Types¶

Solidity is a statically typed language, which means that the type of eachvariable (state and local) needs to be specified (or at least known -see Type Deduction below) atcompile-time. Solidity provides several elementary types which can be combinedto form complex types.

In addition, types can interact with each other in expressions containingoperators. For a quick reference of the various operators, see Order of Precedence of Operators.

Value Types¶

The following types are also called value types because variables of thesetypes will always be passed by value, i.e. they are always copied when theyare used as function arguments or in assignments.

Booleans¶

bool: The possible values are constants true and false.

Operators:

• ! (logical negation)
• && (logical conjunction, “and”)
• || (logical disjunction, “or”)
• == (equality)
• != (inequality)
  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.

Integers¶

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.

Operators:

• Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)
• Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation)
• Arithmetic operators: +, -, unary -, unary +, , /, % (remainder), * (exponentiation), << (left shift), >> (right shift)
  Division always truncates (it is just compiled to the DIV opcode of the EVM), but it does not truncate if bothoperators are literals (or literal expressions).

Division by zero and modulus with zero throws a runtime exception.

The result of a shift operation is the type of the left operand. Theexpression x << y is equivalent to x 2*y, and x >> y isequivalent to x / 2y. This means that shifting negative numberssign extends. Shifting by a negative amount throws a runtime exception.

Warning

The results produced by shift right of negative values of signed integer types is different from those producedby other programming languages. In Solidity, shift right maps to division so the shifted negative valuesare going to be rounded towards zero (truncated). In other programming languages the shift right of negative valuesworks like division with rounding down (towards negative infinity).

Fixed Point Numbers¶

Warning

Fixed point numbers are not fully supported by Solidity yet. They can be declared, butcannot be assigned to or from.

fixed / ufixed: Signed and unsigned fixed point number of various sizes. Keywords ufixedMxN and fixedMxN, where M represents the number of bits taken bythe type and N represents how many decimal points are available. M must be divisible by 8 and goes from 8 to 256 bits. N must be between 0 and 80, inclusive.ufixed and fixed are aliases for ufixed128x19 and fixed128x19, respectively.

Operators:

• Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)
• Arithmetic operators: +, -, unary -, unary +, *, /, % (remainder)

Note

The main difference between floating point (float and double in many languages, more precisely IEEE 754 numbers) and fixed point numbers isthat the number of bits used for the integer and the fractional part (the part after the decimal dot) is flexible in the former, while it is strictlydefined in the latter. Generally, in floating point almost the entire space is used to represent the number, while only a small number of bits definewhere the decimal point is.

Address¶

address: Holds a 20 byte value (size of an Ethereum address). Address types also have members and serve as a base for all contracts.

Operators:

• <=, <, ==, !=, >= and >

Note

Starting with version 0.5.0 contracts do not derive from the address type, but can still be explicitly converted to address.

Members of Addresses¶

• balance and transfer
  For a quick reference, see Address Related.

It is possible to query the balance of an address using the property balanceand to send Ether (in units of wei) to an address using the transfer function:

address x = 0x123;
address myAddress = this;
if (x.balance < 10 && myAddress.balance >= 10) x.transfer(10);

Note

If x is a contract address, its code (more specifically: its fallback function, if present) will be executed together with the transfer call (this is a feature 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 and the current contract will stop with an exception.

• send
  Send is the low-level counterpart of transfer. If the execution fails, the current contract will not stop with an exception, but send will return false.

Warning

There are some dangers in using send: The transfer fails if the call stack depth is at 1024(this can always be forced by the caller) and it also fails if the recipient runs out of gas. So in orderto make safe Ether transfers, always check the return value of send, use transfer or even better:use a pattern where the recipient withdraws the money.

• call, callcode and delegatecall
  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.

address nameReg = 0x72ba7d8e73fe8eb666ea66babc8116a41bfb10e2;
nameReg.call("register", "MyName");
nameReg.call(bytes4(keccak256("fun(uint256)")), a);

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).

