SPDX license and pragma version in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the SPDX license identifier in a Solidity file?

The SPDX license identifier specifies the license under which the Solidity code is released. It helps tools and users understand the legal terms quickly.

beginner

Where should the SPDX license identifier be placed in a Solidity file?

It should be placed as the very first line in the Solidity file, starting with // SPDX-License-Identifier: followed by the license type.

beginner

What does the pragma version statement do in Solidity?

The pragma version statement tells the compiler which versions of Solidity the code is compatible with, ensuring the code compiles correctly.

intermediate

Explain the difference between pragma solidity ^0.8.0; and pragma solidity 0.8.0;.

pragma solidity ^0.8.0; means the code works with version 0.8.0 and any newer 0.8.x versions but not 0.9.0 or above. pragma solidity 0.8.0; means it only works with exactly version 0.8.0.

intermediate

Why is it important to specify both SPDX license and pragma version in Solidity files?

Specifying SPDX license clarifies legal usage rights, and pragma version ensures the code compiles with the intended compiler version, preventing errors and misunderstandings.

What is the correct way to specify an MIT license in a Solidity file?

A // SPDX-License-Identifier: MIT

B // License: MIT

C pragma license MIT;

D license MIT;

The SPDX license identifier must be written as a comment starting with // SPDX-License-Identifier: followed by the license name.

What does pragma solidity ^0.8.0; mean?

A Compatible with Solidity version 0.8.0 and above, but less than 0.9.0

B Compatible only with Solidity version 0.8.0

C Compatible with any Solidity version

D Compatible with Solidity versions below 0.8.0

The caret (^) means any version from 0.8.0 up to but not including 0.9.0.

Where should the pragma version statement be placed in a Solidity file?

A Inside the contract functions

B Anywhere in the contract code

C At the very end of the file

D After the SPDX license identifier and before the contract code

The pragma statement should be near the top, after the SPDX license line, to tell the compiler which version to use.

What happens if you omit the SPDX license identifier in your Solidity file?

A The code will automatically be licensed under MIT

B The compiler will throw an error and refuse to compile

C There is no legal information about the code license, which may cause confusion

D The pragma version will be ignored

Omitting SPDX means no license info is declared, which can cause legal uncertainty.

Which of the following is a valid SPDX license identifier?

A GPLv3

B GPL-3.0-or-later

C GNU Public License 3

D GPL version 3

SPDX license identifiers follow a specific format like GPL-3.0-or-later.

Describe the role of the SPDX license identifier and where it should be placed in a Solidity file.

Think about legal clarity and file structure.

Explain how the pragma version statement controls which Solidity compiler versions can compile your code.

Consider compatibility and preventing errors.

Practice

1.

What is the purpose of the SPDX-License-Identifier line in a Solidity smart contract?

easy

A. To specify the license type of the contract's source code

B. To set the Solidity compiler version

C. To declare the contract's owner

D. To define the contract's storage variables

Solution

1. Step 1: Understand SPDX-License-Identifier usage

   The SPDX-License-Identifier line is a comment that clearly states the license under which the source code is released.

2. Step 2: Differentiate from pragma version

   Unlike pragma which sets compiler version, SPDX is about legal licensing information.

3. Final Answer:

   To specify the license type of the contract's source code -> Option A

4. Quick Check:

   SPDX-License-Identifier = License type [OK]

Hint: SPDX line always shows license info, not compiler version [OK]

Common Mistakes:

• Confusing SPDX line with pragma version
• Thinking SPDX sets compiler behavior
• Ignoring SPDX line as just a comment

2.

Which of the following is the correct way to specify the Solidity compiler version to accept any version from 0.8.0 up to but not including 0.9.0?

pragma solidity ?;

easy

A. pragma solidity ^0.8.19;

B. pragma solidity 0.8.0;

C. pragma solidity >=0.8.0 <0.9.0;

D. pragma solidity >0.8.0 <=0.9.0;

Solution

1. Step 1: Understand version range syntax

   The syntax >=0.8.0 <0.9.0 means any version from 0.8.0 up to but not including 0.9.0.

