Explanation to Expressions and Control Structures in Solidity

Expressions and Control Structures in Solidity

  • Control Structures
  • Function Calls
  • Creating Contracts via new
  • Order of Evaluation of Expressions
  • Assignment
  • Scoping and Declarations
  • Error handling: Assert, Require, Revert and Exceptions

Control Structures

Most of the control structures known from curly-braces languages are available in Solidity:

There is: if, else, while, do, for, break, continue, return, with the usual semantics known from C or JavaScript.

Solidity also supports exception handling in the form of try/catch-statements, but only for external function calls and contract creation calls.

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.

Function Calls

Internal Function Calls

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

pragma solidity >=0.4.16 <0.7.0;

contract C {
    function g(uint a) public pure returns (uint ret) { return a + f(); }
    function f() internal pure 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 instance can be called internally.

You should still avoid excessive recursion, as every internal function call uses up at least one stack slot and there are only 1024 slots available.

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.

Note

A function call from one contract to another does not create its own transaction, it is a message call as part of the overall transaction.

When calling functions of other contracts, you can specify the amount of Wei or gas sent with the call with the special options .value() and .gas(), respectively. Any Wei you send to the contract is added to the total balance of the contract:

pragma solidity >=0.4.0 <0.7.0;

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

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

You need to use the modifier payable with the info function because otherwise, the .value() option would not be available.

Warning

Be careful that feed.info.value(10).gas(800) only locally sets the value and amount of gas sent with the function call, and the parentheses at the end perform the actual call. So in this case, the function is not called and the value and gas settings are lost.

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 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 <0.7.0;

contract C {
    mapping(uint => uint) data;

    function f() public {
        set({value: 2, key: 3});
    }

    function set(uint key, uint value) public {
        data[key] = value;
    }

}

Omitted Function Parameter Names

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

pragma solidity >=0.4.16 <0.7.0;

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

Creating Contracts via new

A contract can create other contracts using the new keyword. The full code of the contract being created has to be known when the creating contract is compiled so recursive creation-dependencies are not possible.

pragma solidity >=0.5.0 <0.7.0;

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

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

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

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

As seen in the example, it is possible to send Ether while creating an instance of D using the .value() option, 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.

Assignment

Destructuring Assignments and Returning Multiple Values

Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose number is a constant at compile-time. Those tuples can be used to return multiple values at the same time. These can then either be assigned to newly declared variables or to pre-existing variables (or LValues in general).

Tuples are not proper types in Solidity, they can only be used to form syntactic groupings of expressions.

pragma solidity >0.4.23 <0.7.0;

contract C {
    uint index;

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

    function g() public {
        // Variables declared with type and assigned from the returned tuple,
        // not all elements have to be specified (but the number must match).
        (uint x, , uint y) = f();
        // 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).
        (index, , ) = f(); // Sets the index to 7
    }
}

It is not possible to mix variable declarations and non-declaration assignments, i.e. the following is not valid: (x, uint y) = (1, 2);

Note

Prior to version 0.5.0 it was possible to assign to tuples of smaller size, either filling up on the left or on the right side (which ever was empty). This is now disallowed, so both sides have to have the same number of components.

Warning

Be careful when assigning to multiple variables at the same time when reference types are involved, because it could lead to unexpected copying behaviour.

Complications for Arrays and Structs

The semantics of assignments 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.

In the example below the call to g(x) has no effect on x because it creates an independent copy of the storage value in memory. However, h(x) successfully modifies x because only a reference and not a copy is passed.

pragma solidity >=0.4.16 <0.7.0;

contract C {
    uint[20] x;

    function f() public {
        g(x);
        h(x);
    }

    function g(uint[20] memory y) internal pure {
        y[2] = 3;
    }

    function h(uint[20] storage y) internal {
        y[3] = 4;
    }
}

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. For dynamically-sized arrays, bytes and string, the default value is an empty array or string. For the enum type, the default value is its first member.

Scoping in Solidity follows the widespread scoping rules of C99 (and many other languages): Variables are visible from the point right after their declaration until the end of the smallest { }-block that contains the declaration. As an exception to this rule, variables declared in the initialization part of a for-loop are only visible until the end of the for-loop.

Variables that are parameter-like (function parameters, modifier parameters, catch parameters, …) are visible inside the code block that follows – the body of the function/modifier for a function and modifier parameter and the catch block for a catch parameter.

