Multiple inheritance in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Multiple inheritance

Start

↓

Define BaseContractA

↓

Define BaseContractB

↓

Define DerivedContract inheriting A and B

↓

Create DerivedContract instance

↓

Call methods from A and B

↓

Use combined features

↓

End

Shows how a contract inherits from two base contracts, combining their features.

Execution Sample

Blockchain / Solidity

contract A {
  function foo() public pure returns (string memory) {
    return "A";
  }
}

contract B {
  function bar() public pure returns (string memory) {
    return "B";
  }
}

contract C is A, B {
  function combined() public pure returns (string memory) {
    return string(abi.encodePacked(foo(), bar()));
  }
}

Defines two base contracts A and B, then a contract C inherits both and combines their functions.

Execution Table

Step	Action	Contract	Function Called	Return Value	Notes
1	Deploy contract A	A	-	-	Base contract A deployed
2	Deploy contract B	B	-	-	Base contract B deployed
3	Deploy contract C inheriting A and B	C	-	-	Derived contract C deployed with A and B features
4	Call foo() from C	C	foo()	"A"	Inherited from A
5	Call bar() from C	C	bar()	"B"	Inherited from B
6	Call combined() from C	C	combined()	"AB"	Combines foo() and bar() results
7	End	-	-	-	Execution complete

💡 All functions called successfully, demonstrating multiple inheritance.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	Final
foo() return	-	"A"	"A"	"A"	"A"
bar() return	-	-	"B"	"B"	"B"
combined() return	-	-	-	"AB"	"AB"

Key Moments - 3 Insights

How does contract C access functions from both A and B?

Contract C inherits both A and B, so it can call their public functions directly, as shown in steps 4 and 5.

What happens if A and B have functions with the same name?

Solidity requires explicit override to resolve conflicts; without it, compilation fails. This example avoids that by using different function names.

Why does combined() use abi.encodePacked to join strings?

Because Solidity strings cannot be concatenated with +, abi.encodePacked combines them into one string, shown in step 6.

Visual Quiz - 3 Questions

Look at the execution table, what does foo() return when called from contract C?

A "AB"

B "B"

C "A"

D ""

💡 Hint

Check step 4 in the execution table where foo() is called from C.

At which step does the combined() function return the concatenated string?

A Step 5

B Step 6

C Step 4

D Step 7

💡 Hint

Look at the execution table row where combined() is called.

If contract B also had a function named foo(), what would happen without explicit override?

A Compilation error due to function name conflict

B Contract C would compile and use B's foo()

C Contract C would compile and use A's foo()

D Contract C would ignore both foo() functions

💡 Hint

Refer to the key moment about function name conflicts in multiple inheritance.

Concept Snapshot

Multiple inheritance lets a contract inherit from two or more contracts.
Syntax: contract Derived is Base1, Base2 { ... }
Derived contract can call public functions from all bases.
If functions share names, explicit override is needed.
Use combined features by calling inherited functions.

Full Transcript

This example shows multiple inheritance in blockchain smart contracts. Two base contracts A and B each have a function returning a string. Contract C inherits both and can call foo() from A and bar() from B. The combined() function in C joins these results. The execution table traces deployment and function calls step-by-step. Key points include how C accesses both base functions, the need for explicit overrides if function names clash, and string concatenation using abi.encodePacked. The visual quiz tests understanding of return values and inheritance behavior.

Practice

1.

What is the main purpose of multiple inheritance in blockchain smart contracts?

easy

A. To restrict a contract to inherit from only one contract

B. To allow a contract to use code from more than one contract

C. To make contracts run faster on the blockchain

D. To prevent contracts from sharing any code

Solution

1. Step 1: Understand inheritance in contracts

   Inheritance allows a contract to reuse code from another contract.

2. Step 2: Identify multiple inheritance meaning

   Multiple inheritance means inheriting from more than one contract to combine features.

3. Final Answer:

   To allow a contract to use code from more than one contract -> Option B

4. Quick Check:

   Multiple inheritance = use code from multiple contracts [OK]

Hint: Multiple inheritance means using code from many contracts [OK]

Common Mistakes:

• Thinking multiple inheritance restricts to one contract
• Confusing inheritance with contract speed
• Believing inheritance prevents code sharing

2.

