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Clarify creating a contract example as reported in #2661

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Chris Ward 2018-09-24 14:10:01 +02:00 committed by chriseth
parent 288582e38e
commit 144909dfb8
1 changed files with 20 additions and 13 deletions
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33
docs/contracts.rst
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@ -50,7 +50,7 @@ This means that cyclic creation dependencies are impossible.
pragma solidity >=0.4.22 <0.6.0; pragma solidity >=0.4.22 <0.6.0;
contract OwnedToken { contract OwnedToken {
// TokenCreator is a contract type that is defined below. // `TokenCreator` is a contract type that is defined below.
// It is fine to reference it as long as it is not used // It is fine to reference it as long as it is not used
// to create a new contract. // to create a new contract.
TokenCreator creator; TokenCreator creator;
@ -61,14 +61,18 @@ This means that cyclic creation dependencies are impossible.
// creator and the assigned name. // creator and the assigned name.
constructor(bytes32 _name) public { constructor(bytes32 _name) public {
// State variables are accessed via their name // State variables are accessed via their name
// and not via e.g. this.owner. This also applies // and not via e.g. `this.owner`. Functions can
// to functions and especially in the constructors, // be accessed directly or through `this.f`,
// you can only call them like that ("internally"), // but the latter provides an external view
// because the contract itself does not exist yet. // to the function. Especially in the constructor,
// you should not access functions externally,
// because the function does not exist yet.
// See the next section for details.
owner = msg.sender; owner = msg.sender;
// We do an explicit type conversion from `address` // We do an explicit type conversion from `address`
// to `TokenCreator` and assume that the type of // to `TokenCreator` and assume that the type of
// the calling contract is TokenCreator, there is // the calling contract is `TokenCreator`, there is
// no real way to check that. // no real way to check that.
creator = TokenCreator(msg.sender); creator = TokenCreator(msg.sender);
name = _name; name = _name;
@ -86,10 +90,11 @@ This means that cyclic creation dependencies are impossible.
// Only the current owner can transfer the token. // Only the current owner can transfer the token.
if (msg.sender != owner) return; if (msg.sender != owner) return;
// We also want to ask the creator if the transfer // We ask the creator contract if the transfer
// is fine. Note that this calls a function of the // should proceed by using a function of the
// contract defined below. If the call fails (e.g. // `TokenCreator` contract defined below. If
// due to out-of-gas), the execution also fails here. // the call fails (e.g. due to out-of-gas),
// the execution also fails here.
if (creator.isTokenTransferOK(owner, newOwner)) if (creator.isTokenTransferOK(owner, newOwner))
owner = newOwner; owner = newOwner;
} }
@ -100,8 +105,8 @@ This means that cyclic creation dependencies are impossible.
public public
returns (OwnedToken tokenAddress) returns (OwnedToken tokenAddress)
{ {
// Create a new Token contract and return its address. // Create a new `Token` contract and return its address.
// From the JavaScript side, the return type is simply // From the JavaScript side, the return type is
// `address`, as this is the closest type available in // `address`, as this is the closest type available in
// the ABI. // the ABI.
return new OwnedToken(name); return new OwnedToken(name);
@ -113,12 +118,14 @@ This means that cyclic creation dependencies are impossible.
tokenAddress.changeName(name); tokenAddress.changeName(name);
} }
// Perform checks to determine if transferring a token to the
// `OwnedToken` contract should proceed
function isTokenTransferOK(address currentOwner, address newOwner) function isTokenTransferOK(address currentOwner, address newOwner)
public public
pure pure
returns (bool ok) returns (bool ok)
{ {
// Check some arbitrary condition. // Check an arbitrary condition to see if transfer should proceed
return keccak256(abi.encodePacked(currentOwner, newOwner))[0] == 0x7f; return keccak256(abi.encodePacked(currentOwner, newOwner))[0] == 0x7f;
} }
} }