The goal of this lab is to provide enough exposure to C++ that you understand how to explore more on your own. It assumes you learned all of the topics in last week’s lab.

1 Three Language Features

1.1 Reference arguments

In C, if you have an argument of type int * you don’t know if it is an array or just a value you are supposed to set. C++ adds syntax to allow these to look different the reference argument type.

If you write a function where an argument name is followed by an ampersand (e.g., int &x) then C++ will compile it to be a pointer, but have your syntax look like it is a variable.

int f(int *x) {
    int ans = *x + 2;
    *x = 3;
    return ans;
}

int f(int &x) {
    int ans = x + 2
    x = 3;
    return ans;
}

1.2 Templates

As discussed last week, C++ allows function names to be overloaded using name mangling. It also allows more complicated overloads.

A template in C++ is an outline of how to create code if it is needed. Templates uses angle-brackets, like generics in Java. Java, however, discards the generics during compilation as part of the type checking step. C++ instead uses them to generate special code for each template argument created. Thus, C++ code like

template <typename T> T foo(T x) { /* ... */ }

does not create any code, by itself. Instead, each time you use the template with different template parameters the compiler creates code for that parameterization. Thus, if you later have

int x = foo(3);
double y = foo(3.14);

the compiler will create two different foo functions: one that has an int parameter and return type and another that has double.

Note that the compiler will try to guess what template parameters were missing; if you want to be explicit, you can also write

int x0 = foo(3); // compiler figures out it is a template, and deduces type
int x1 = foo<>(3); // told it is a template, compiler deduces type
int x2 = foo<int>(3); // told it is a template and type

{.aside} Another difference from Java is that the parameters of a template do not need to be type names; they could also be values, like integers. Value templates are unusual enough in C++ we’ll say no more about them here.

{.aside} C++ template metaprogramming is a complicated topic on which entire books have been written. Some libraries, such as boost and FFTW are well-known for complicated template usage. If you find yourself intrigued by templates, I recommend the D language as having a richer, more programmer-friendly template mechanism than C++.

1.3 Operator Overloading and Friends

C++ recognizes that there is no magic to operators, and it is awkward to write your matrix class with x.add(y) when you write your numbers as x + y instead, so it lets you overload operators. In essence, this lets you add, to any class you want to work with, functions that are the implementation of operators. This lets you write math with your custom matrix class just like you do with the built-in number types.

Once you have this, though, it becomes tempting to use in other ways. One of the most famous is the ostream type, C’s wrapper over file writing, which overloads the left-shift operator to mean write.

#include <iostream>
using namespace std;

int main() {
    cout << "Hello, world!" << endl;
}

#include <iostream>
int main() {
    std::cout << "Hello, world!" << std::endl;
}

#include <iostream>
int main() {
    std::cout << "Hello, all " << (3 + 1) << " dimensions!" << std::endl;
}

Not all ostreams will flush output as promptly as others. For debugging, you should definitely use cerr instead of cout as it is better about showing everything when you ask it to be shown instead of delaying until later.

C++ also allows what it calls friend functions, which are used to add functions and operators to existing classes in new files. A common example is to add ostream << mytype operators for new types.

2 STL and standard library

The C++ Standard Library has a variety of types and algorithms in it. Some of the best known of these are part of the Standard Template Library (STL), and sometimes people use STL informally to mean the full C++ Standard Lbirary. There is more in the library than we can fully cover, but the following will get you going:

There are many others not listed above; see wikipedia for a reasonable overview.

Note that many C++ types overload == to compare values the way that .equals() does in Java. Hence "Hi" == "Hi" compares pointers, but string("Hi") == string("Hi") compares words.

3 Your Task

The input processing cal look like a loop which, as long as cin is .good() (i.e., not closed and with no read errors)

1. >> a string from cin

2. make a stringstream and << a string into it

3. >> a double from the stringstream

4. if that >>ing into the stringstream causes the stringstream to .fail(),

   1. check if the string is == to an operator, and

      1. if so apply the operator (ending if there are fewer than 2 operands)

         otherwise, end the loop

      otherwise, push the double

Use a stack<double> as your stack. Note that pop returns void; you’ll need to use top to retrieve a value before poping it off the stack.

Displaying a stack is not something the STL natively supports, so you’ll need to do that manually.

If you have time, implement your own ostream & operator<<(ostream & o, stack<double> s) which repeatedly displays (with <<) the top() and then pop()s until the stack is .empty(). Note that we pass in the ouput stream by reference (with &), but pass the stack by value. Passing by value means passing a copy, so poping s does not pop the stack that was passed in, only the local copy of it that the operator<< function contains.