We showed you some basic C++ last semester, including using templates like Java uses generics. But that is far from all C++ templates can do… This lab will help you explore this topic in more detail.

You’ll work in pairs via discord for this lab, like those before it.

1 Caveat

There exists a subset of programmers who believe that C++ templates beyond type generics are an abomination and should never be used. I hope this lab will help you make your own opinion, but be aware that if you work for a company that uses C++ and suggest some of these techniques, you might get some blow-back…

2 Review

Begin by reviewing the C++ labs from COA1:

• name overloading, namespaces, references, methods
• STL, references, templates, operator overloading, iostream

We’ll be picking up from where those left off in this lab.

3 A Mathematical Vector type

In mathematics, a vector has fixed length and adding vectors of different length is meaningless. In this lab you’ll use C++ templates to make a mathematical vector class that uses the type checker to ensure at compile time that no wrong-sized operations occur.

We’ll also use a struct, not a class, to encourage passing small vectors by value instead of by pointer.

typedef vec<double, 2> vd2;
typedef vec<double, 3> vd3;
typedef vec<int,4> vi4;

int main() {
    vd3 a;
    std::cout << a << std::endl; // prints (0, 0, 0)
    
    a[2] = 2.5;
    std::cout << a << std::endl; // prints (0, 0, 2.5)

    std::cout << (a + a) << std::endl; // prints (0, 0, 5)
    std::cout << a << std::endl; // prints (0, 0, 2.5)

    vd2 b;
    std::cout << b << std::endl; // prints (0, 0)
    vi4 c;
    std::cout << c << std::endl; // prints (0, 0, 0, 0)
    
    std::cout << (a + b) << std::endl; // does not compile
    std::cout << (a + c) << std::endl; // does not compile
    
    
    b[2] = 2.5; // assigns off the end of b...
    c[2] = 2.5; // implicitly casts a double to an int...
    b[1] = 2.5;
    c[2] = 2;

    std::cout << b << std::endl; // prints (0, 2.5)
    std::cout << c << std::endl; // prints (0, 1074003968, 2, 0)
                                 // -- the 1074003968 is from b[2] overflow
    vd3 x = {1.5, 2.5, 3.5};
    std::cout << x << std::endl; // prints (1.5, 2.5, 3.5)

    vd3 y = {1, 2, 3};
    std::cout << y << std::endl; // prints (1, 2, 3)
    
    vd3 z = {1, 2.3, 4.5}; // does not compile
}

4 Guides

• Precede your struct with template <typename N, int n> so it will work with multiple types and lengths

• Use a static array N data[n] (or the like) as the only field so avoid heap memory allocations

• Constructors have no return type and the same name as the struct

  • your default constructor should sets the data to all 0s
  • the other should accept an initializer list, which is what the {1,2,3} turns into at compile time
    • #include <initializer_list>
    • 1 argument, a std::initializer_list<R> where R is the type of value you expect to be passed in.
    • a std::initializer_list<R> is a collection, meaning you access it by iterator
    • you’ll need to verify that the initializer list has the right number of values (it’s size() method should help)
    • we recommend using a template to pick the initializer list contained type, e.g. with template<typename R> before the function

• Operator overloading uses special function names

  • operator + should accept only vectors of the same length
  • operator [] needs two variants
    • N& operator[] (int idx) – allows setting by index by returning a reference
    • N operator[] (int idx) const – allows reading when no reference exists
  • Printing needs a special friend notation to let you access between the ostream and vec classes:
    • friend std::ostream& operator << (std::ostream& out, const vec<N,n>& x)
    • this function should use out << thing and then return out

• It is best practice (but not technically required) to

  • put template declarations on the line before the struct or function they modify

    template <typename R>
    R dostuff(const R& t) { /*...*/ }

  • use const for all arguments you will not modify

    double sqrt(const double x)

  • use reference types const mytype& for struct arguments you don’t want to copy

    double length(const vec<double,3>& x)

  • use const for any method that does not modify this’s fields

    double length() const

5 C++ Iterators

Many STL structures in C++ use the iterator pattern to allow access. This has a somewhat different look than it does in Java or the like.

The collection offers two functions of note:

• .begin() returns an iterator pointing to the first element of the collection
• .end() returns an iterator pointing to the past the end element of the collection

The iterator overloads three operators of note:

• operator != tells if two iterators point to distinct entries in the collection
• operator * gets the item pointed to out of the iterator
• operator ++ moves the item to the next spot in the collection

Thus, a loop that prints all items in a collection might look like

for(auto it = mycollection.begin(); // create an iterator
    it != mycollection.end();       // and while it's not off the end
    ++it) {                         // move it forward
    std::cout << *it << std::endl;  // derreference to print
}

Note in the above that when you combine a declaration and initialization, you can declare the type to be auto meaning use whatever type the initialization gives me.