Variables and other items declared outside of a code block, for example functions, contracts, user-defined types, etc., are visible even before they were declared. This means you can use state variables before they are declared and call functions recursively.

As a consequence, the following examples will compile without warnings, since the two variables have the same name but disjoint scopes.

pragma solidity >=0.5.0 <0.7.0;
contract C {
    function minimalScoping() pure public {
        {
            uint same;
            same = 1;
        }

        {
            uint same;
            same = 3;
        }
    }
}

As a special example of the C99 scoping rules, note that in the following, the first assignment to x will actually assign the outer and not the inner variable. In any case, you will get a warning about the outer variable being shadowed.

pragma solidity >=0.5.0 <0.7.0;
// This will report a warning
contract C {
    function f() pure public returns (uint) {
        uint x = 1;
        {
            x = 2; // this will assign to the outer variable
            uint x;
        }
        return x; // x has value 2
    }
}

Warning

Before version 0.5.0 Solidity followed the same scoping rules as JavaScript, that is, a variable declared anywhere within a function would be in scope for the entire function, regardless where it was declared. The following example shows a code snippet that used to compile but leads to an error starting from version 0.5.0.

pragma solidity >=0.5.0 <0.7.0;
// This will not compile
contract C {
    function f() pure public returns (uint) {
        x = 2;
        uint x;
        return x;
    }
}

Error handling: Assert, Require, Revert and Exceptions

Solidity uses state-reverting exceptions to handle errors. Such an exception undoes all changes made to the state in the current call (and all its sub-calls) and flags an error to the caller.

When exceptions happen in a sub-call, they “bubble up” (i.e., exceptions are rethrown) automatically. Exceptions to this rule are send and the low-level functions call, delegatecall and staticcall: they return false as their first return value in case of an exception instead of “bubbling up”.

Warning

The low-level functions call, delegatecall and staticcall return true as their first return value if the account called is non-existent, as part of the design of the EVM. Account existence must be checked prior to calling if needed.

Exceptions can be caught with the try/catch statement.

assert and require

The convenience functions assert and require can be used to check for conditions and throw an exception if the condition is not met.

The assert function should only be used to test for internal errors, and to check invariants. Properly functioning code should never reach a failing assert statement; if this happens there is a bug in your contract which you should fix. Language analysis tools can evaluate your contract to identify the conditions and function calls which will reach a failing assert.

An assert-style exception is generated in the following situations:

  1.  you access an array or an array slice 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 divide or modulo by zero (e.g. 5 / 0 or 23 % 0).
  4. If you shift by a negative amount.
  5. If you convert a value too big or negative into an enum type.
  6. If you call a zero-initialized variable of internal function type.
  7. If you call assert with an argument that evaluates to false.

The require function should be used to ensure valid conditions that cannot be detected until execution time. This includes conditions on inputs or return values from calls to external contracts.

A require-style exception is generated in the following situations:

  1. Calling require with an argument that evaluates to false.
  2. 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, callcode or staticcall is used. The low level operations never throw exceptions but indicate failures by returning false.
  3. If you create a contract using the new keyword but the contract creation does not finish properly.
  4. If you perform an external function call targeting a contract that contains no code.
  5. If your contract receives Ether via a public function without payable modifier (including the constructor and the fallback function).
  6. If your contract receives Ether via a public getter function.
  7. If a .transfer() fails.

You can optionally provide a message string for require, but not for assert.

The following example shows how you can use require to check conditions on inputs and assert for internal error checking.

pragma solidity >=0.5.0 <0.7.0;

contract Sharer {
    function sendHalf(address payable addr) public payable returns (uint balance) {
        require(msg.value % 2 == 0, "Even value required.");
        uint balanceBeforeTransfer = address(this).balance;
        addr.transfer(msg.value / 2);
        // Since transfer throws an exception on failure and
        // cannot call back here, there should be no way for us to
        // still have half of the money.
        assert(address(this).balance == balanceBeforeTransfer - msg.value / 2);
        return address(this).balance;
    }
}

Internally, Solidity performs a revert operation (instruction 0xfd) for a require-style exception and executes an invalid operation (instruction 0xfe) to throw an assert-style exception. In both cases, this causes the EVM to revert all changes made to the state. The reason for reverting is that there is no safe way to continue execution, because an expected effect did not occur. Because we want to keep the atomicity of transactions, the safest action is to revert all changes and make the whole transaction (or at least call) without effect.

