C++ Modes for Passing Arguments

• By Value
  • A copy of variable will be created
  • Any modification of the variable inside the function have no effect on original variable
• By Reference
  • A reference to the original variable is created (i.e. alias)
  • Any modification of the variable inside the function happens also on original variable
• By Pointer
  • A pointer to the original variable is passed
  • Any modification to the pointed object inside the function happens also on the original variable
  • Access to the pointed variable must happen using pointer syntax

Example

• By Value

int foo(int x) // pass x by value, it is copied
{
    return ++x; // x is modified
}

int main()
{
    int y = 5;
    cout << y << endl; // 5
    cout << foo(y) << endl; // 6
    cout << y << endl; // 5
    return 0;
}

If x was a large object instead of being an int, then passing it by value would be expensive. Better to pass it by const reference.

• By Reference

int foo(int &x) // pass x by reference
{
    return ++x; // x is modified
}

int main()
{
    int y = 5;
    cout << y << endl; // 5
    cout << foo(y) << endl; // 6
    cout << y << endl; // 6
    return 0;
}

• By Pointer

int foo(int *x) // pass x by pointer
{
    return ++ *x; // x is modified
}

int main()
{
    int y = 5;
    cout << y << endl; // 5
    cout << foo(y) << endl; // 6
    cout << y << endl; // 6
    return 0;
}

For Struct

• If passed by value, the struct is copied into the argument, which might be costly for large structures
• Generally we pass struct arguments by:
  • reference, if we intend to modify them inside the function
  • const reference otherwise

struct ComplexNumber 
{
    double r, i; 
};

// we pass the complex numbers by const reference, so we avoid the cost of copying them 
// we specify them as const, to make it clear that they will not be modified in the function 

ComplexNumber mult(const ComplexNumber& a, const ComplexNumber& b) 
{
    ComplexNumber c; 
    c.r = a.r * b.r - a.i * b.i;
    c.i = a.r * b.i + a.i * b.r; 
    return c;
}
int main() 
{
    ComplexNumber x = {1, 2}, y = {2, -3}, z = mult(x, y);
    std::cout << z.r << "+" << z.i << "i\n"; 
}