the contract is computed from
the address of the creating contract and a counter that is increased with
each contract creation.
If you specify the option ``salt`` (a bytes32 value), then contract creation will
use a different mechanism to come up with the address of the new contract:
It will compute the address from the address of the creating contract,
the given salt value, the (creation) bytecode of the created contract and the constructor
arguments.
In particular, the counter ("nonce") is not used. This allows for more flexibility
in creating contracts: You are able to derive the address of the
new contract before it is created. Furthermore, you can rely on this address
also in case the creating
contracts creates other contracts in the meantime.
The main use-case here is contracts that act as judges for off-chain interactions,
which only need to be created if there is a dispute.
::
pragma solidity >0.6.1 <0.8.0;
contract D {
uint public x;
constructor(uint a) public {
x = a;
}
}
contract C {
function createDSalted(bytes32 salt, uint arg) public {
/// This complicated expression just tells you how the address
/// can be pre-computed. It is just there for illustration.
/// You actually only need ``new D{salt: salt}(arg)``.
address predictedAddress = address(bytes20(keccak256(abi.encodePacked(
byte(0xff),
address(this),
salt,
keccak256(abi.encodePacked(
type(D).creationCode,
arg
))
))));
D d = new D{salt: salt}(arg);
require(address(d) == predictedAddress);
}
}
.. warning::
There are some peculiarities in relation to salted creation. A contract can be
re-created at the same address after having been destroyed. Yet, it is possible
for that newly created contract to have a different deployed bytecode even
though the creation bytecode has been the same (which is a requirement because
otherwise the address would change). This is due to the fact that the compiler
can query external state that might have changed between the two creations
and incorporate that into the deployed bytecode before it is stored.
Order of Evaluation of Expressions
==================================
@@ -273,18 +343,8 @@ i.e. the following is not valid: ``(x, uint y) = (1, 2);``
Complications for Arrays and Structs
------------------------------------
The semantics of assignments 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.
The semantics of assignments are more complicated for non-value types like arrays and structs,
including ``bytes`` and ``string``, see :ref:`Data location and assignment behaviour <data-location-assignment>` for details.
In the example below the call to ``g(x)`` has no effect on ``x`` because it creates
an independent copy of the storage value in memory. However, ``h(x)`` successfully modifies ``x``
@@ -647,3 +707,13 @@ in scope in the block that follows.
in a catch block or the execution of the try/catch statement itself
reverts (for example due to decoding failures as noted above or
due to not providing a low-level catch clause).
.. note::
The reason behind a failed call can be manifold. Do not assume that
the error message is coming directly from the called contract:
The error might have happened deeper down in the call chain and the
called contract just forwarded it. Also, it could be due to an
out-of-gas situation and not a deliberate error condition:
The caller always retains 63/64th of the gas in a call and thus
even if the called contract goes out of gas, the caller still
has some gas left.