class: title 5CCYB041 # OBJECT-ORIENTED PROGRAMMING ### Week 5, session 2 ## pointers, iterators<br>and lambda functions --- # Picking up where we left off We continue working on our [fMRI analysis project](https://github.com/KCL-BMEIS/OOP/blob/main/projects/fMRI/assignment.md) You can find the most up to date version in [the project's `solution/` folder](https://github.com/KCL-BMEIS/OOP/tree/fmri_solution/projects/fMRI/solution) .explain-bottom[ Make sure your code is up to date now! ] --- # Pointers and iterators Many algorithms in the standard template library require an understanding of [iterators](https://www.geeksforgeeks.org/iterators-c-stl/) - we have already come across a few places where we needed to interact with them -- Iterators are inspired from the concept of *pointers* - pointers are an old concept, predating C++ - pointers are a powerful tool, but difficult to understand - and very difficult to use safely! -- To understand iterators, we need to start with pointers --- # Pointers A pointer is a special data type for variables that *point to* other variables. Consider a regular variable of type `int`, initialised to 10: ``` int x = 10; ``` -- We can declare a variable `p` as a *pointer* to an `int` like so: ``` int* p; ``` -- We can then assign the *address* of the regular `x` variable using the [*address-of* operator](https://www.geeksforgeeks.org/cpp-pointer-operators/) to `p`: ``` p = &x; ``` -- The pointer `p` now contains the *address* of `x` - we can say that `p` *points to* `x` --- # Pointers and addresses Pointers contain the *memory location* of the variable they point to - on modern systems, this is a 64-bit unsigned integer -- The *value* held by a pointer variable is always an integer – an address - regardless of which type of variable it points to -- To read the value that the pointer refers to, we need to use the [dereferencing operator](https://www.w3schools.com/cpp/cpp_pointers_dereference.asp): ``` int y = *p; ``` --- # Pointers and addresses To illustrate: ``` int x = 10; int*p = &x; std::cerr << "pointer value: " << p << ", value pointed to: " << *p << "\n"; ``` gives: ``` pointer value: 0x7ffffcc34, value pointed to: 10 ```