When you write smart contracts in Solidity you will use modifier for each function that indicates accessibility for the functions.
We will see the difference between public and external modifier.
Let’s see the following smart contract to feel the difference in practice.
pragma solidity^0.4.24;
contract SimpleContract {
function test(uint[20] a) public returns (uint){
return a[10]*2;
}
function test2(uint[20] a) external returns (uint){
return a[10]*2;
}
}Here is distinct difference between public and external functions.
Public function uses 496 gas while external function uses only 261 gas.
The difference is because in public functions, Solidity immediately copies array arguments to memory, while external functions can read directly from calldata. Memory allocation is expensive, whereas reading from calldata is cheap.
The reason that public functions need to write all of the arguments to memory is that public functions may be called internally, which is actually an entirely different process than external calls. Internal calls are executed via jumps in the code, and array arguments are passed internally by pointers to memory. Thus, when the compiler generates the code for an internal function, that function expects its arguments to be located in memory.
For external functions, the compiler doesn’t need to allow internal calls, and so it allows arguments to be read directly from calldata, saving the copying step.
As for best practices, you should use external if you expect that the function will only ever be called externally, and use public if you need to call the function internally. It almost never makes sense to use the this.f() pattern, as this requires a real CALL to be executed, which is expensive. Also, passing arrays via this method would be far more expensive than passing them internally.
You will essentially see performance benefits with external any time you are only calling a function externally, and passing in large arrays.
Examples to differentiate:
public – all can access
external – Cannot be accessed internally, only externally
internal – only this contract and contracts deriving from it can access
private – can be accessed only from this contract
