Where are local variables of Primitive (Elementary) data types stored in Solidity

Question

Though in the latest solidity compiler versions, it has been made clear that for reference variables local to a function, one needs to now explicitly mention its location modifier (memory, storage), thereby removing the whole confusion about its default nature.

But it is still unclear where are all the local variables defined as uint, uint8, bool, address, enum etc. of a function stored ? is it stored in storage, memory or stack ?

Answer 1

The primitive scalar types:

• bool
• uint
• address
• etc.

reside in storage when declared globally, and in memory when declared in functions.

contract Locations {

  // storage
  bool switch;
  uint number;
  address account;

  function doSomething() public pure {
    // memory
    bool switch2;
    uint number2;
    address account2;
  }
}

Hope it helps.

Answer 2

Variables can be stored in 3 ways in solidity: storage, memory, and stack.

Storage: Storage is where all the contract state variables reside. Every contract has its own storage and it is persistent between function calls and quite expensive to use.

Memory: Memory is cheaper to use as it stores values temporarily. It is erased between external function calls. Memory is not persistent.

Stack: Almost free to use. Stack can be used to hold small local variables, but only limited in number.

Complex variable types, which don't always fit into 256 bits have to be handled carefully. For example, arrays and structs have an additional annotation, called 'data location' that indicates where it is stored in memory or storage.

So, you can use the following concept:

Forced data location:

• parameters (not return) of external functions: calldata

• state variables: storage

Default data location:

• parameters (also return) of functions: memory

• all other local variables: storage

Also, take a look at: Storage and Memory - Solidity Docs