Rvalues redefined

In this post we will explore what I consider the most significant language change in C++17. I call it the most significant because it changes the way you design your resource-managing types and how you think about initialization. It is often called “guaranteed copy elision”, but I will not use that name (except for this single time) because it does not reflect what this feature is. C++ has completely changed the meaning of rvalue (actually, prvalue).

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Testing C++ online

In this short post I wanted to share links to three online services that I use for testing different aspects of C++ and C++ compilers.

• https://wandbox.org/ — this allows you to compile and observe the output of programs compiled in different versions of GCC and Clang, with different versions of Boost libraries;
• https://godbolt.org/ — this allows you to see the assembly of C++ programs generated with different compilers on different platforms;
• http://quick-bench.com/ — this allows you to micro-benchmark C++ functions on different versions of GCC and Clang using Google’s benchmark library.

String’s competing constructors

Let’s start with the problem. I want to check whether a program received a text message that consists of four consecutive zeroes. Not '0', but the numeric zero. I will create a constant std::string representing the special sequence and compare the messages (also stored as std::strings) I receive. I cannot just type:

const std::string Special = "\0\0\0\0";

As this would be interpreted, according to C rules, as a zero-length string. But there is another constructor exactly for our case, where I can say “character C repeated N times”. So, I type:

const std::string Special ('\0', size_t(4));

I could have used braces for the initialization, but since I am aware of the std::initializer_list gotchas (like the one described here), I am following my rule of thumb, “if you suspect the type might expose an initializer-list constructor, and you want to initialize through a different constructor, do not use braces”.

Now I can test my program. It still doesn’t work correctly for some reasons. It does not recognize the special messages as special, even though I would expect it to. I test and debug it for quite some time, until finally do I realize that my string Special has a different value than I think.

Help the compiler warn you

Compiler warnings are a very useful tool for detecting bugs in your program. Because you can enable them selectively, and because you can choose to turn them into hard errors on your compiler, you can in fact build a dialect, which is a safer subset of C++. You can enable even more warning if you assist your compiler a bit and instruct it about your intentions. We will see how to do it in one case.

A friendly type predicate

This is a sequel to the previous post on writing a custom type predicate. One of the readers on Reddit made a very insightful observation. The user has implemented a type that she intends to use with our library, call it Calc. She passes it to the library function, and she gets the compiler error:

static assertion failed: X does not have a desired interface

But what is our type missing? In the previous post we were describing 3 constraints. A concept could have far more of them. The user has already made an effort to have Calc comply with the constraints, so there must be something tiny missing. Maybe there is a bug in the implementation of the predicate? But it is difficult to track what it is that the predicate does not like about our type. We could use some more specific information.

Your own type predicate

In this post we will see how to define a type trait or a type predicate or a meta-function that would allow us to check at compile time whether a type exposes an interface that we need. That is, we want to check if a given type T has:

• nested typename result_type,
• static member function set_limit that takes one argument of type int,
• member function get_result that returns type const result_type& and that is declared not to throw exceptions,

In other words, the following expressions and constructs need to be valid for any variable x of type T:

T::result_type	is a type
T::set_limit(1)	return type unimportant
x.get_result()	returns const result_type&, throws nothing

By “checking” I mean being able to test it at compile time, for instance like this:

static_assert(is_acceptable<X>::value,
              "X does not have a desired interface");

We will do it in C++11. This is also going to be an introduction to “metafunctions” in C++.

error codes — some clarifications

In this post I would like to discuss two issues brought up by the readers regarding previous posts on using <system_error>:

1. Storing 0 in error codes, and using non-0 success values.
2. Concerns about using globals.