There are three types of variables in Solidity:<\/p>\n
block.number<\/code>, block.timestamp<\/code>, or msg.sender<\/code><\/li>\n<\/ul>\n<\/li>\n- State variables\n
\n- Declared outside a function<\/li>\n
- Stored on the blockchain<\/li>\n
- Also called "storage"<\/li>\n<\/ul>\n<\/li>\n
- Local variables\n
\n- Declared inside a function<\/li>\n
- Not stored on the blockchain, stored in memory instead<\/li>\n
- Erased between function calls<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n
References:
\nhttps:\/\/docs.soliditylang.org\/en\/v0.7.6\/units-and-global-variables.html<\/a>
\nhttps:\/\/solidity-by-example.org\/variables<\/a><\/p>\nData Types<\/h2>\n
There are two types of data:<\/p>\n
\n- Value types:
YourContract<\/code>, address<\/code>, bool<\/code>, uint256<\/code>, int256<\/code>, enum<\/code>, and bytes32<\/code> (fixed-size byte arrays)<\/li>\n- Reference types:
array<\/code>, mapping<\/code>, and struct<\/code><\/li>\n<\/ul>\nIt's worth noting that bytes<\/code> and string<\/code> are dynamically-sized byte arrays and are considered reference types. However, byte<\/code> (bytes1<\/code>), bytes2<\/code>, ..., bytes32<\/code> are value types since they're fixed-size byte arrays.<\/p>\nReferences:
\nhttps:\/\/docs.soliditylang.org\/en\/v0.7.6\/types.html#value-types<\/a>
\nhttps:\/\/docs.soliditylang.org\/en\/v0.7.6\/types.html#reference-types<\/a><\/p>\nData Location<\/h2>\n
When using a reference type, you must explicitly provide the data location where the type is stored. There are three data locations:<\/p>\n
\nstorage<\/code> is where the state variables are stored, and its lifetime is the lifetime of the contract<\/li>\nmemory<\/code> means variables are temporary and erased between external function calls<\/li>\ncalldata<\/code> behaves mostly like memory<\/code>, but is immutable<\/li>\n<\/ul>\nFor reference types in function arguments, you must declare them as memory<\/code> or calldata<\/code>.<\/p>\nIf possible, use calldata<\/code> as the data location because it avoids copying, reduces gas usage, and ensures that the data cannot be modified. Arrays and structs with calldata<\/code> data location can also be returned from functions, but it is not possible to allocate such types.<\/p>\nReferences:
\nhttps:\/\/docs.soliditylang.org\/en\/v0.7.6\/types.html#data-location<\/a>
\nhttps:\/\/medium.com\/coinmonks\/solidity-storage-vs-memory-vs-calldata-8c7e8c38bce<\/a>
\nhttps:\/\/gist.github.com\/hrkrshnn\/ee8fabd532058307229d65dcd5836ddc<\/a><\/p>\nIt is also a best practice to use external<\/code> if you expect that the function will only ever be called externally, and use public<\/code> if you need to call the function internally. The difference between both is that public<\/code> function arguments are copied to memory<\/code>, while in external<\/code> functions, arguments are read directly from calldata<\/code>, which is cheaper than memory allocation. external<\/code> functions are sometimes more efficient when they receive large arrays.<\/p>\n