Expressions and Control Structures

Input Parameters and Output Parameters

As in Javascript, functions may take parameters as input; unlike in Javascript and C, they may also return arbitrary number of parameters as output.

Input Parameters

The input parameters are declared the same way as variables are. As an exception, unused parameters can omit the variable name. For example, suppose we want our contract to accept one kind of external calls with two integers, we would write something like:

contract Simple {
    function taker(uint _a, uint _b) {
        // do something with _a and _b.
    }
}

Output Parameters

The output parameters can be declared with the same syntax after the returns keyword. For example, suppose we wished to return two results: the sum and the product of the two given integers, then we would write:

contract Simple {
    function arithmetics(uint _a, uint _b) returns (uint o_sum, uint o_product) {
        o_sum = _a + _b;
        o_product = _a * _b;
    }
}

The names of output parameters can be omitted. The output values can also be specified using return statements. The return statements are also capable of returning multiple values, see Returning Multiple Values. Return parameters are initialized to zero; if they are not explicitly set, they stay to be zero.

Input parameters and output parameters can be used as expressions in the function body. There, they are also usable in the left-hand side of assignment.

Control Structures

Most of the control structures from JavaScript are available in Solidity except for switch and goto. So there is: if, else, while, do, for, break, continue, return, ? :, with the usual semantics known from C or JavaScript.

Parentheses can not be omitted for conditionals, but curly brances can be omitted around single-statement bodies.

Note that there is no type conversion from non-boolean to boolean types as there is in C and JavaScript, so if (1) { ... } is not valid Solidity.

Returning Multiple Values

When a function has multiple output parameters, return (v0, v1, ..., vn) can return multiple values. The number of components must be the same as the number of output parameters.

Function Calls

Internal Function Calls

Functions of the current contract can be called directly (“internally”), also recursively, as seen in this nonsensical example:

contract C {
    function g(uint a) returns (uint ret) { return f(); }
    function f() returns (uint ret) { return g(7) + f(); }
}

These function calls are translated into simple jumps inside the EVM. This has the effect that the current memory is not cleared, i.e. passing memory references to internally-called functions is very efficient. Only functions of the same contract can be called internally.

External Function Calls

The expressions this.g(8); and c.g(2); (where c is a contract instance) are also valid function calls, but this time, the function will be called “externally”, via a message call and not directly via jumps. Please note that function calls on this cannot be used in the constructor, as the actual contract has not been created yet.

Functions of other contracts have to be called externally. For an external call, all function arguments have to be copied to memory.

When calling functions of other contracts, the amount of Wei sent with the call and the gas can be specified:

contract InfoFeed {
    function info() payable returns (uint ret) { return 42; }
}


contract Consumer {
    InfoFeed feed;
    function setFeed(address addr) { feed = InfoFeed(addr); }
    function callFeed() { feed.info.value(10).gas(800)(); }
}

The modifier payable has to be used for info, because otherwise, we would not be able to send Ether to it in the call feed.info.value(10).gas(800)().

Note that the expression InfoFeed(addr) performs an explicit type conversion stating that “we know that the type of the contract at the given address is InfoFeed” and this does not execute a constructor. Explicit type conversions have to be handled with extreme caution. Never call a function on a contract where you are not sure about its type.

We could also have used function setFeed(InfoFeed _feed) { feed = _feed; } directly. Be careful about the fact that feed.info.value(10).gas(800) only (locally) sets the value and amount of gas sent with the function call and only the parentheses at the end perform the actual call.

Function calls cause exceptions if the called contract does not exist (in the sense that the account does not contain code) or if the called contract itself throws an exception or goes out of gas.

Warning

Any interaction with another contract imposes a potential danger, especially if the source code of the contract is not known in advance. The current contract hands over control to the called contract and that may potentially do just about anything. Even if the called contract inherits from a known parent contract, the inheriting contract is only required to have a correct interface. The implementation of the contract, however, can be completely arbitrary and thus, pose a danger. In addition, be prepared in case it calls into other contracts of your system or even back into the calling contract before the first call returns. This means that the called contract can change state variables of the calling contract via its functions. Write your functions in a way that, for example, calls to external functions happen after any changes to state variables in your contract so your contract is not vulnerable to a reentrancy exploit.

Named Calls and Anonymous Function Parameters

Function call arguments can also be given by name, in any order, if they are enclosed in { } as can be seen in the following example. The argument list has to coincide by name with the list of parameters from the function declaration, but can be in arbitrary order.

pragma solidity ^0.4.0;

contract C {
    function f(uint key, uint value) { ... }

    function g() {
        // named arguments
        f({value: 2, key: 3});
    }
}

Omitted Function Parameter Names

The names of unused parameters (especially return parameters) can be omitted. Those names will still be present on the stack, but they are inaccessible.

pragma solidity ^0.4.0;

contract C {
    // omitted name for parameter
    function func(uint k, uint) returns(uint) {
        return k;
    }
}

Creating Contracts via new

A contract can create a new contract using the new keyword. The full code of the contract being created has to be known in advance, so recursive creation-dependencies are not possible.

pragma solidity ^0.4.0;

contract D {
    uint x;
    function D(uint a) payable {
        x = a;
    }
}


contract C {
    D d = new D(4); // will be executed as part of C's constructor

    function createD(uint arg) {
        D newD = new D(arg);
    }

    function createAndEndowD(uint arg, uint amount) {
        // Send ether along with the creation
        D newD = (new D).value(amount)(arg);
    }
}

As seen in the example, it is possible to forward Ether to the creation, 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.

