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Can you declare a constant variable in Solidity?

const uint answer = 42;

Or is the only way to do it with a const function?

function GetAnswer() constant returns(uint ret) {
  return 42;
}

I know you could always create a normal public variable and then just not have any setters in the code, but this is not immediate obvious from the declaration, so it seems preferable to explicitly declare a variable as a constant or read-only if the language supports it.

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1
  • minor nitpick: do you mean to cast a float to an uint here? – Joris Bontje Jul 8 '16 at 6:38
33

Found the answer:

State variables can be declared as constant (this is not yet implemented for array and struct types and not possible for mapping types).

contract C {
    uint constant x = 32**22 + 8;
    string constant text = "abc"; 
}

This has the effect that the compiler does not reserve a storage slot for these variables and every occurrence is replaced by their constant value.

The value expression can only contain integer arithmetics.

http://solidity.readthedocs.io/en/latest/contracts.html#constants

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2
  • In addition to Raine's answer, while declaring constant - you must not modify the state. – Ashok Mahalik Jan 25 '17 at 4:44
  • " the compiler does not reserve a storage slot for these variables" - does it mean we pay no gas for constants? 1 byte is 1 gas, so if I use a constant that normally takes 5 bytes we pay no gas. Is it right? – Dany D Mar 16 '18 at 12:33

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