A condition that always hold true for one or more fields, if those fields are *[[Validity|Valid]]*. >[!example] > Invariant: All values of type [[Reference]] in [[C++]] and [[Rust]] are non null > > *[[C++]]*: >```cpp >void goofy(const int& number) { > // number is a non null reference / a pointer to a valid integer. > std::cout << number << std::endl; >} >``` > *[[Rust]]*: >```rust >fn goofy(number: &i32) { > println!("{number}"); >} >```