Library contracts in Blockchain / Solidity - Mini Project: Build & Apply

Library Contracts

📖 Scenario: You are building a simple blockchain project where you want to reuse common functions across multiple smart contracts. Instead of copying code, you will create a library contract that other contracts can call. This helps keep your code clean and efficient.

🎯 Goal: Create a library contract with a function to add two numbers. Then create a main contract that uses this library to add two numbers and show the result.

📋 What You'll Learn

Create a library contract named MathLib with a function add(uint a, uint b) returns (uint) that returns the sum of a and b.

Create a contract named Calculator that uses MathLib to add two numbers.

In Calculator, create a function calculateSum(uint x, uint y) returns (uint) that calls MathLib.add and returns the result.

Write a function to get the sum and print it.

💡 Why This Matters

🌍 Real World

Library contracts help developers reuse common code in blockchain projects, making smart contracts smaller and easier to maintain.

💼 Career

Understanding library contracts is important for blockchain developers to write efficient and modular smart contracts used in decentralized applications.

1

Create the library contract

Create a library contract called MathLib with a public function add that takes two uint parameters a and b and returns their sum as uint.

Blockchain / Solidity

// Your code here to create the MathLib library contract

Hint

Use the library keyword to create a library contract. The add function should be public and pure because it does not change or read state.

2

Create the main contract and import the library

Create a contract called Calculator. Inside it, add a function calculateSum that takes two uint parameters x and y. This function will use MathLib.add to add the two numbers and return the result.

Blockchain / Solidity

library MathLib {
    function add(uint a, uint b) public pure returns (uint) {
        return a + b;
    }
}

// Your code here to create the Calculator contract

Hint

Define the Calculator contract and inside it create the calculateSum function. Call the library function using MathLib.add(x, y).

3

Add a function to call and print the sum

Inside the Calculator contract, add a public function called getSum that calls calculateSum with the numbers 5 and 7 and returns the result.

Blockchain / Solidity

library MathLib {
    function add(uint a, uint b) public pure returns (uint) {
        return a + b;
    }
}

contract Calculator {
    function calculateSum(uint x, uint y) public pure returns (uint) {
        return MathLib.add(x, y);
    }

    // Your code here to add getSum function
}

Hint

Create a simple function getSum that calls calculateSum with fixed numbers and returns the result.

4

Print the result of the sum

Write a line of code to call getSum() from the Calculator contract and print the result.

Blockchain / Solidity

library MathLib {
    function add(uint a, uint b) public pure returns (uint) {
        return a + b;
    }
}

contract Calculator {
    function calculateSum(uint x, uint y) public pure returns (uint) {
        return MathLib.add(x, y);
    }

    function getSum() public pure returns (uint) {
        return calculateSum(5, 7);
    }
}

// Your code here to print the result of getSum()

Hint

Solidity contracts do not print like normal programs. You get the result by calling the function from outside the contract (like a test or blockchain interface). The sum of 5 and 7 is 12.

Practice

1. What is the main purpose of a library contract in blockchain development?

easy

A. To reuse code across multiple smart contracts

B. To store user data permanently

C. To mine new blocks

D. To create new tokens

Solution

1. Step 1: Understand the role of library contracts

   Library contracts contain reusable code that can be called by other contracts to avoid duplication.

2. Step 2: Compare options with library purpose

   Only To reuse code across multiple smart contracts correctly describes code reuse, while others describe unrelated blockchain functions.

3. Final Answer:

   To reuse code across multiple smart contracts -> Option A

4. Quick Check:

   Library = Code reuse [OK]

Hint: Library contracts are for sharing code, not storing data [OK]

Common Mistakes:

• Confusing libraries with data storage contracts
• Thinking libraries create tokens
• Assuming libraries mine blocks

2. Which of the following is the correct way to declare a library in Solidity?

easy

A. library MyLib {}

B. contract MyLib {}

C. interface MyLib {}

D. struct MyLib {}

Solution

1. Step 1: Recall Solidity keywords for libraries

   Solidity uses the keyword library to declare a library contract.

2. Step 2: Check each option's keyword

   Only library MyLib {} uses library. Others declare contract, interface, or struct, which are different constructs.

