Abstract contracts in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract contract in blockchain programming?

An abstract contract is a contract that cannot be deployed on its own because it has at least one function without implementation. It serves as a base for other contracts to inherit and implement missing functions.

beginner

Why do we use abstract contracts?

We use abstract contracts to define common interfaces and shared behavior for multiple contracts. They help organize code and enforce that child contracts implement required functions.

beginner

How do you declare an abstract contract in Solidity?

You declare an abstract contract by using the keyword <code>abstract</code> before <code>contract</code>. For example: <pre>abstract contract MyBase { function doSomething() public virtual; }</pre>

intermediate

What happens if a contract inherits an abstract contract but does not implement all its functions?

The inheriting contract also becomes abstract and cannot be deployed until it implements all the abstract functions from its base contracts.

intermediate

Can abstract contracts have implemented functions?

Yes, abstract contracts can have both implemented and unimplemented (abstract) functions. The unimplemented functions must be implemented by child contracts.

What keyword is used to declare an abstract contract in Solidity?

A interface

B virtual

C abstract

D override

The keyword abstract is used before contract to declare an abstract contract.

Can you deploy an abstract contract directly?

A Yes, always

B No, never

C Only if all functions are implemented

D Only if it has no functions

Abstract contracts cannot be deployed directly because they have unimplemented functions.

If a contract inherits an abstract contract but does not implement all abstract functions, what happens?

A It becomes abstract and cannot be deployed

B It deploys but throws errors at runtime

C It automatically implements missing functions

D It ignores the abstract functions

The contract remains abstract and cannot be deployed until all abstract functions are implemented.

Which of these is true about abstract contracts?

A They must be deployed before use

B They cannot have any implemented functions

C They are the same as interfaces

D They can have both implemented and unimplemented functions

Abstract contracts can have implemented functions and also declare abstract functions without implementation.

Why use abstract contracts instead of interfaces?

A To share code and define some implemented functions

B Because interfaces cannot be inherited

C Because abstract contracts are cheaper to deploy

D Because interfaces do not support functions

Abstract contracts allow sharing implemented code, while interfaces only declare function signatures.

Explain what an abstract contract is and why it is useful in blockchain programming.

Think about contracts that set rules but don’t do everything themselves.

Describe how inheritance works with abstract contracts and what happens if child contracts don’t implement all abstract functions.

Consider what happens if you inherit a contract with missing pieces.

Practice

1.

What is the main purpose of an abstract contract in blockchain programming?

easy

A. To define functions without implementation that child contracts must complete

B. To deploy a contract directly on the blockchain

C. To store data permanently without functions

D. To create a user interface for blockchain apps

Solution

1. Step 1: Understand abstract contract role

   Abstract contracts declare functions without full code, acting as templates.

2. Step 2: Identify deployment capability

   They cannot be deployed alone but require child contracts to implement missing parts.

3. Final Answer:

   To define functions without implementation that child contracts must complete -> Option A

4. Quick Check:

   Abstract contract = function templates [OK]

Hint: Abstract contracts declare incomplete functions for children [OK]

Common Mistakes:

• Thinking abstract contracts can be deployed directly
• Confusing abstract contracts with interfaces
• Believing abstract contracts store data only

2.

Which of the following is the correct syntax to declare an abstract contract in Solidity?

?

easy

A. contract MyContract abstract { function foo() public; }

B. abstract contract MyContract { function foo() public virtual; }

C. abstract MyContract contract { function foo() public; }

D. contract MyContract { abstract function foo(); }

Solution

1. Step 1: Check abstract contract declaration

   In Solidity, use 'abstract contract' before the contract name.

2. Step 2: Verify function declaration

   Abstract functions are declared with 'virtual' and no body.

3. Final Answer:

   abstract contract MyContract { function foo() public virtual; } -> Option B

4. Quick Check:

   Correct abstract syntax = abstract contract MyContract { function foo() public virtual; } [OK]

Hint: Use 'abstract contract' and 'virtual' for abstract functions [OK]

Common Mistakes:

• Placing 'abstract' after 'contract'
• Missing 'virtual' keyword on abstract functions
• Trying to define function body in abstract contract

3.

Given the following Solidity code, what will be the output when child.getValue() is called?

abstract contract Parent {
    function getValue() public virtual returns (string memory);
}

contract Child is Parent {
    function getValue() public override returns (string memory) {
        return "Hello from Child";
    }
}

medium

A. Compilation error due to missing function body in Parent

B. "Hello from Parent"

C. "Hello from Child"

D. Runtime error because Parent is abstract

Solution

1. Step 1: Understand abstract function in Parent

   Parent declares getValue() as abstract (no body), so Child must implement it.

2. Step 2: Check Child's implementation

   Child overrides getValue() and returns "Hello from Child" string.

3. Final Answer:

   "Hello from Child" -> Option C

4. Quick Check:

   Child's override runs = "Hello from Child" [OK]

Hint: Child must implement abstract functions; output from child method [OK]

Common Mistakes:

• Expecting Parent's abstract function to run
• Thinking abstract contracts cause runtime errors
• Confusing compilation errors with runtime errors

4.

Identify the error in this Solidity code snippet:

abstract contract Base {
    function doSomething() public virtual;
}

contract Derived is Base {
    // Missing implementation of doSomething
}

medium

A. Derived contract must implement doSomething() or be declared abstract

B. Base contract should not have virtual functions

C. Derived contract cannot inherit from abstract contracts

D. doSomething() must have a function body in Base

Solution

1. Step 1: Check abstract function in Base

   doSomething() is abstract (no body), so Derived must implement it or be abstract.

2. Step 2: Analyze Derived contract

   Derived does not implement doSomething() and is not abstract, causing error.

3. Final Answer:

   Derived contract must implement doSomething() or be declared abstract -> Option A

4. Quick Check:

   Abstract functions require implementation in non-abstract children [OK]

Hint: Non-abstract child must implement all abstract functions [OK]

Common Mistakes:

• Assuming virtual means implemented
• Forgetting to mark Derived as abstract if no implementation
• Thinking abstract contracts can be instantiated

5.

You want to create a contract system where multiple child contracts share a common function signature but implement it differently. How should you design the parent contract?

hard

A. Use an interface with implemented functions

B. Use a regular contract with fully implemented functions

C. Use a library instead of a contract

D. Use an abstract contract with the function declared as virtual and no body

Solution

1. Step 1: Understand shared function signature need

   Multiple children must share function names but implement differently.

2. Step 2: Choose abstract contract design

   Abstract contracts declare functions without bodies, forcing children to implement them.

3. Step 3: Exclude other options

   Regular contracts have implementations, libraries can't enforce function signatures, interfaces can't have implementations.

4. Final Answer:

   Use an abstract contract with the function declared as virtual and no body -> Option D

5. Quick Check:

   Abstract contract enforces function signature for children [OK]

Hint: Abstract contracts enforce shared function signatures [OK]

Common Mistakes:

• Using regular contracts which fix implementation
• Confusing interfaces with implemented functions
• Trying to use libraries for function enforcement