It is possible to adjust the supplied gas with the .gas() modifier:

namReg.call.gas(1000000)("register", "MyName");

Similarly, the supplied Ether value can be controlled too:

nameReg.call.value(1 ether)("register", "MyName");

Lastly, these modifiers can be combined. Their order does not matter:

nameReg.call.gas(1000000).value(1 ether)("register", "MyName");

Note

It is not yet possible to use the gas or value modifiers on overloaded functions.

A workaround is to introduce a special case for gas and value and just re-checkwhether they are present at the point of overload resolution.

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.

The .gas() option is available on all three methods, while the .value() option is not supported for delegatecall.

Note

All contracts inherit the members of address, so it is possible to query the balance of thecurrent contract using this.balance.

Note

The use of callcode is discouraged and will be removed in the future.

Warning

All these functions are low-level functions and should be used with care.Specifically, any unknown contract might be malicious and if you call it, youhand over control to that contract which could in turn call back intoyour contract, so be prepared for changes to your state variableswhen the call returns.

Fixed-size byte arrays¶

bytes1, bytes2, bytes3, …, bytes32. byte is an alias for bytes1.

Operators:

• Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)
• Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation), << (left shift), >> (right shift)
• Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).
  The shifting operator works with any integer type as right operand (but willreturn the type of the left operand), which denotes the number of bits to shift by.Shifting by a negative amount will cause a runtime exception.

Members:

• .length yields the fixed length of the byte array (read-only).

Note

It is possible to use an array of bytes as byte[], but it is wasting a lot of space, 31 bytes every element,to be exact, when passing in calls. It is better to use bytes.

Dynamically-sized byte array¶

bytes:

Dynamically-sized byte array, see Arrays. Not a value-type!

string:

Dynamically-sized UTF-8-encoded string, see Arrays. Not a value-type!

uint128 a = 1;
uint128 b = 2.5 + a + 0.5;

pragma solidity ^0.4.16;

contract test {
    enum ActionChoices { GoLeft, GoRight, GoStraight, SitStill }
    ActionChoices choice;
    ActionChoices constant defaultChoice = ActionChoices.GoStraight;

    function setGoStraight() public {
        choice = ActionChoices.GoStraight;
    }

    // Since enum types are not part of the ABI, the signature of "getChoice"
    // will automatically be changed to "getChoice() returns (uint8)"
    // for all matters external to Solidity. The integer type used is just
    // large enough to hold all enum values, i.e. if you have more values,
    // uint16 will be used and so on.
    function getChoice() public view returns (ActionChoices) {
        return choice;
    }

    function getDefaultChoice() public pure returns (uint) {
        return uint(defaultChoice);
    }
}

function (<parameter types>) {internal|external} [pure|constant|view|payable] [returns (<return types>)]

pragma solidity ^0.4.16;

contract Selector {
  function f() public view returns (bytes4) {
    return this.f.selector;
  }
}

pragma solidity ^0.4.16;

library ArrayUtils {
  // internal functions can be used in internal library functions because
  // they will be part of the same code context
  function map(uint[] memory self, function (uint) pure returns (uint) f)
    internal
    pure
    returns (uint[] memory r)
  {
    r = new uint;
    for (uint i = 0; i < self.length; i++) {
      r[i] = f(self[i]);
    }
  }
  function reduce(
    uint[] memory self,
    function (uint, uint) pure returns (uint) f
  )
    internal
    pure
    returns (uint r)
  {
    r = self[0];
    for (uint i = 1; i < self.length; i++) {
      r = f(r, self[i]);
    }
  }
  function range(uint length) internal pure returns (uint[] memory r) {
    r = new uint;
    for (uint i = 0; i < r.length; i++) {
      r[i] = i;
    }
  }
}

contract Pyramid {
  using ArrayUtils for ;
  function pyramid(uint l) public pure returns (uint) {
    return ArrayUtils.range(l).map(square).reduce(sum);
  }
  function square(uint x) internal pure returns (uint) {
    return x  x;
  }
  function sum(uint x, uint y) internal pure returns (uint) {
    return x + y;
  }
}

