Solidity Tutorial: All About Types Conversion | by Jean Cvllr | Apr, 2022

Implicit vs express and examples to know conversion between sorts

Photograph by Jamie Templeton on Unsplash

As we speak’s article is kind of in-depth, technical and detailed! We discover conversions between differing types in Solidity.

We essential goal of this text is so that you can perceive “what’s the distinction between implicit conversion vs express conversion in Solidity”.

We’ll then cowl every attainable conversion between sorts, nonetheless utilizing some code examples. So seize a espresso or tea (or a glass of whisky) and let’s get began!

  • Implicit vs Specific — Definition
  • Implicit vs Specific Conversions in Solidity
  • Conversion between unsigned integers uintN
  • Conversion between bytesN (eg: bytes4 <-> bytes16)
  • Conversions from uintM to bytesN
  • Conversion from bytesN to uintM
  • Conversion from bytes to string
  • Desk Abstract — uintM ← → bytesN
  • Literal assignments to bytesN
  • Decimals and Hex Literals assignments uintN
  • Conversion from bytes to bytesN
  • Conversions to tackle kind
  • Conversion between tackle and tackle payable
  • Conversion between Contract and tackle sorts
  • References

Earlier than diving into implicit vs express conversion in Solidity, let’s first perceive the distinction in human language.

Lexico.com provides the next definitions of implicit and express.

Implicit = one thing is usually recommended, however circuitously expressed.

Explicit = one thing is acknowledged clearly and intimately, leaving no room for confusion or doubt.

When one thing is express, it is rather clear. There is no such thing as a obscure understanding or ambiguity

When one thing is implicit, it’s implied. One thing is known from the wording, it isn’t instantly acknowledged or clearly described.

Let’s illustrate with an instance. Alice is an worker of Bob. She is trying to specific her choice to depart the corporate if she doesn’t get hold of a pay rise. She will be able to specific this both:

implicitly: “if I’m not being proven extra appreciation, I’ll overview my choices.”

explicitly: “if I don’t get hold of a pay rise, I’ll go away.”

All programming languages assist some kind of conversion.

Solidity additionally permits kind conversion. Kind conversion in Solidity can happen in three essential situations:

  • by means of variable assignments.
  • when passing arguments to features.
  • when making use of operators.

Kind conversion might be completed both implicitly (= the compiler derives the sort robotically) or by being express to the compiler (= by telling the compiler which sort to transform to).

Let’s look intimately on the underlying guidelines of implicit vs express conversions for the Solidity compiler.

Implicit conversion between two information sorts is allowed in Solidity if:

  • it is sensible semantically (what does that imply?)
  • no info is misplaced within the course of.

Examples:

  • uint8 to uint16 = ✅
  • int120 to int256 = ✅
  • int8 to uint256 = ❌ (as a result of an uint256 can not maintain adverse values that would doubtlessly come from the int8).

You’ll be able to see from the final instance above that the Solidity compiler doesn’t enable implicit conversion when some info can doubtlessly be misplaced.

For instance utilizing the final instance, if the worth of the int8 was -5, the compiler must drop the negation as a way to convert it to an allowed uint256 quantity. This conversion results in info loss within the information, and the Solidity compiler is wise sufficient to know that and warn you.

Nonetheless, in case you “don’t agree with the compiler”, or if you wish to “implement some conversions”, you may all the time inform the compiler what to do by being express.

Specific conversion

If the compiler doesn’t enable implicit conversion however what you’re doing, an express kind conversion is usually attainable.

Specific conversion might be completed by means of casting or a constructor-like syntax.

uint256 a = 12345;
bytes32 b = bytes32(a);

Nonetheless, express conversion might be dangerous, as described within the Solidity docs.

This will likely lead to sudden behaviour and permits you to bypass some security measures of the compiler, so you should definitely check that the result’s what you need and count on!

Definitions abstract

Here’s a two sentences abstract of implicit vs express conversion in Solidity.

When changing from kind A to kind B in Solidity, some info within the information might be misplaced throughout the conversion course of.

  • with implicit conversion: you won’t pay attention to the (potential) info being misplaced.

If some info is definitely misplaced, the compiler will refuse to compile and throws an error ❌

  • with express conversion: you’re absolutely conscious that some info may very well be misplaced.

Since you’re being express to the compiler, it can permit you to compile, and permits the (potential) lack of info ✅

Unsigned integers in Solidity exist with completely different bits order, in sequences of 8 bits. Instance: uint8, uint16, uint24, uint32, … as much as uint256 .

