By default c++ uses call-by-value, but copying large arguments can be expensive so we can also use call by:

1. pointer
2. reference
3. const reference

Since as a user we do not see if a function that takes an argument actually modifies it, we can use const references to have the compiler guarantee that an argument that is passed is not modified.

The syntax looks as follows: Note that changing a const reference gives a compile error.

#include <iostream>
    
void CallByPointer(int *i)
{
    *i = *i + 1;
}

void CallByReference(int &i)
{
    i = i + 1;
}

void CallByReferenceConst(int const &i) {
    //error C3892: 'i': you cannot assign to a variable that is const
    i = i + 1;
}

int main()
{
    int i = 0;
    
    CallByPointer(&i);
    std::cout << i << std::endl; //i = 1

    CallByReference(i);
    std::cout << i << std::endl; //i = 2

    CallByReferenceConst(i);
    std::cout << i << std::endl; //error: see function definition

    return 0;
}