2. Step 2: Compare with caret (^) operator

   The ^0.8.19 syntax allows >=0.8.19 <0.9.0, excluding earlier 0.8.x versions like 0.8.0. The explicit range >=0.8.0 <0.9.0 matches exactly.

3. Final Answer:

   pragma solidity >=0.8.0 <0.9.0; -> Option C

4. Quick Check:

   Version range syntax = >=0.8.0 <0.9.0 [OK]

Hint: Use >= and < to specify exact version range [OK]

Common Mistakes:

• Using exact version without range
• Misplacing comparison operators
• Confusing caret (^) with range syntax

3.

What will happen if you compile this Solidity code snippet?

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;
contract Test {
  uint public x = 5;
}

Assuming you use Solidity compiler version 0.8.10.

medium

A. Compilation fails due to pragma version mismatch

B. Compilation fails due to SPDX license missing

C. Compilation succeeds but with a warning

D. Compilation succeeds without errors

Solution

1. Step 1: Check pragma version compatibility

   The pragma ^0.7.0 means compiler versions from 0.7.0 up to but not including 0.8.0 are allowed.

2. Step 2: Compare with compiler version used

   Compiler version 0.8.10 is outside the allowed range, so compilation will fail due to version mismatch.

3. Final Answer:

   Compilation fails due to pragma version mismatch -> Option A

4. Quick Check:

   Pragma ^0.7.0 excludes 0.8.10 = mismatch [OK]

Hint: Compiler version must fit pragma range exactly [OK]

Common Mistakes:

• Ignoring pragma version mismatch
• Thinking SPDX license affects compilation
• Assuming caret (^) allows all newer versions

4.

Identify the error in this Solidity contract header:

// SPDX-License-Identifier MIT
pragma solidity ^0.8.0;
contract Sample {}

medium

A. SPDX license identifier must be in quotes

B. Incorrect pragma version syntax

C. Contract name must be lowercase

D. Missing colon ':' after SPDX-License-Identifier

Solution

1. Step 1: Check SPDX license line format

   The SPDX license line must be in the format // SPDX-License-Identifier: LicenseName with a colon after the identifier.

2. Step 2: Verify other parts

   The pragma syntax and contract name are correct; SPDX license identifier does not require quotes.

3. Final Answer:

   Missing colon ':' after SPDX-License-Identifier -> Option D

4. Quick Check:

   SPDX line requires colon ':' [OK]

Hint: SPDX line always has colon after identifier [OK]

Common Mistakes:

• Omitting colon after SPDX-License-Identifier
• Adding quotes around license name
• Thinking contract names must be lowercase

5.

You want to write a Solidity contract that is compatible with any compiler version from 0.6.0 up to but not including 0.9.0, and you want to specify the MIT license. Which of the following headers is correct?

hard

A. // SPDX-License-Identifier: MIT
pragma solidity 0.6.0;

B. // SPDX-License-Identifier: MIT
pragma solidity >=0.6.0 <0.9.0;

C. // SPDX-License-Identifier MIT
pragma solidity ^0.6.0;

D. // SPDX-License-Identifier: MIT
pragma solidity >=0.6.0 <=0.9.0;

Solution

1. Step 1: Check SPDX license line correctness

   The SPDX line must have a colon after the identifier and specify the license, so // SPDX-License-Identifier: MIT is correct.

2. Step 2: Verify pragma version range

   The pragma >=0.6.0 <0.9.0 correctly specifies compatibility from 0.6.0 up to but not including 0.9.0. Using <=0.9.0 would include 0.9.0, which is not desired.

3. Final Answer:

   // SPDX-License-Identifier: MIT pragma solidity >=0.6.0 <0.9.0; -> Option B

4. Quick Check:

   SPDX with colon + correct pragma range = // SPDX-License-Identifier: MIT pragma solidity >=0.6.0 <0.9.0; [OK]

Hint: Use colon in SPDX and < for upper pragma bound [OK]

Common Mistakes:

• Omitting colon in SPDX line
• Using exact version instead of range
• Including upper bound version with <= instead of <