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February 2018

Batteries not included: what should go in the C++ standard library?

By Guy | Feb 21, 2018 09:44 AM | Tags: None

With the next standardisation meeting coming up, now is a good time to consider what the limits of the standard library should be. 

Batteries not included: what should go in the C++ standard library?

By Guy Davidson

From the article:

About forty Christmases ago I was absolutely delighted to open a slot car racing set from my parents. I feverishly set everything up, created a track reminiscent of Brands Hatch, and went to plug everything in, only to discover that I needed batteries to operate the controllers. This was Great Britain in the 1970s...

Quick Q: How can a class template store either reference or value?

By Adrien Hamelin | Feb 20, 2018 08:48 AM | Tags: advanced

Quick A: This happen by default.

Recently on SO:

How can a class template store either reference or value?

You already wrote it (minus the required template <typename T>). The deduction rules for a forwarding reference preserve value category as follows:

  1. If t is bound to an lvalue of type T2, then T = T2&.
  2. If t is bound to an rvalue of type T2, then T = T2.

It's those deduction rules that std::forward relies on to do its job. And why we need to pass the type to it as well.

The above means that you instantiate holder directly with T2 in the rvalue case. Giving you exactly what you want. A copy is made.

As a matter of fact, two copies are made. Once to create the constructor argument t, and the other copy is to initialize obj_m from it. But we can get rid of it with some clever use of type_traits:

template <class T>
class holder {
    T  obj_m;  // should be a reference if possible...
public:
    holder(std::add_rvalue_reference_t<T> t) :obj_m { std::forward<T>(t) } {}
};

template<typename T>
auto hold_this(T && t) { return holder<T>(std::forward<T>(t)); }

See it live. We use add_rvalue_reference_t to make t be of the correct reference type in each case. And "simulate" the argument deduction which would make obj_m { std::forward<T>(t) } resolve to initializing obj_m from the correct reference type.

I say "simulate" because it's important to understand the constructor argument for holder cannot be a forwarding reference because the constructor itself is not templated.

By the way, since you tagged c++17, we can also add a deduction guide to your example. If we define it as follows (with the feedback from T.C. incorporated):

template <class T>
class holder {
    T  obj_m;  // should be a reference if possible...
public:
    holder(T&& t) :obj_m { std::forward<T>(t) } {}
};

template<typename T>
holder(T&&) -> holder<T>;

Then this live example shows you can define variables as hold h1{t}; and hold h2{test()};, with the same deduced types as the function return values from before.

Quick Q: In C++ are static member functions inherited? If yes why ambiguity error does not arise?

By Adrien Hamelin | Feb 20, 2018 08:44 AM | Tags: intermediate

Quick A: Yes, and there are no ambiguity with static members.

Recently on SO:

In C++ are static member functions inherited? If yes why ambiguity error does not arise?

It's fine according to the lookup rules. You see, when you write member access (obj.display();), the member display is looked up not just in the scope of the class and its base classes. Base class sub-objects are taken into consideration as well.

If the member being looked up is not static, since base class sub-objects are part of the consideration, and you have two sub-objects of the same type, there's an ambiguity in the lookup.

But when they are static, there is no ambiguity. And to make it perfectly clear, the C++ standard even has a (non-normative) example when it describes class member lookup (in the section [class.member.lookup])

Quick Q: Most concise way to disable copy and move semantics

By Adrien Hamelin | Feb 20, 2018 08:40 AM | Tags: c++11 advanced

Quick A: Delete the move assignment.

Recently on SO:

Most concise way to disable copy and move semantics

According to this chart (by Howard Hinnant):

The most concise way is to =delete move assignment operator (or move constructor, but it can cause problems mentioned in comments).

Though, in my opinion the most readable way is to =delete both copy constructor and copy assignment operator.

Webinar - Demystifying C++ Lambdas -- Glennan Carnie

By JoCool | Feb 18, 2018 11:06 AM | Tags: None

Following the success of our first webinar on 'Measuring software quality', we are pleased to announce the second in our series:

Demystifying C++ Lambdas

by Glennan Carnie

About the webinar:

Lambdas are one of the features of Modern C++ that seem to cause considerable consternation amongst many programmers.

In this 45 minute webinar, Feabhas Technical Consultant Glennan Carnie, will have a look at the syntax and underlying implementation of lambdas to try and put them into some sort of context.

The Webinar runs twice

10am GMT
4pm GMT

Choose a webinar and reserve a free place on our website: