A few C++ tricks

So far, all the things we have learned have been part of the vanilla C language. C++ is an extension to C that adds some object-oriented programming features; this is a collection of programming methods that are mostly useful to allow large groups of programmers to collaborate on huge projects (say, Skyrim).

In our class we are not going to be using object-oriented programming. However, there are some C++ tricks that can make your lives much simpler. To use these, you’ll need to compile your code with g++ rather than gcc.

Trick #1: initializing variables inside a for loop

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for (int i=0; i<100; i++)
  printf("%d squared is %d.\n",i,i*i);

Trick #2: ``pass-by-reference’’ to functions

You know how to use a function that takes a bunch of inputs and returns an output:

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double hypotenuse(double a, double b)
{
  return sqrt(a*a + b*b);
}

Sometimes, though, you want to write a function that changes its inputs, or that returns more than one thing. You can’t do this with return, since this only lets you send one value back.

Ordinary parameters are passed by value. This means that C takes the number and creates new variables (in the function above, a and b) that hold those numbers inside the function.

But in C++ you can also pass variables themselves by putting an ampersand before the variable name. When you do this, there is no second copy created; this means that if you change the variable inside the function, it “stays changed”. Consider the following function:

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void leapfrog(double &theta, double &omega, double dt)
{
  theta = theta + omega * dt/2;
  omega = omega - sin(theta) * dt; // take g and L to be 1
  theta = theta + omega * dt/2;
}

The utility of something like this should be obvious.