For understanding, we outline: (must be rephrased)

  • lower-order quantity: a quantity nearer to the bottom bits vary uint8.
  • high-order quantity: a quantity nearer to the very best bits vary uint256.

The conversion for unsigned integers can go each methods:

Changing to the next kind

(Rephrase) That is the situation if you find yourself changing a quantity to a brand new kind that has the next order of bits: e.g: uint64 to uint128

When changing an unsigned integer to the next kind, left-padding happens, which means zeroes (= 0 bits) are added to the left. e.g: uint256 to uint128 .

uint16 a = 0x1234;
uint32 b = uint32(a); // b = 0x00001234

Changing to a decrease kind

(rephrase) That is the situation if you find yourself changing a quantity to a brand new kind that has a decrease order of bits

When changing an unsigned integer to a smaller kind, the excessive order bits (the bits “essentially the most on the left”) find yourself being discarded.

Instance:

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

That is the equal of doing a modulo of the quantity we need to convert with the upper variety of the vary of the brand new bits.

Let’s take the next examples:

uint32 a = 100000;
uint16 public b = uint16(a); //b = a % 65536
uint8 public c = uint8(a); //c = a % 256

Within the above instance:

  • uint16 b might be calculated by doing a % 65536 = 34,464
  • uint8 c might be calculated by doing a % 256 = 160

Solidity permits changing between completely different fixed-size bytes. A number of situations exist. These are coated under.

Changing to a smaller bytes vary

When explicitly changing to a smaller-bytes vary, the right-most bytes are discarded (= the “larger order bytes”).

bytes2 a = 0x1234;
bytes1 b = bytes1(a); // b = 0x12

This principally imply that Solidity truncates from the appropriate hand facet, till the size in bytes is the same as new size of the bytes specified within the kind casting.

Changing to the next bytes vary

When changing to bigger bytes, zero padding is added to the appropriate.

bytes2 a = 0x1234;
bytes4 b = bytes4(a); // b = 0x12340000

Beneath are the foundations for changing two values a and b. Let’s use the next template to raised perceive the long run attainable conversions.

uintM a , the place M = a 8-bits vary between uint8 ... uint256

bytesN b, the place N = a 1-byte vary between bytes1 ... bytes32

Implicit conversion

uintM and bytesN can’t be implicitly transformed between one another ❌

uint32 a = 0xcafecafe;
bytes4 b = a; // TypeError
// TypeError: Kind uint32 is just not implicitly convertible to anticipated kind bytes4.
bytes4 c = 0xbeefbeef;
uint32 d = c; // TypeError
// TypeError: Kind bytes4 is just not implicitly convertible to anticipated kind uint32.

Specific conversion

Specific conversion is allowed so long as each theuintM and bytesN have the identical measurement (the variety of bits M is equal to the variety of bytes N) ✅

e.g: M-bits = N-bytes

uint32 a = 0xcafecafe;
bytes4 b = bytes4(a); // OK
bytes4 c = 0xbeefbeef;
uint32 d = uint32(c); // OK

e.g.: M-bits > N-bytes

uint32 a = 0xcafecafe;
bytes3 b = bytes3(a); // TypeError
// TypeError: Specific kind conversion not allowed from "uint32" to "bytes3".

e.g.: M-bits < N-bytes

uint32 a = 0xcafecafe;
bytes5 b = bytes5(a); // TypeError
// TypeError: Specific kind conversion not allowed from "uint32" to "bytes5".

Identical guidelines as earlier than however within the different order.

Specific conversion is allowed so long as the bytesN and uintM are of the identical measurement (variety of bits M is equal to the variety of bytes N)✅

e.g: N-bytes = M-bits

bytes4 a = 0xbeefbeef;
uint32 b = uint32(a);

e.g: N-bytes > M-bits

bytes4 a = 0xbeefbeef;
uint24 b = uint24(a); // TypeError
// TypeError: Specific kind conversion not allowed from "bytes4" to "uint24".

e.g: N-bytes < M-bits

bytes4 a = 0xbeefbeef;
uint40 b = uint40(a); // TypeError
// TypeError: Specific kind conversion not allowed from "bytes4" to "uint40".

