Constants in programming are definitions whose value is fixed throughout a program's execution. Literals in most languages are constants, for example. In referentially transparent programming styles, all definitions are constant.
State variables can be declared as constant. In this case, they have to be assigned from an expression which is a constant at compile time. Any expression that accesses storage, blockchain data (e.g. now, address(this).balance or block.number) or execution data (msg.value or gasleft()) or makes calls to external contracts is disallowed. Expressions that might have a side-effect on memory allocation are allowed, but those that might have a side-effect on other memory objects are not. The built-in functions keccak256, sha256, ripemd160, ecrecover, addmod and mulmod are allowed (even though, with the exception of keccak256, they do call external contracts).
The reason behind allowing side-effects on the memory allocator is that it should be possible to construct complex objects like e.g. lookup-tables. This feature is not yet fully usable.
The compiler does not reserve a storage slot for these variables, and every occurrence is replaced by the respective constant expression (which might be computed to a single value by the optimizer).
Not all types for constants are implemented at this time. The only supported types are value types and strings.
pragma solidity >=0.4.0 <0.7.0;
contract C {
uint constant x = 32**22 + 8;
string constant text = "abc";
bytes32 constant myHash = keccak256("abc");
}
