How can a variable be passed to a function in C++: by value, by reference, or by pointer?

In C++, a variable can be passed to a function in three main ways:

1. By value — the function receives a copy of the variable. Changes inside the function do not affect the original.

void foo(int x) {
    x = 10; // change to local copy
}

2. By reference — the function receives a reference to the original variable. Changes inside the function affect the original variable.

void foo(int& x) {
    x = 10; // will change the original
}

3. By pointer — the function receives the address of the variable. To access the value, dereference the pointer. Allows passing nullptr and modifying the original.

void foo(int* x) {
    if (x != nullptr) {
        *x = 10; // will change the original
    }
}

The choice of method depends on the task: if you need to modify the original variable — it's better to use a reference or a pointer; if not — pass by value.