First glance: C++17, part 2

Last time, we got a glimpse of what the future of C++ will look like from the language perspective. But programming isn’t just about the language, so here are some of the highlights of the C++ Standard Library. It’s getting a bit of a makeover, as you’ll see, but not enough to cover its roots.


Without even looking through the whole list of changes and additions, I already knew this was the big one, at least for me. The variant is a type-safe (or tagged) union. It’s an object that holds a value of one type chosen from a compile-time list. You know, like C’s union. Except variant keeps track of what kind of value it’s holding at the moment, and it’ll stop you from doing bad things:

variant<int, double> v;
v = 42;

// This works...
auto w = get<int> v;    // w = 42

// ...but this one throws an error
auto u = get<double> v; // nope!

Optional values

This one’s similar, and it was supposed to be in C++14. An optional is either a value, or it’s not. In that, it’s like Haskell’s Maybe. You can use it to hold the value of a function that can fail, and it works like an error signal. When converted to a boolean (as in an if), it acts as true if it contains a value, or false if it doesn’t. Not huge, but a bit of a time-saver:

optional<unsigned int> i;

// some function that can return an int or fail
i = f();

if (i)
    // work with a proper value
    // handle an error condition

Any values

The third of this little trinity is any, an object that can—as you might imagine—hold a value of any type. You’re expected to access the value through the any_cast function, which will throw an exception if you try the wrong type. It’s not quite a dynamic variable, but it’s pretty close, and it’ll likely be faster.


If you’ve ever used JavaScript, you know about its apply method. Well, C++ will soon have something similar, but it’s a free function. It calls a function (or object or lambda or whatever) with a tuple of arguments, expanding the tuple as if it were a parameter pack.


Yes, C++ lacked a standard way of searching a sequence for a value until now. Rather, it lacked a general way of searching for a value. Some searches can be made faster by using a different algorithm, and that’s how C++17 upgrades std::search. And they’re nice enough to give you a couple to get started: bayer_moore_searcher and bayer_moore_horspool_searcher. No points for guessing which algorithms those use.


It’s common to need to clamp a value to within certain bounds, but programming languages don’t seem to realize this. Libraries have functions to this, but languages rarely do. Well, C++ finally did it. That’ll instantly shave off 50% of the lines of code working with lighting and colors, and the rest of us will find some way to benefit.

Mathematical special functions

C++ is commonly used for numeric computation, but this set of functions is something else. They likely won’t be of interest to most programmers, but if you ever need a quick beta function or exponential integral, C++17 has got you covered.


Alright, I’ll admit, I was holding back. Everything above is great, but the real jewel in the C++17 Standard Library is the Filesystem library. If you’ve ever used Boost.Filesystem, you’re in luck! It’s the same thing, really, but it’s now standard. So everybody gets to use it. Files, paths, directories, copying, moving, deleting…it’s all here. It certainly took long enough.

Still not done

That’s not nearly everything, but those are my favorite parts. In next week’s finale, we’ll switch to the lowlights. We’ll see those features that just didn’t make the cut.

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