The desk under summarizes the equivalence between uintN and bytesN. Simply do not forget that the quantity for uintM is the variety of bits, the quantity for bytesN is the variety of bytes, and one byte N = 8 bits M.

bytes ← → unit desk equivalence for Solidity

Implicit task

Any hexadecimal literal might be implicitly assigned to abytesN so long as the literal has the identical variety of bytes talked about within the kind.

bytes4 a = 0xcafecafe;bytes4 b = 0xcafe; // TypeError: Kind [...] not implicitly convertible to anticipated kind bytes4.bytes4 c = 0xcafecafecafe; // TypeError: Kind [...] is just not implicitly convertible to anticipated kind bytes4.

Implicit conversion

Decimals or hexadecimal quantity literals might be implicitly transformed to any uintN , however has to comply with one of many following two guidelines:

  • the uintN is similar measurement because the literal quantity ✅
  • the uintN is of bigger measurement than the literal quantity ✅

In abstract, the rule is that integer kind(= the vary of the bits) needs to be giant sufficient to symbolize + maintain worth with out truncation.

Right here is the instance from the Solidity docs:

uint8 a = 12; // no error
uint32 b = 1234; // no error
uint16 c = 0x123456; // error, as truncation required to 0x3456

Specific conversion

Previous to Solidity 0.8.0. it was attainable to explicitly convert any decimal or hexadecimal literal to any integer kind (irrespective of the bits vary). See the instance under.

// this is able to compile as much as solc 0.7.6
uint8 a = uint8(300); // a = 44

The results of this express conversion would have been a equal to calculating the modulo of 300, as 300 % 256 = 44.

Since Solidity 0.8.0, the code above would consequence within the error:

TypeError: Specific kind conversion not allowed from "int_const 300" to "uint8".

There such express conversions for literals are as strict as implicit conversions ranging from 0.8.0. That means they’re solely allowed if the literal suits within the ensuing vary.

  • Implicit conversion is just not allowed ❌
  • Specific conversion is allowed since Solidity 0.8.5 🙌 🙂

Beneath is an instance:

bytes reminiscence information = new bytes(5);
bytes2 firstTwoBytes = bytes2(information);
  • Implicit conversion is just not allowed ❌ on both facet (bytes to string, or string to bytes)
  • Specific conversion is allowed ✅
string reminiscence a = "All About Solidity";
bytes reminiscence b = bytes(a);
bytes reminiscence c = new bytes(5);
string reminiscence d = string(c);

Here’s a sensible instance of a contract that makes use of this express kind of conversion to transform from uncooked bytes to string: the LSP4Compatibility.sol contract from @lukso/lsp-smart-contracts.

LSP4 is a Metadata Standard used to explain a token or NFT on LUKSO (see LSP7 or LSP8 for extra particulars about this new era of tokens and NFTs on EVM primarily based chains).

In LSP4, the essential info of a token or NFT (like its identify or image) is saved below particular “information keys” within the ERC725Y key-value retailer of the token /NFT.

These keys are talked about within the code snippet under as _LSP4_TOKEN_NAME_KEY and _LSP4_TOKEN_SYMBOL_KEY.

supply: Github — lukso-network/lsp-smart-contracts, LSP4Compatibility.sol, lines 21–37

The code above is from the LSP4Compatibility.sol contract. This contract allows to create LSP7 tokens and LSP8 NFTs which might be backwards appropriate, which means any ERC20 and ERC721 tokens can work together with them like they might with common ERC20 / ERC721 tokens. (solely distinction is that LSP7 and LSP8 have better built-in security + more extensible metadata! 😉)

Let’s return to bytes to string conversion. Within the code snippet above, the features identify() and image() retrieve information from the underlying ERC725Y key-value retailer, the place all the information is saved as uncooked bytes.

To allow backwards compatibility, these bytes are explicitly transformed to string (consider it like casting). This express conversion leads to these two features like those from the ERC20 / 721 requirements, whereas the identify and symbols are literally not saved below variables, however below the key-value storage abstraction obtained due to ERC725Y 🗄

Conversion from hex literals to deal with

Beneath are the foundations for changing a hexadecimal literal to an tackle, both implicitly by way of task or explicitly by way of kind casting like tackle(0x…) .

Implicit conversion

Any hex literal might be implicitly transformed to an tackle kind if it passes the next two necessities:

rule 1: will need to have the proper measurement: 20 bytes lengthy.