pragma solidity ^0.4.21;

contract Oracle {
  struct Request {
    bytes data;
    function(bytes memory) external callback;
  }
  Request[] requests;
  event NewRequest(uint);
  function query(bytes data, function(bytes memory) external callback) public {
    requests.push(Request(data, callback));
    emit NewRequest(requests.length - 1);
  }
  function reply(uint requestID, bytes response) public {
    // Here goes the check that the reply comes from a trusted source
    requests[requestID].callback(response);
  }
}

contract OracleUser {
  Oracle constant oracle = Oracle(0x1234567); // known contract
  function buySomething() {
    oracle.query("USD", this.oracleResponse);
  }
  function oracleResponse(bytes response) public {
    require(msg.sender == address(oracle));
    // Use the data
  }
}

pragma solidity ^0.4.0;

contract C {
    uint[] x; // the data location of x is storage

    // the data location of memoryArray is memory
    function f(uint[] memoryArray) public {
        x = memoryArray; // works, copies the whole array to storage
        var y = x; // works, assigns a pointer, data location of y is storage
        y[7]; // fine, returns the 8th element
        y.length = 2; // fine, modifies x through y
        delete x; // fine, clears the array, also modifies y
        // The following does not work; it would need to create a new temporary /
        // unnamed array in storage, but storage is "statically" allocated:
        // y = memoryArray;
        // This does not work either, since it would "reset" the pointer, but there
        // is no sensible location it could point to.
        // delete y;
        g(x); // calls g, handing over a reference to x
        h(x); // calls h and creates an independent, temporary copy in memory
    }

    function g(uint[] storage storageArray) internal {}
    function h(uint[] memoryArray) public {}
}

Forced data location:

- parameters (not return) of external functions: calldata
- state variables: storage

Default data location:

- parameters (also return) of functions: memory
- all other local variables: storage

Arrays¶

Arrays can have a compile-time fixed size or they can be dynamic.For storage arrays, the element type can be arbitrary (i.e. also otherarrays, mappings or structs). For memory arrays, it cannot be a mapping andhas to be an ABI type if it is an argument of a publicly-visible function.

An array of fixed size k and element type T is written as T[k],an array of dynamic size as T[]. As an example, an array of 5 dynamicarrays of uint is uint[][5] (note that the notation is reversed whencompared to some other languages). To access the second uint in thethird dynamic array, you use x[2][1] (indices are zero-based andaccess works in the opposite way of the declaration, i.e. x[2]shaves off one level in the type from the right).

Variables of type bytes and string are special arrays. A bytes is similar to byte[],but it is packed tightly in calldata. string is equal to bytes but does not allowlength or index access (for now).

So bytes should always be preferred over byte[] because it is cheaper.

Note

If you want to access the byte-representation of a string s, usebytes(s).length / bytes(s)[7] = 'x';. Keep in mindthat you are accessing the low-level bytes of the UTF-8 representation,and not the individual characters!

It is possible to mark arrays public and have Solidity create a getter.The numeric index will become a required parameter for the getter.

Allocating Memory Arrays¶

Creating arrays with variable length in memory can be done using the new keyword.As opposed to storage arrays, it is not possible to resize memory arrays by assigning tothe .length member.

pragma solidity ^0.4.16;
 
contract C {
    function f(uint len) public pure {
        uint[] memory a = new uint[](7);
        bytes memory b = new bytes(len);
        // Here we have a.length == 7 and b.length == len
        a[6] = 8;
    }
}

Array Literals / Inline Arrays¶

Array literals are arrays that are written as an expression and are notassigned to a variable right away.

pragma solidity ^0.4.16;
 
contract C {
    function f() public pure {
        g([uint(1), 2, 3]);
    }
    function g(uint[3] _data) public pure {
        // ...
    }
}

The type of an array literal is a memory array of fixed size whose basetype is the common type of the given elements. The type of [1, 2, 3] isuint8[3] memory, because the type of each of these constants is uint8.Because of that, it was necessary to convert the first element in the exampleabove to uint. Note that currently, fixed size memory arrays cannotbe assigned to dynamically-sized memory arrays, i.e. the following is notpossible:

// This will not compile.
 
pragma solidity ^0.4.0;
 
contract C {
    function f() public {
        // The next line creates a type error because uint[3] memory
        // cannot be converted to uint[] memory.
        uint[] x = [uint(1), 3, 4];
    }
}

It is planned to remove this restriction in the future but currently createssome complications because of how arrays are passed in the ABI.

Members¶

length:

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 returndynamic content from external function calls. The function f incontract C { function f() returns (uint[]) { … } } will returnsomething if called from web3.js, but not if called from Solidity.

The only workaround for now is to use large statically-sized arrays.

pragma solidity ^0.4.16;
 
contract ArrayContract {
    uint[2**20] m_aLotOfIntegers;
    // Note that the following is not a pair of dynamic arrays but a
    // dynamic array of pairs (i.e. of fixed size arrays of length two).
    bool[2][] m_pairsOfFlags;
    // newPairs is stored in memory - the default for function arguments
 
    function setAllFlagPairs(bool[2][] newPairs) public {
        // assignment to a storage array replaces the complete array
        m_pairsOfFlags = newPairs;
    }
 
    function setFlagPair(uint index, bool flagA, bool flagB) public {
        // access to a non-existing index will throw an exception
        m_pairsOfFlags[index][0] = flagA;
        m_pairsOfFlags[index][1] = flagB;
    }
 
    function changeFlagArraySize(uint newSize) public {
        // if the new size is smaller, removed array elements will be cleared
        m_pairsOfFlags.length = newSize;
    }
 
    function clear() public {
        // these clear the arrays completely
        delete m_pairsOfFlags;
        delete m_aLotOfIntegers;
        // identical effect here
        m_pairsOfFlags.length = 0;
    }
 
    bytes m_byteData;
 
    function byteArrays(bytes data) public {
        // byte arrays ("bytes") are different as they are stored without padding,
        // but can be treated identical to "uint8[]"
        m_byteData = data;
        m_byteData.length += 7;
        m_byteData[3] = byte(8);
        delete m_byteData[2];
    }
 
    function addFlag(bool[2] flag) public returns (uint) {
        return m_pairsOfFlags.push(flag);
    }
 
    function createMemoryArray(uint size) public pure returns (bytes) {
        // Dynamic memory arrays are created using `new`:
        uint[2][] memory arrayOfPairs = new uint[2][](size);
        // Create a dynamic byte array:
        bytes memory b = new bytes(200);
        for (uint i = 0; i < b.length; i++)
            b[i] = byte(i);
        return b;
    }
}

Structs¶

Solidity provides a way to define new types in the form of structs, which isshown in the following example:

pragma solidity ^0.4.11;
 
contract CrowdFunding {
    // Defines a new type with two fields.
    struct Funder {
        address addr;
        uint amount;
    }
 
    struct Campaign {
        address beneficiary;
        uint fundingGoal;
        uint numFunders;
        uint amount;
        mapping (uint => Funder) funders;
    }
 
    uint numCampaigns;
    mapping (uint => Campaign) campaigns;
 
    function newCampaign(address beneficiary, uint goal) public returns (uint campaignID) {
        campaignID = numCampaigns++; // campaignID is return variable
        // Creates new struct and saves in storage. We leave out the mapping type.
        campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0);
    }
 
    function contribute(uint campaignID) public payable {
        Campaign storage c = campaigns[campaignID];
        // Creates a new temporary memory struct, initialised with the given values
        // and copies it over to storage.
        // Note that you can also use Funder(msg.sender, msg.value) to initialise.
        c.funders[c.numFunders++] = Funder({addr: msg.sender, amount: msg.value});
        c.amount += msg.value;
    }
 
    function checkGoalReached(uint campaignID) public returns (bool reached) {
        Campaign storage c = campaigns[campaignID];
        if (c.amount < c.fundingGoal)
            return false;
        uint amount = c.amount;
        c.amount = 0;
        c.beneficiary.transfer(amount);
        return true;
    }
}

The contract does not provide the full functionality of a crowdfundingcontract, but it contains the basic concepts necessary to understand structs.Struct types can be used inside mappings and arrays and they can itselfcontain 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 tomembers of the local variable actually write to the state.

Of course, you can also directly access the members of the struct withoutassigning it to a local variable, as incampaigns[campaignID].amount = 0.

Mappings¶

Mapping types are declared as mapping(_KeyType => _ValueType).Here _KeyType can be almost any type except for a mapping, a dynamically sized array, a contract, an enum and a struct._ValueType can actually be any type, including mappings.

Mappings can be seen as hash tables which are virtually initialized such thatevery possible key exists and is mapped to a value whose byte-representation isall zeros: a type’s default value. The similarity ends here, though: The key data is not actually storedin a mapping, only its keccak256 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 typesin internal functions).

It is possible to mark mappings public and have Solidity create a getter.The _KeyType will become a required parameter for the getter and it willreturn _ValueType.

The _ValueType can be a mapping too. The getter will have one parameterfor each _KeyType, recursively.

pragma solidity ^0.4.0;
 
contract MappingExample {
    mapping(address => uint) public balances;
 
    function update(uint newBalance) public {
        balances[msg.sender] = newBalance;
    }
}
 
contract MappingUser {
    function f() public returns (uint) {
        MappingExample m = new MappingExample();
        m.update(100);
        return m.balances(this);
    }
}

Note

Mappings are not iterable, but it is possible to implement a data structure on top of them.For an example, see iterable mapping.

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 -=, *=, /=, %=, |=, &= 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.

pragma solidity ^0.4.0;
 
contract DeleteExample {
    uint data;
    uint[] dataArray;
 
    function f() public {
        uint x = data;
        delete x; // sets x to 0, does not affect data
        delete data; // sets data to 0, does not affect x which still holds a copy
        uint[] storage y = dataArray;
        delete dataArray; // this sets dataArray.length to zero, but as uint[] is a complex object, also
        // y is affected which is an alias to the storage object
        // On the other hand: "delete y" is not valid, as assignments to local variables
        // referencing storage objects can only be made from existing storage objects.
    }
}

Conversions between Elementary Types¶

Implicit Conversions¶

If an operator is applied to different types, the compiler tries toimplicitly convert one of the operands to the type of the other (the same istrue for assignments). In general, an implicit conversion between value-typesis possible if itmakes sense semantically and no information is lost: uint8 is convertible touint16 and int128 to int256, but int8 is not convertible to uint256(because uint256 cannot hold e.g. -1).Furthermore, unsigned integers can be converted to bytes of the same or largersize, but not vice-versa. Any type that can be converted to uint160 can alsobe converted to address.

Explicit Conversions¶

If the compiler does not allow implicit conversion but you know what you aredoing, an explicit type conversion is sometimes possible. Note that this maygive you some unexpected behaviour so be sure to test to ensure that theresult is what you want! Take the following example where you are convertinga negative int8 to a uint:

int8 y = -3;
uint x = uint(y);

At the end of this code snippet, x will have the value 0xfffff..fd (64 hexcharacters), which is -3 in the two’s complement representation of 256 bits.

If a type is explicitly converted to a smaller type, higher-order bits arecut off:

uint32 a = 0x12345678;
uint16 b = uint16(a); // b will be 0x5678 now

Type Deduction¶

For convenience, it is not always necessary to explicitly specify the type of avariable, the compiler automatically infers it from the type of the firstexpression that is assigned to the variable:

uint24 x = 0x123;
var y = x;

Here, the type of y will be uint24. Using var is not possible for functionparameters or return parameters.

Warning

The type is only deduced from the first assignment, sothe loop in the following snippet is infinite, as i will have the typeuint8 and the highest value of this type is smaller than 2000.for (var i = 0; i < 2000; i++) { … }

原文: http://solidity.apachecn.org/cn/doc/v0.4.21/types.html