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Contract in Solidity is similar to a Class in C++. A Contract have following properties.
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Constructor − A special function declared with constructor keyword which will be executed once per contract and is invoked when a contract is created.
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State Variables − Variables per Contract to store the state of the contract.
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Functions − Functions per Contract which can modify the state variables to alter the state of a contract.
Visibility Quantifiers
Following are various visibility quantifiers for functions/state variables of a contract.
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external − External functions are meant to be called by other contracts. They cannot be used for internal call. To call external function within contract this.function_name() call is required. State variables cannot be marked as external.
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public − Public functions/ Variables can be used both externally and internally. For public state variable, Solidity automatically creates a getter function.
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internal − Internal functions/ Variables can only be used internally or by derived contracts.
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private − Private functions/ Variables can only be used internally and not even by derived contracts.
Example
pragma solidity ^0.5.0; contract C { //private state variable uint private data; //public state variable uint public info; //constructor constructor() public { info = 10; } //private function function increment(uint a) private pure returns(uint) { return a + 1; } //public function function updateData(uint a) public { data = a; } function getData() public view returns(uint) { return data; } function compute(uint a, uint b) internal pure returns (uint) { return a + b; } } //External Contract contract D { function readData() public returns(uint) { C c = new C(); c.updateData(7); return c.getData(); } } //Derived Contract contract E is C { uint private result; C private c; constructor() public { c = new C(); } function getComputedResult() public { result = compute(3, 5); } function getResult() public view returns(uint) { return result; } function getData() public view returns(uint) { return c.info(); } }
Run the above program using steps provided in Solidity First Application chapter. Run various method of Contracts. For E.getComputedResult() followed by E.getResult() shows −
Output
0: uint256: 8
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