Payable functions in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Payable functions

Call function

↓

Is function payable?

No→Reject transaction

Yes↓

Accept Ether sent

↓

Execute function code

↓

Update contract balance

↓

Return success

When a function is called, the system checks if it is payable. If yes, it accepts Ether and runs the function; otherwise, it rejects the transaction.

Execution Sample

Blockchain / Solidity

contract Example {
  function deposit() public payable {
    // Accept Ether
  }
}

This contract has a payable function 'deposit' that can receive Ether when called.

Execution Table

Step	Function Called	Is Payable?	Ether Sent (wei)	Action	Contract Balance (wei)
1	deposit()	Yes	1000	Accept Ether and execute	1000
2	deposit()	Yes	500	Accept Ether and execute	1500
3	withdraw()	No	100	Reject transaction	1500

💡 At step 3, 'withdraw' is not payable, so transaction is rejected and balance stays the same.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
contract_balance	0	1000	1500	1500

Key Moments - 2 Insights

Why does the contract reject Ether sent to a non-payable function?

Because only payable functions can accept Ether. As shown in execution_table step 3, calling 'withdraw' which is not payable causes rejection.

What happens to the contract balance when a payable function is called with Ether?

The contract balance increases by the Ether amount sent, as seen in steps 1 and 2 where balance updates from 0 to 1000 and then to 1500.

Visual Quiz - 3 Questions

Look at the execution_table, what is the contract balance after step 2?

A 1500 wei

B 1000 wei

C 500 wei

D 0 wei

💡 Hint

Check the 'Contract Balance (wei)' column at step 2 in the execution_table.

At which step is a non-payable function called and what happens?

A Step 2, transaction accepted

B Step 1, transaction accepted

C Step 3, transaction rejected

D Step 3, transaction accepted

💡 Hint

Look at 'Is Payable?' and 'Action' columns in execution_table for step 3.

If the deposit function was not marked payable, what would happen when sending Ether?

A Ether accepted and balance updated

B Transaction rejected

C Function runs but no Ether accepted

D Contract balance resets

💡 Hint

Refer to the concept_flow where non-payable functions reject Ether.

Concept Snapshot

Payable functions allow contracts to receive Ether.
Only functions marked 'payable' can accept Ether.
Sending Ether to non-payable functions rejects the transaction.
Contract balance updates when payable functions receive Ether.
Use 'payable' keyword in function declaration.

Full Transcript

Payable functions in blockchain contracts are special functions that can receive Ether when called. When a function is called, the system checks if it is marked payable. If it is, the Ether sent with the call is accepted and the function code runs. The contract's balance increases by the amount of Ether received. If the function is not payable, the transaction is rejected and no Ether is accepted. For example, a deposit function marked payable can receive Ether and increase the contract balance. Calling a non-payable function with Ether causes rejection. This behavior ensures contracts only accept Ether where intended.