Order of Evaluation of Expressions

The evaluation order of expressions is not specified (more formally, the order in which the children of one node in the expression tree are evaluated is not specified, but they are of course evaluated before the node itself). It is only guaranteed that statements are executed in order and short-circuiting for boolean expressions is done. See Order of Precedence of Operators for more information.

Assignment

Destructuring Assignments and Returning Multiple Values

Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose size is a constant at compile-time. Those tuples can be used to return multiple values at the same time and also assign them to multiple variables (or LValues in general) at the same time:

contract C {
    uint[] data;

    function f() returns (uint, bool, uint) {
        return (7, true, 2);
    }

    function g() {
        // Declares and assigns the variables. Specifying the type explicitly is not possible.
        var (x, b, y) = f();
        // Assigns to a pre-existing variable.
        (x, y) = (2, 7);
        // Common trick to swap values -- does not work for non-value storage types.
        (x, y) = (y, x);
        // Components can be left out (also for variable declarations).
        // If the tuple ends in an empty component,
        // the rest of the values are discarded.
        (data.length,) = f(); // Sets the length to 7
        // The same can be done on the left side.
        (,data[3]) = f(); // Sets data[3] to 2
        // Components can only be left out at the left-hand-side of assignments, with
        // one exception:
        (x,) = (1,);
        // (1,) is the only way to specify a 1-component tuple, because (1) is
        // equivalent to 1.
    }
}

Complications for Arrays and Structs

The semantics of assignment 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.

Scoping and Declarations

A variable which is declared will have an initial default value whose byte-representation is all zeros. The “default values” of variables are the typical “zero-state” of whatever the type is. For example, the default value for a bool is false. The default value for the uint or int types is 0. For statically-sized arrays and bytes1 to bytes32, each individual element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, bytes and string, the default value is an empty array or string.

A variable declared anywhere within a function will be in scope for the entire function, regardless of where it is declared. This happens because Solidity inherits its scoping rules from JavaScript. This is in contrast to many languages where variables are only scoped where they are declared until the end of the semantic block. As a result, the following code is illegal and cause the compiler to throw an error, Identifier already declared:

pragma solidity ^0.4.0;

contract ScopingErrors {
    function scoping() {
        uint i = 0;

        while (i++ < 1) {
            uint same1 = 0;
        }

        while (i++ < 2) {
            uint same1 = 0;// Illegal, second declaration of same1
        }
    }

    function minimalScoping() {
        {
            uint same2 = 0;
        }

        {
            uint same2 = 0;// Illegal, second declaration of same2
        }
    }

    function forLoopScoping() {
        for (uint same3 = 0; same3 < 1; same3++) {
        }

        for (uint same3 = 0; same3 < 1; same3++) {// Illegal, second declaration of same3
        }
    }
}

In addition to this, if a variable is declared, it will be initialized at the beginning of the function to its default value. As a result, the following code is legal, despite being poorly written:

function foo() returns (uint) {
    // baz is implicitly initialized as 0
    uint bar = 5;
    if (true) {
        bar += baz;
    } else {
        uint baz = 10;// never executes
    }
    return bar;// returns 5
}

Exceptions

There are some cases where exceptions are thrown automatically (see below). You can use the throw instruction to throw an exception manually. The effect of an exception is that the currently executing call is stopped and reverted (i.e. all changes to the state and balances are undone) and the exception is also “bubbled up” through Solidity function calls (exceptions are send and the low-level functions call, delegatecall and callcode, those return false in case of an exception).

Catching exceptions is not yet possible.

In the following example, we show how throw can be used to easily revert an Ether transfer and also how to check the return value of send:

pragma solidity ^0.4.0;

contract Sharer {
    function sendHalf(address addr) payable returns (uint balance) {
        if (!addr.send(msg.value / 2))
            throw; // also reverts the transfer to Sharer
        return this.balance;
    }
}

Currently, Solidity automatically generates a runtime exception in the following situations:

  1. If you access an array at a too large or negative index (i.e. x[i] where i >= x.length or i < 0).
  2. If you access a fixed-length bytesN at a too large or negative index.
  3. If you call a function via a message call but it does not finish properly (i.e. it runs out of gas, has no matching function, or throws an exception itself), except when a low level operation call, send, delegatecall or callcode is used. The low level operations never throw exceptions but indicate failures by returning false.
  4. If you create a contract using the new keyword but the contract creation does not finish properly (see above for the definition of “not finish properly”).
  5. If you divide or modulo by zero (e.g. 5 / 0 or 23 % 0).
  6. If you shift by a negative amount.
  7. If you convert a value too big or negative into an enum type.
  8. If you perform an external function call targeting a contract that contains no code.
  9. If your contract receives Ether via a public function without payable modifier (including the constructor and the fallback function).
  10. If your contract receives Ether via a public getter function.
  11. If you call a zero-initialized variable of internal function type.
  12. If a .transfer() fails.
  13. If you call assert with an argument that evaluates to false.

While a user-provided exception is generated in the following situations: #. Calling throw. #. Calling require with an argument that evaluates to false.

Internally, Solidity performs a revert operation (instruction 0xfd) when a user-provided exception is thrown or the condition of a require call is not met. In contrast, it performs an invalid operation (instruction 0xfe) if a runtime exception is encountered or the condition of an assert call is not met. In both cases, this causes the EVM to revert all changes made to the state. The reason for this is that there is no safe way to continue execution, because an expected effect did not occur. Because we want to retain the atomicity of transactions, the safest thing to do is to revert all changes and make the whole transaction (or at least call) without effect.

If contracts are written so that assert is only used to test internal conditions and require is used in case of malformed input, a formal analysis tool that verifies that the invalid opcode can never be reached can be used to check for the absence of errors assuming valid inputs.