3. Final Answer:

   library MyLib {} -> Option A

4. Quick Check:

   Keyword for library = library [OK]

Hint: Use 'library' keyword to declare libraries in Solidity [OK]

Common Mistakes:

• Using 'contract' instead of 'library'
• Confusing interfaces with libraries
• Trying to declare libraries as structs

3. Given the following Solidity code, what will be the output of test.getDouble(5)?

library MathLib {
    function double(uint x) internal pure returns (uint) {
        return x * 2;
    }
}

contract Test {
    using MathLib for uint;
    function getDouble(uint value) public pure returns (uint) {
        return value.double();
    }
}

medium

A. 5

B. 10

C. Compilation error

D. 0

Solution

1. Step 1: Understand the library function call

   The library MathLib defines a function double that multiplies input by 2.

2. Step 2: Analyze the usage in contract Test

   Using using MathLib for uint; allows calling double() on uint variables. So value.double() returns 5 * 2 = 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   5 * 2 = 10 [OK]

Hint: Library functions called with 'using for' act on the type [OK]

Common Mistakes:

• Expecting the original value instead of doubled
• Thinking 'using for' is optional for calling library functions
• Confusing internal and public function visibility

4. Identify the error in the following Solidity code using a library:

library Utils {
    function increment(uint x) public pure returns (uint) {
        return x + 1;
    }
}

contract Counter {
    using Utils for uint;
    uint count;
    function increase() public {
        count = count.increment();
    }
}

medium

A. Cannot use 'using for' with uint type

B. Missing constructor in contract

C. Library function must be internal, not public

D. Function increase should be view

Solution

1. Step 1: Check library function visibility

   Library functions used with 'using for' must be internal, not public.

2. Step 2: Verify other options

   No constructor is needed here, 'using for' works with uint, and increase modifies state so cannot be view.

3. Final Answer:

   Library function must be internal, not public -> Option C

4. Quick Check:

   Library function visibility = internal [OK]

Hint: Library functions called via 'using for' cannot be public [OK]

Common Mistakes:

• Declaring library functions as public
• Thinking 'using for' can't be used with uint
• Marking state-changing functions as view

5. You want to create a library that provides a function to safely add two unsigned integers without overflow. Which of the following code snippets correctly implements this using Solidity 0.8+ features and can be used with using for in other contracts?

hard

A. library SafeMath {
function safeAdd(uint a, uint b) internal view returns (uint) {
return a + b;
}
}

B. library SafeMath {
function safeAdd(uint a, uint b) public pure returns (uint) {
return a + b;
}
}

C. library SafeMath {
function safeAdd(uint a, uint b) internal pure returns (uint) {
unchecked {
return a + b;
}
}
}

D. library SafeMath {
function safeAdd(uint a, uint b) internal pure returns (uint) {
uint c = a + b;
require(c >= a, "Overflow");
return c;
}
}

Solution

1. Step 1: Check function visibility and usage

   Library functions used with 'using for' must be internal. library SafeMath { function safeAdd(uint a, uint b) public pure returns (uint) { return a + b; } } uses public (invalid), D uses view which is incorrect for pure functions. library SafeMath { function safeAdd(uint a, uint b) internal pure returns (uint) { uint c = a + b; require(c >= a, "Overflow"); return c; } } uses internal pure.

2. Step 2: Verify overflow check and Solidity 0.8+ behavior

   Solidity 0.8+ has built-in overflow checks, so unchecked block in C disables protection and returns wrapped value on overflow. library SafeMath { function safeAdd(uint a, uint b) internal pure returns (uint) { uint c = a + b; require(c >= a, "Overflow"); return c; } } correctly relies on built-in checks with additional require guard and no unchecked block.

3. Final Answer:

   library SafeMath { function safeAdd(uint a, uint b) internal pure returns (uint) { uint c = a + b; require(c >= a, "Overflow"); return c; } } -> Option D

4. Quick Check:

   Internal pure function with overflow check = library SafeMath { function safeAdd(uint a, uint b) internal pure returns (uint) { uint c = a + b; require(c >= a, "Overflow"); return c; } } [OK]

Hint: Use internal pure functions with require for safe math [OK]

Common Mistakes:

• Using public visibility in library functions
• Using view instead of pure for math functions
• Unnecessary unchecked blocks in Solidity 0.8+