Workshop: Ethereum e Smart contracts

Notas do Editor

• #5: Também funciona como incentivo para que os desenvolvedores escrevam aplicativos de qualidade (o desperdício de código é custoso) e que a rede permaneça saudável (as pessoas são compensadas pelos recursos contribuídos).
• #10: You can think of it as a single slot in a database that can be queried and altered by calling functions of the code that manages the database. In the case of Ethereum, this is always the owning contract. This contract does not do much yet (due to the infrastructure built by Ethereum) apart from allowing anyone to store a single number that is accessible by anyone in the world without a (feasible) way to prevent you from publishing this number. Of course, anyone could just call set again with a different value and overwrite your number, but the number will still be stored in the history of the blockchain. Line 1: Tells that the source code is written for Solidity version 0.4.0 or anything newer that does not break functionality Line 3: Declares a state variable called storedData of type "uint" (unsigned integer of 256 bits) Line 5-11: And in this case, the functions set and get can be used to modify or retrieve the value of the variable. Line 11: To access a state variable, you do not need the prefix this. as is common in other languages.To access a state variable, you do not need the prefix this. as is common in other languages.
• #12: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.
• #13: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.
• #14: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.
• #15: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.
• #16: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.
• #17: The following contract will implement the simplest form of a cryptocurrency. It is possible to generate coins out of thin air, but only the person that created the contract will be able to do that (it is trivial to implement a different issuance scheme). Furthermore, anyone can send coins to each other without any need for registering with username and password - all you need is an Ethereum keypair.Linha 4: Declares a state variable of type address that is publicly accessible. The keyword publicautomatically generates a function that allows you to access the current value of the state variable. Without this keyword, other contracts have no way to access the variable.The function will look something like this: function minter() returns (address) { return minter; } Linha 5: creates a public state variable, but it is a more complex datatype. The type maps addresses to unsigned integers Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. This analogy does not go too far, though, as it is neither possible to obtain a list of all keys of a mapping, nor a list of all values. So either keep in mind (or better, keep a list or use a more advanced data type) what you added to the mapping or use it in a context where this is not needed, like this one. The getter function created by the publickeyword is a bit more complex in this case. It roughly looks like the following: function balances(address _account) returns (uint) { return balances[_account];} As you see, you can use this function to easily query the balance of a single account. Linha 7: declares a so-called “event” which is fired in the last line of the function send. Linha 9: is the constructor which is run during creation of the contract and cannot be called afterwards. It permanently stores the address of the person creating the contract: msg (together with tx and block) is a magic global variable that contains some properties which allow access to the blockchain. msg.sender is always the address where the current (external) function call came from.