Solidity changes in version 0.6.0

Wednesday, 13/05/2020

Tram Ho

Preamble

Solidity is a programming language familiar to blockchain application developers, used to build smart contracts. In this 0.6 version, solidity has made significant changes to support the construction of smart contracts.

Major changes

Abstract

The abstract keyword used to denote a contract is abstract (only declare the function, not implement its functions) – similar to the abstract class in Java. Contract must be declared abstract when at least 1 function is not implemented, but it is still possible to declare a contract as abstract even though all of its functions have been implemented.

Need to distinguish abstract from interface . All functions in the interface are not implemented.

For example:

pragma solidity <span class="token operator">^</span> <span class="token number">0.6</span> <span class="token number">.0</span> <span class="token punctuation">;</span>

abstract contract Feline <span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">utterance</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token keyword">public</span> virtual <span class="token function">returns</span> <span class="token punctuation">(</span> bytes32 <span class="token punctuation">)</span> <span class="token punctuation">;</span>
<span class="token punctuation">}</span>

contract Cat is Feline <span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">utterance</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token keyword">public</span> override <span class="token function">returns</span> <span class="token punctuation">(</span> bytes32 <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token string">"miaow"</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

pragma solidity ^ 0.6 .0 ;

abstract contract Feline {

function utterance ( ) public virtual returns ( bytes32 ) ;

}

contract Cat is Feline {

function utterance ( ) public override returns ( bytes32 ) { return "miaow" ; }

}

Note: When a contract inherits from 1 abstract contract. and do not execute all non-implemented functions by overriding it, the contract must also be declared as abstract .

Virtual and override

A function can now only be overridden if it is declared with virtual keyword or defined in interface or abstract contract. When we want to override or change a function, we use the override keyword. For example:

pragma solidity <span class="token operator">&gt;=</span> <span class="token number">0.6</span> <span class="token number">.0</span> <span class="token operator">&lt;</span> <span class="token number">0.7</span> <span class="token number">.0</span> <span class="token punctuation">;</span>

contract Base
<span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">foo</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> virtual <span class="token keyword">public</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

contract Middle is Base <span class="token punctuation">{</span> <span class="token punctuation">}</span>

contract Inherited is Middle
<span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">foo</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token keyword">public</span> override <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

pragma solidity >= 0.6 .0 < 0.7 .0 ;

contract Base

{

}

contract Middle is Base { }

contract Inherited is Middle

{

}

In case of multiple inheritance, the original function with the same name needs to be explicitly specified after the override keyword. For example:

pragma solidity <span class="token operator">&gt;=</span> <span class="token number">0.6</span> <span class="token number">.0</span> <span class="token operator">&lt;</span> <span class="token number">0.7</span> <span class="token number">.0</span> <span class="token punctuation">;</span>

contract Base1
<span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">foo</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> virtual <span class="token keyword">public</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

contract Base2
<span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">foo</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> virtual <span class="token keyword">public</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

contract Inherited is Base1 <span class="token punctuation">,</span> Base2
<span class="token punctuation">{</span>
    <span class="token comment">// Derives from multiple bases defining foo(), so we must explicitly</span>
    <span class="token comment">// override it</span>
    <span class="token keyword">function</span> <span class="token function">foo</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> <span class="token keyword">public</span> <span class="token function">override</span> <span class="token punctuation">(</span> Base1 <span class="token punctuation">,</span> Base2 <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

contract Base1

{

}

contract Base2

{

}

contract Inherited is Base1 , Base2

{

// Derives from multiple bases defining foo(), so we must explicitly

// override it

function foo ( ) public override ( Base1 , Base2 ) { }

}

Note: the private function cannot be declared as virtual.

Length of array

Access to the length of arrays will always be read-only, even for storage arrays. Now we cannot change the length of the array by assigning a new value to its length. Instead, we'll use the push and pop functions, or you can assign a whole new array to the old array.

The function push(value) for dynamic arrays no longer returns the new length of the array. Example of old usage:

uint length <span class="token operator">=</span> zombies <span class="token punctuation">.</span> <span class="token function">push</span> <span class="token punctuation">(</span> <span class="token function">Zombie</span> <span class="token punctuation">(</span> _name <span class="token punctuation">,</span> _dna <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">;</span>

uint length = zombies . push ( Zombie ( _name , _dna ) ) ;

Now the push function merely adds a new element to the array and returns nothing else.