Which of the following is the correct syntax to inherit from two contracts A and B in Solidity?

contract C is ? {}

easy

A. A + B

B. A & B

C. A | B

D. A, B

Solution

1. Step 1: Recall Solidity inheritance syntax

   In Solidity, multiple inheritance uses commas to separate parent contracts.

2. Step 2: Check the options

   Only A, B uses commas correctly: contract C is A, B {}.

3. Final Answer:

   A, B -> Option D

4. Quick Check:

   Use commas to list parents in Solidity inheritance [OK]

Hint: Use commas to list multiple parents in Solidity [OK]

Common Mistakes:

• Using symbols like & or + instead of commas
• Forgetting to separate parent contracts
• Writing inheritance without any separator

3.

Consider the following Solidity contracts:

contract A {
    function greet() public pure virtual returns (string memory) {
        return "Hello from A";
    }
}

contract B {
    function greet() public pure virtual returns (string memory) {
        return "Hello from B";
    }
}

contract C is A, B {
    function greet() public pure override(A, B) returns (string memory) {
        return super.greet();
    }
}

What will C().greet() return?

medium

A. "Hello from A"

B. "Hello from C"

C. Compilation error due to ambiguity

D. "Hello from B"

Solution

1. Step 1: Understand method resolution order

   In Solidity, the order of inheritance matters. The first parent contract's function is called by super.

2. Step 2: Check inheritance order and super call

   Contract C inherits A then B. Calling super.greet() calls A's greet due to C3 linearization order C, A, B.

3. Final Answer:

   "Hello from A" -> Option A

4. Quick Check:

   First parent function called by super = "Hello from A" [OK]

Hint: In Solidity, super calls first inherited (leftmost) contract's method [OK]

Common Mistakes:

• Assuming super calls last parent
• Expecting compilation error without override
• Thinking super calls child's own method

4.

Given the contracts below, what is the error in contract C?

contract A {
    function foo() public pure returns (string memory) {
        return "A";
    }
}

contract B {
    function foo() public pure returns (string memory) {
        return "B";
    }
}

contract C is A, B {
    // Missing override keyword
    function foo() public pure returns (string memory) {
        return super.foo();
    }
}

medium

A. Missing override keyword causes compilation error

B. super.foo() calls an undefined function

C. No error, code compiles and runs fine

D. Function foo() must be private

Solution

1. Step 1: Check function overriding rules

   When overriding functions from multiple parents, Solidity requires the override keyword.

2. Step 2: Identify missing override keyword

   Contract C's foo lacks override(A, B), causing a compilation error.

3. Final Answer:

   Missing override keyword causes compilation error -> Option A

4. Quick Check:

   Override keyword required when overriding multiple parents [OK]

Hint: Always add override keyword when overriding multiple parents [OK]

Common Mistakes:

• Forgetting override keyword in child function
• Assuming super.foo() is undefined
• Thinking function visibility must be private

5.

You have two contracts:

contract Token {
    function transfer() public pure virtual returns (string memory) {
        return "Token transfer";
    }
}

contract Mintable {
    function transfer() public pure virtual returns (string memory) {
        return "Mintable transfer";
    }
}

contract MyToken is Token, Mintable {
    function transfer() public pure override(Token, Mintable) returns (string memory) {
        return string.concat(Token.transfer(), " & ", Mintable.transfer());
    }
}

What will MyToken().transfer() return?

hard

A. "Mintable transfer & Token transfer"

B. Compilation error due to ambiguous override

C. "Token transfer & Mintable transfer"

D. "Token transfer"

Solution

1. Step 1: Understand explicit calls to parent functions

   MyToken's transfer calls both Token.transfer() and Mintable.transfer() explicitly.

2. Step 2: Check string concatenation order

   The returned string concatenates Token.transfer() first, then Mintable.transfer() with " & " in between.

3. Final Answer:

   "Token transfer & Mintable transfer" -> Option C

4. Quick Check:

   Explicit calls combine both parents' transfer strings [OK]

Hint: Explicitly call each parent to combine their outputs [OK]

Common Mistakes:

• Assuming super calls only one parent
• Expecting compilation error without override
• Mixing order of concatenation