Answer by BugShotGG for What's the difference between passing by reference vs. passing by value?

Here is an example that demonstrates the differences between pass by value - pointer value - reference:

void swap_by_value(int a, int b){
    int temp;

    temp = a;
    a = b;
    b = temp;
}   
void swap_by_pointer(int *a, int *b){
    int temp;

    temp = *a;
    *a = *b;
    *b = temp;
}    
void swap_by_reference(int &a, int &b){
    int temp;

    temp = a;
    a = b;
    b = temp;
}

int main(void){
    int arg1 = 1, arg2 = 2;

    swap_by_value(arg1, arg2);
    cout << arg1 << " " << arg2 << endl;    //prints 1 2

    swap_by_pointer(&arg1, &arg2);
    cout << arg1 << " " << arg2 << endl;    //prints 2 1

    arg1 = 1;                               //reset values
    arg2 = 2;
    swap_by_reference(arg1, arg2);
    cout << arg1 << " " << arg2 << endl;    //prints 2 1
}

The “passing by reference” method has an important limitation. If a parameter is declared as passed by reference (so it is preceded by the & sign) its corresponding actual parameter must be a variable.

An actual parameter referring to “passed by value” formal parameter may be an expression in general, so it is allowed to use not only a variable but also a literal or even a function invocation's result.

The function is not able to place a value in something other than a variable. It cannot assign a new value to a literal or force an expression to change its result.

PS: You can also check Dylan Beattie answer in the current thread that explains it in plain words.