Fallback function

The function without a previous name implies that the fallback function in this version will be split as fallback and receive , without the preceding function keyword. For example:

How to declare old fallback:

contract Test <span class="token punctuation">{</span>
    <span class="token comment">// This function is called for all messages sent to</span>
    <span class="token comment">// this contract (there is no other function).</span>
    <span class="token comment">// Sending Ether to this contract will cause an exception,</span>
    <span class="token comment">// because the fallback function does not have the `payable`</span>
    <span class="token comment">// modifier.</span>
    <span class="token keyword">function</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> external <span class="token punctuation">{</span> x <span class="token operator">=</span> <span class="token number">1</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span>
    uint x <span class="token punctuation">;</span>
<span class="token punctuation">}</span>

contract Test {

// This function is called for all messages sent to

// this contract (there is no other function).

// Sending Ether to this contract will cause an exception,

// because the fallback function does not have the `payable`

// modifier.

function ( ) external { x = 1 ; }

uint x ;

}

New declaration method:

contract Test <span class="token punctuation">{</span>
    <span class="token comment">// This function is called for all messages sent to</span>
    <span class="token comment">// this contract (there is no other function).</span>
    <span class="token comment">// Sending Ether to this contract will cause an exception,</span>
    <span class="token comment">// because the fallback function does not have the `payable`</span>
    <span class="token comment">// modifier.</span>
    <span class="token function">fallback</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> external <span class="token punctuation">{</span> x <span class="token operator">=</span> <span class="token number">1</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span>
    uint x <span class="token punctuation">;</span>
<span class="token punctuation">}</span>

contract Test {

// This function is called for all messages sent to

// this contract (there is no other function).

// Sending Ether to this contract will cause an exception,

// because the fallback function does not have the `payable`

// modifier.

fallback ( ) external { x = 1 ; }

uint x ;

}

Receive function: A contract has at least 1 receive function, using the syntax receive() external payable { ... } . This function has no parameters nor returns anything. It is called when calling the contract without any data transfer but only eth transfer. If no functions receive , it will fall into the fallback is declared as payable . Without both the receive and fallback functions with payable , the contract will not be able to accept eth and fire an exception.

pragma solidity <span class="token operator">^</span> <span class="token number">0.6</span> <span class="token number">.0</span> <span class="token punctuation">;</span>

<span class="token comment">// This contract keeps all Ether sent to it with no way</span>
<span class="token comment">// to get it back.</span>
contract Sink <span class="token punctuation">{</span>
    event <span class="token function">Received</span> <span class="token punctuation">(</span> address <span class="token punctuation">,</span> uint <span class="token punctuation">)</span> <span class="token punctuation">;</span>
    <span class="token function">receive</span> <span class="token punctuation">(</span> <span class="token punctuation">)</span> external payable <span class="token punctuation">{</span>
        emit <span class="token function">Received</span> <span class="token punctuation">(</span> msg <span class="token punctuation">.</span> sender <span class="token punctuation">,</span> msg <span class="token punctuation">.</span> value <span class="token punctuation">)</span> <span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

pragma solidity ^ 0.6 .0 ;

// This contract keeps all Ether sent to it with no way

// to get it back.

contract Sink {

event Received ( address , uint ) ;

receive ( ) external payable {

emit Received ( msg . sender , msg . value ) ;

}

Try / Catch

An external call upon failure can be captured by the try/catch as shown in the following example:

pragma solidity <span class="token operator">^</span> <span class="token number">0.6</span> <span class="token number">.0</span> <span class="token punctuation">;</span>

<span class="token keyword">interface</span> <span class="token class-name">DataFeed</span> <span class="token punctuation">{</span> <span class="token keyword">function</span> <span class="token function">getData</span> <span class="token punctuation">(</span> address token <span class="token punctuation">)</span> external <span class="token function">returns</span> <span class="token punctuation">(</span> uint value <span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token punctuation">}</span>

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

pragma solidity ^ 0.6 .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 ++ ;

}

Struct and Enum:

Struct and enum can now be declared at the file level.

Address:

Cannot convert from external function type to address type. Instead, the external function type will have 1 component as the address. For example:

address(f) converts to f.address with f being the external function type.

It is possible to convert the address type to the address payable type using the payable (x) syntax, where x is the address type.

Conclude

Above are the noticeable changes in version 0.6.0 of Solidity. Hopefully the above article will help you in developing your Dapp.

Source:

https://solidity.readthedocs.io/en/v0.6.7/060-breaking-changes.html

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Source : Viblo

Solidity changes in version 0.6.0

Preamble

Major changes

Abstract

Virtual and override

Length of array

Fallback function

Try / Catch

Struct and Enum:

Address:

Conclude

Source:

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