// not lengthy sufficient
tackle vanityAddress = 0xfccfdadf3acefddcdebdefad8d0e7cbb96eeee;
// TypeError: Kind int_const 5637...(38 digits omitted)...7022 is just not implicitly convertible to anticipated kind tackle.

rule 2: will need to have a sound checksum

// invalid checksum
tackle vanityAddress = 0xfccfdadf3acefddcdebdefad8d0e7cbb96eeeebf;
// SyntaxError: This appears like an tackle however has an invalid checksum. Appropriate checksummed tackle: "0xFCCfDadf3acEFDdcdeBdefaD8d0e7Cbb96eeEeBf". If this isn't used as an tackle, please prepend '00'. For extra info please see https://docs.soliditylang.org/en/develop/sorts.html#address-literals

As you may see from above, the Solidity compiler gives you an error again but additionally the tackle literal again with a sound checksum.

The ultimate code snippet will compile efficiently, because it follows the two guidelines:

tackle vanityAddress = 0xFCCfDadf3acEFDdcdeBdefaD8d0e7Cbb96eeEeBf;

Specific conversion

You’ll be able to convert any hex literal explicitly to an tackle as proven under. That is allowed as lengthy the literal is much less or equal to twenty bytes.

If the hex literal is lower than 20 bytes, it can left-zero pad the tackle + check-sum it.

tackle example1 = tackle(0xcafecafe)
// 0x00000000000000000000000000000000CaFECAfE
tackle example2 = tackle(0xca11ab1e00beef010101)
// 0x00000000000000000000ca11AB1e00BEEF010101

If the hex literal is precisely 20 bytes lengthy, the literal will need to have a sound checksum. In any other case, the Solidity compiler will return an error and provide you with again the legitimate checksummed literal.

tackle instance = tackle(0xcafecafecafecafecafecafecafecafecafecafe);// SyntaxError: This appears like an tackle however has an invalid checksum. Appropriate checksummed tackle: "0xCAfEcAfeCAfECaFeCaFecaFecaFECafECafeCaFe". If this isn't used as an tackle, please prepend '00'. For extra info please see https://docs.soliditylang.org/en/develop/types.html#address-literals

Conversion from uint160 to deal with

  • Implicit conversion is just not allowed from uint160 to tackle
  • Specific conversion is allowed from uint160 to tackle
uint160 someNumber = 5_689_454_112;tackle convertedAddress = tackle(someNumber);

NB: previous to Solidity 0.8.0 (as much as Solidity 0.7.6), it was attainable to transform explicitly any integer kind uintN to an tackle (by way of casting). Since Solidity 0.8.0, express conversion is just allowed with uint160.

Conversion from tackle to uint160

  • Implicit conversion is just not allowed from tackle to uint160
  • Specific conversion is allowed from tackle to uint160

This might sound an odd case, and never a quite common one (so far as I’m conscious, I’ve by no means seen such implementation). The code under provides an illustrative instance

operate addressToUint160() public view returns (uint160) 

tackle from = msg.sender;
uint160 consequence = uint160(from);
return consequence;

// instance with msg.sender = 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4
// consequence = 520786028573371803640530888255888666801131675076

An tackle from might be explicitly transformed to tackle payable by way of payable(from)

tackle payable from = msg.sender;// TypeError: Kind tackle is just not implicitly convertible to anticipated kind tackle payable.
tackle payable from = payable(msg.sender);

That is particularly related to our earlier examples, as any express conversion into tackle kind (utilizing tackle(…)) all the time returns a non-payable tackle kind.

Subsequently, any Tackle Literal, bytes20 or uint160 worth might be explicitly transformed to an tackle payable as comply with:

// conversion from Tackle Literal to deal with payabletackle to = 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4;
tackle payable payableTo = payable(to);
// conversion from bytes20 to deal with payablebytes20 from = bytes20(0x5B38Da6a701c568545dCfcB03FcB875f56beddC4);
tackle payable payableFrom = payable(tackle(from));
// conversion from uint160 to deal with payableuint160 u = 12345;
tackle payable transformed = payable(tackle(u));

Specific conversion might be completed from an tackle to a Contract kind.

Let’s take the instance under from the Solidity docs:

tackle creator = TokenCreator(msg.sender);

By way of this express conversion, we assume right here that the kind of msg sender (= the calling contract) is TokenCreator. Nonetheless, there is no such thing as a actual method to confirm this other than utilizing ERC165 Commonplace Interface detection (assuming that msg.sender right here implements this normal).

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