Payable functions in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a payable function in blockchain smart contracts?

A payable function is a special type of function in smart contracts that can receive cryptocurrency (like Ether) when called. It allows the contract to accept and handle payments.

beginner

Why do we need to mark a function as payable?

Marking a function as payable tells the blockchain that this function can accept money. Without it, sending money to the function will cause the transaction to fail.

beginner

What happens if you send Ether to a non-payable function?

The transaction will be rejected and the Ether will not be transferred. The contract will not accept the payment because the function is not marked as payable.

intermediate

How can you access the amount of Ether sent to a payable function?

Inside a payable function, you can use msg.value to get the amount of Ether (in wei) sent with the transaction.

beginner

Show a simple example of a payable function in Solidity.

Example:<br><pre>function deposit() public payable {
    // Function can receive Ether
    balance[msg.sender] += msg.value;
}</pre>This function accepts Ether and updates the sender's balance.

What keyword must a Solidity function have to accept Ether?

A payable

B external

C view

D pure

Only functions marked as payable can receive Ether.

What does msg.value represent inside a payable function?

A The block timestamp

B The sender's address

C The gas price

D The amount of Ether sent with the transaction

msg.value holds the amount of Ether (in wei) sent to the function.

What happens if you send Ether to a function NOT marked payable?

A Transaction fails and reverts

B Ether is accepted anyway

C Function runs but ignores Ether

D Ether is sent back automatically

The transaction will fail because the function is not payable.

Which of these is a correct way to declare a payable function in Solidity?

A function buy() public {}

B function buy() public payable {}

C function buy() payable {}

D function buy() external view payable {}

The correct syntax is to add payable after visibility like public payable.

Can a constructor be payable in Solidity?

A No, constructors cannot be payable

B Only if marked external

C Yes, constructors can be payable

D Only in older Solidity versions

Constructors can be marked payable to accept Ether during contract creation.

Explain what a payable function is and why it is important in smart contracts.

Think about how contracts get money from users.

Describe how to access the amount of Ether sent to a payable function and give a simple example.

Look inside the function for the special variable.