In both cases, the caller can react on such failures using try/catch (in the failing assert-style exception only if enough gas is left), but the changes in the caller will always be reverted.

Note

assert-style exceptions consume all gas available to the call, while require-style exceptions do not consume any gas starting from the Metropolis release.

revert

The revert function is another way to trigger exceptions from within other code blocks to flag an error and revert the current call. The function takes an optional string message containing details about the error that is passed back to the caller.

The following example shows how to use an error string together with revert and the equivalent require:

pragma solidity >=0.5.0 <0.7.0;

contract VendingMachine {
    function buy(uint amount) public payable {
        if (amount > msg.value / 2 ether)
            revert("Not enough Ether provided.");
        // Alternative way to do it:
        require(
            amount <= msg.value / 2 ether,
            "Not enough Ether provided."
        );
        // Perform the purchase.
    }
}

The two syntax options are equivalent, it’s developer preference which to use.

The provided string is abi-encoded as if it were a call to a function Error(string). In the above example, revert("Not enough Ether provided."); returns the following hexadecimal as error return data:

0x08c379a0                                                         // Function selector for Error(string)
0x0000000000000000000000000000000000000000000000000000000000000020 // Data offset
0x000000000000000000000000000000000000000000000000000000000000001a // String length
0x4e6f7420656e6f7567682045746865722070726f76696465642e000000000000 // String data

The provided message can be retrieved by the caller using try/catch as shown below.

Note

There used to be a keyword called throw with the same semantics as revert() which was deprecated in version 0.4.13 and removed in version 0.5.0.

try/catch

A failure in an external call can be caught using a try/catch statement, as follows:

pragma solidity >=0.5.0 <0.7.0;

interface DataFeed { function getData(address token) external returns (uint value); }

contract FeedConsumer {
    DataFeed feed;
    uint errorCount;
    function rate(address token) public returns (uint value, bool success) {
        // Permanently disable the mechanism if there are
        // more than 10 errors.
        require(errorCount < 10);
        try feed.getData(token) returns (uint v) {
            return (v, true);
        } catch Error(string memory /*reason*/) {
            // This is executed in case
            // revert was called inside getData
            // and a reason string was provided.
            errorCount++;
            return (0, false);
        } catch (bytes memory /*lowLevelData*/) {
            // This is executed in case revert() was used
            // or there was a failing assertion, division
            // by zero, etc. inside getData.
            errorCount++;
            return (0, false);
        }
    }
}

The try keyword has to be followed by an expression representing an external function call or a contract creation (new ContractName()). Errors inside the expression are not caught (for example if it is a complex expression that also involves internal function calls), only a revert happening inside the external call itself. The returns part (which is optional) that follows declares return variables matching the types returned by the external call. In case there was no error, these variables are assigned and the contract’s execution continues inside the first success block. If the end of the success block is reached, execution continues after the catch blocks.

Currently, Solidity supports different kinds of catch blocks depending on the type of error. If the error was caused by revert("reasonString") or require(false, "reasonString") (or an internal error that causes such an exception), then the catch clause of the type catch Error(string memory reason) will be executed.

It is planned to support other types of error data in the future. The string Error is currently parsed as is and is not treated as an identifier.

The clause catch (bytes memory lowLevelData) is executed if the error signature does not match any other clause, there was an error during decoding of the error message, if there was a failing assertion in the external call (for example due to a division by zero or a failing assert()) or if no error data was provided with the exception. The declared variable provides access to the low-level error data in that case.

If you are not interested in the error data, you can just use catch { ... } (even as the only catch clause).

In order to catch all error cases, you have to have at least the clause catch { ...} or the clause catch (bytes memory lowLevelData) { ... }.

The variables declared in the returns and the catch clause are only in scope in the block that follows.

Note

If an error happens during the decoding of the return data inside a try/catch-statement, this causes an exception in the currently executing contract and because of that, it is not caught in the catch clause. If there is an error during decoding of catch Error(string memory reason) and there is a low-level catch clause, this error is caught there.

Note

If execution reaches a catch-block, then the state-changing effects of the external call have been reverted. If execution reaches the success block, the effects were not reverted. If the effects have been reverted, then execution either continues in a catch block or the execution of the try/catch statement itself reverts (for example due to decoding failures as noted above or due to not providing a low-level catch clause).

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