Is constructible?

Today I want to share with you something that really surprised me. Currently, Tomasz Kamiński and Ville Voutilainen are working on fixing a certain issue with std::optional’s converting constructors (which deserves a separate post). At some point, in the solution, they perform the following type-trait test:

is_constructible<T, U>::value || is_convertible<U, T>::value

If the varying order of T and U upsets you, rest assured that this is correct. This is how these traits are defined: is_constructible takes the created type first, whereas is_convertible takes the converted-to type second. But what really struck me here is the apparent redundancy. When type U is convertible to T does it not imply that T is constructible from U? Or in other words, if the following copy-initialization works:

T v = u;

The following direct-initialization:

T v (u);

should also work? Well, this is C++. It turns out that such expectation is not necessarily correct. Continue reading

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The cost of std::initializer_list

C++ aims to be a language for people working close to metal. It offers abstractions that cost no (or minimum) run-time or memory overhead. If you require zero-overhead abstractions, std::initializer_list may not be a tool for you. Continue reading

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Competing constructors

We start with a known C++ gotcha:

std::vector<int> v (size_t(4), 2); // parentheses
std::vector<int> u {size_t(4), 2}; // braces
 
assert (v.size() == 4);
assert (u.size() == 2);
 
assert (v[0] == 2); // elements: {2, 2, 2, 2}
assert (u[0] == 4); // elements: {4, 2}

In this post I want to analyze the source of the problem a bit further, and offer some suggestions on class design. Continue reading

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Diagnosable validity

Certain combinations of types and expressions can make a C++ program ill-formed. “Ill-formed” is a term taken from the C++ Standard and it means that a program is not valid, and compiler must (in most of the cases) reject it. This is quite obvious:

int main()
{
  auto i = "some text".size(); // invalid expression
};

String literals do not have member functions, therefore compiler cannot accept this program, and must report an error. This puts a responsibility on programmers to learn which expressions and types are valid in a given context and use only these. Again, I am saying a very obvious thing.

What is less obvious is that there is a way in C++ to enter a type or expression of which we do not know if it is valid or not, in an isolated environment, where it does not render the entire program ill-formed, but instead it returns a yes-no (or rather valid-invalid) answer, which we can use at compile-time to make a decision how we want the program to behave. When requested, compiler can analyze all the declarations it has seen so far, and make an approximated judgement whether a given type or expression would make the program ill-formed or not, if used outside the isolated environment. The compiler’s approximated answer is not always correct, but it is just enough most of the time. Continue reading

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Sessions and object lifetimes

In this post we will see how C++ object lifetime can be used to control the duration of sessions: time spent owing and using a resource. The goal is to get a better understanding of what tools the language offers for using and sharing resources efficiently. Continue reading

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Concepts without Concepts

“Concept” can mean two things in the context of C++ generic libraries:

  1. Something informal: something we know about template parameters, and can tell to other human programmers, e.g. in documentation.
  2. A language feature.

This post is about concepts in the first sense. It claims that we had concepts for quite a while already, and shows how we can use them to make generic libraries easier to use. Continue reading

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Another polymorphism

In this post we will try to see by a practical example what Boost.Variant is for. You can sometimes see examples that use type variant<int, double, string>, but to me they are artificial: I never needed to use something that is either a double or int; but I still consider this library useful. Even if you are already familiar with Boost.Variant an its concepts of “never-empty guarantee” and “static visitor”, I made sure there is still something you can get from reading this post. Continue reading

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