using constants external contract ethereum code example
Example: Expected a state variable declaration. If you intended this as a fallback function or a function to handle plain ether transactions, use the "fallback" keyword or the "receive" keyword instead.
pragma solidity >=0.4.22 <0.6.0;
contract OwnedToken {
// `TokenCreator` is a contract type that is defined below.
// It is fine to reference it as long as it is not used
// to create a new contract.
TokenCreator creator;
address owner;
bytes32 name;
// This is the constructor which registers the
// creator and the assigned name.
constructor(bytes32 _name) public {
// State variables are accessed via their name
// and not via e.g. `this.owner`. Functions can
// be accessed directly or through `this.f`,
// but the latter provides an external view
// 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;
// We do an explicit type conversion from `address`
// to `TokenCreator` and assume that the type of
// the calling contract is `TokenCreator`, there is
// no real way to check that.
creator = TokenCreator(msg.sender);
name = _name;
}
function changeName(bytes32 newName) public {
// Only the creator can alter the name --
// the comparison is possible since contracts
// are explicitly convertible to addresses.
if (msg.sender == address(creator))
name = newName;
}
function transfer(address newOwner) public {
// Only the current owner can transfer the token.
if (msg.sender != owner) return;
// We ask the creator contract if the transfer
// should proceed by using a function of the
// `TokenCreator` contract defined below. If
// the call fails (e.g. due to out-of-gas),
// the execution also fails here.
if (creator.isTokenTransferOK(owner, newOwner))
owner = newOwner;
}
}
contract TokenCreator {
function createToken(bytes32 name)
public
returns (OwnedToken tokenAddress)
{
// Create a new `Token` contract and return its address.
// From the JavaScript side, the return type is
// `address`, as this is the closest type available in
// the ABI.
return new OwnedToken(name);
}
function changeName(OwnedToken tokenAddress, bytes32 name) public {
// Again, the external type of `tokenAddress` is
// simply `address`.
tokenAddress.changeName(name);
}
// Perform checks to determine if transferring a token to the
// `OwnedToken` contract should proceed
function isTokenTransferOK(address currentOwner, address newOwner)
public
pure
returns (bool ok)
{
// Check an arbitrary condition to see if transfer should proceed
return keccak256(abi.encodePacked(currentOwner, newOwner))[0] == 0x7f;
}
}