Frenetic Array

A canvas for logic and imagination.

Modern C++ (The Good Parts)

I recently gave a lecture over C++11 and C++14 to an introductory programming course. This briefly covers auto, lambda expressions, constexprs, and the default and deleted keywords. My lecture notes can be found here (alternatively, under projects).

C++ Syntactic Peculiarities

After using a fairly large, matured language for a reasonable period of time, find peculiarities in the language or the libraries is guaranteed to happen. However, given it’s history, I have to say C++ definitely allows for some of the strangest peculiarities in it’s syntax. Below I list three that are my favorite.

Ternaries Returning lvalues

You might be familiar with ternaries as condition ? do something : do something else, and they become quite useful in comparison to the standard if-else. However, if you’ve dealt with ternaries a lot, you might have noticed that ternaries also return lvalues/rvalues. Now, as the name suggests suggests, you can assign to lvalues (lvalues are often referred to as locator values). So something like so is possible:

    std::string x = "foo", y = "bar";

    std::cout << "Before Ternary! ";
    // prints x: foo, y: bar
    std::cout << "x: " << x << ", y: " << y << "\n"; 
    // Use the lvalue from ternary for assignment
    (1 == 1 ? x : y) = "I changed";
    (1 != 1 ? x : y) = "I also changed";

    std::cout << "After Ternary! ";
    // prints x: I changed, y: I also changed
    std::cout << "x: " << x << ", y: " << y << "\n"; 

Although it makes sense, it’s really daunting; I can attest to never seeing it in the wild.

Commutative Bracket Operator

An interesting fact about C++ bracket operator, it’s simply pointer arithmetic. Writing array[42] is actually the same as writing *(array + 42), and thinking in terms of x86/64 assembly, this makes sense! It’s simply an indexed addressing mode, a base (the beginning location of array) followed by an offset (42). If this doesn’t make sense, that’s okay. We will discuss the implications without any need for assembly programming.

So we can do something like *(array + 42), which is interesting; but we can do better. We know addition to be commutative, so wouldn’t saying *(42 + array) be the same? Indeed it is, and by transitivity, array[42] is exactly the same as 42[array]. The following is a more concrete example.

    std::string array[50];
    42[array] = "answer";

    // prints 42 is the answer
    std::cout << "42 is the " << array[42] << ".";

Zero Width Space Identifiers

This one has the least to say, and could cause the most damage. The C++ standard allows for hidden white space in identifiers (i.e. variable names, method/property names, class names, etc.). So this makes the following possible.

    int n​umber = 1;
    int nu​mber = 2;
    int num​ber = 3;

    std::cout << n​umber << std::endl; // prints 1
    std::cout << nu​mber << std::endl; // prints 2
    std::cout << num​ber << std::endl; // prints 3

Using \u as a proxy for hidden whitespace character, the above code can be re-written as such:

    int n\uumber = 1;
    int nu\umber = 2;
    int num\uber = 3;

    std::cout << n\uumber << std::endl; // prints 1
    std::cout << nu\umber << std::endl; // prints 2
    std::cout << num\uber << std::endl; // prints 3

So if you’re feeling like watching the world burn, this would be the way to go.