3.24 Visibility

Remember that functions in Solidity have the notion of visibility where they could be either public, external, internal or private, this affects which users can call these functions.

So public and external functions are callable by anyone depending on the access control that is enforced on top of that, whereas internal and private can be called only from within the contracts or the derived contracts.

Until Solidity version 0.5.0 this visibility specifier was optional and they defaulted to public. This aspect led to vulnerabilities where the developer forgot to mention or specify the visibility in which case it became public by default and resulted in malicious users being able to call these functions and make unauthorized state changes completely unexpected by the developer or the smart partner.

So this optional specification of function visibility defaulting to public visibility was removed as of Solidity version 0.5.2, so this was a big change when it came to increasing the security of smart contracts and since that version function visibility is required to be specified explicitly for every function.

Public Functions

Remember that Solidity has the notion of visibility for functions, there are four visibility specifiers: internal, private, public and external. public functions consume more Gas than external functions.

The reason for this is because the arguments of public functions need to be copied from thecall data component of the EVM to the memory component. This copying produces more bytecode for such public functions which therefore consumes more Gas.

This copying is not required for external functions where their arguments can be left behind in the calldata component of the EVM. This key difference leads to public functions consuming more Gas than external functions in Solidity.

So if there are functions in the contract that are never called from within the contracts themselves, then such functions should be declared with external visibility and not public visibility, which leads to better Gas efficiency.