Solidity is a statically typed language, which means that the type of each variable (state and local) needs to be specified. Solidity provides several elementary types which can be combined to form complex types.

The concept of "undefined" or "null" values does not exist in Solidity, but newly declared variables always have a default value dependent on its type. To handle any unexpected values, you should use the revert function to revert the whole transaction, or return a tuple with a second bool value denoting success.

Value Types

The following types are also called value types because variables of these types will always be passed by value, i.e. they are always copied when they are used as function arguments or in assignments.

Booleans

bool: The possible values are constants true and false.

Operators:

• ! : logical negation
• && : logical conjunction, "and"
• || : logical disjunction, "or"
• == : equality
• != : inequality

Integers

int / uint : Signed and unsigned integers of various sizes.