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Quick Q: std::ignore for ignoring unused variable

By Adrien Hamelin | Sep 29, 2016 11:52 AM | Tags: intermediate c++11

Quick A: While in theory possible, it is not the intended usage and other solutions exist.

Recently on SO:

std::ignore for ignoring unused variable

std::ignore may work but it is intended to be used for tuples. So you need to include the tuple header and who knows what operations are done for the assignment. This also may break in another c++ version because it was never documented to be used that way.

A better way for this is the C++17 attribute [[maybe_unused]]

void func([[maybe_unused]] int i)
{
}

It places the declaration right at the variable declaration, so you don't have to declare it in an extra line/statement.

The same can be used for local (and local-static) variables

...
[[maybe_unused]] static int a = something();
...

And also for many more:

Appears in the declaration of a class, a typedef­, a variable, a non­static data member, a function, an enumeration, or an enumerator. If the compiler issues warnings on unused entities, that warning is suppressed for any entity declared maybe_unused.

See http://en.cppreference.com/w/cpp/language/attributes

As for the people concerned that you can still use the variables after you declare them unused:

Yes, this is possible but (at least with clang) you will get warnings in case you use maybe_unused declared variables.

Quick Q: Initializing a std::string with function return value, is there a copy?

By Adrien Hamelin | Sep 29, 2016 11:47 AM | Tags: c++11 basics

Quick A: No, at worse it will be a move.

Recently on SO:

Initializing a std::string with function return value, is there a copy?

In general, if the std::string is constructed in the call and then returned most modern compilers with apply the return value optimization (a special case of copy elision). Your case in particular is the Named RVO (thanks @NathanOliver for pointing this out), if you want to look up the precise rules. From C++17 on this optimization is guaranteed through the standard.

Whether or not the optimization is applied is hard to tell, however if it is not, then in C++11 and above it is very likely that the std::string object in the scope of the function will be moved from and that the return value will be moved to. You could theoretically call std::move on the return value, but thereby you would also prevent any RVO from happening. While move may be efficient, RVO typically yields faster code, because nothing is moved or copied at all, but the object is constructed in place, so I would advise against doing it, and in favor of relying on your compiler.

Concepts Lite vs enable_if--Andrzej Krzemieński

By Adrien Hamelin | Sep 22, 2016 01:12 PM | Tags: experimental community

Why having concepts?

Concepts Lite vs enable_if

by Andrzej Krzemieński

From the article:

This post contains quite advanced material. I assume you are already familiar with Concepts Lite. For an overview of what Concepts Lite is, I recommend this proposal. Also, I have found this blog very useful regarding the details of and issues with concepts’ mechanics. One could look at Concepts Lite as three features:

  1. A superior alternative to enable_if (or overload hiding).
  2. The subsumption relation that enables the additional control of partial ordering in the overload resolution process.
  3. A convenient tool for building compile-time predicates that check for valid types and expressions.

In this post I will only focus on the first feature, and try to answer the question, “what do we need Concepts Lite for, given that we already have std::enable_if (and SFINAE)?”

How to avoid bugs using modern C++

By Andrey Karpov | Sep 15, 2016 05:11 AM | Tags: c++17 c++11 basics

One of the main problems with C++ is having a huge number of constructions whose behavior is undefined, or is just unexpected for a programmer. Let's see which techniques in modern C++ help writing not only simple and clear code, but make it safer and more reliable.

How to avoid bugs using modern C++

by Pavel Belikov

From the article:

Of course, there are some flaws in the range-based for: it doesn't allow flexible management of the loop, and if there is more complex work with indexes required, then for won't be of much help to us. But such situations should be examined separately. We have quite a simple situation: we have to move along the items in the reverse order. However, at this stage, there are already difficulties. There are no additional classes in the standard library for range-based for. Let's see how it could be implemented.

Exploring std::string--Shahar Mike

By Adrien Hamelin | Sep 13, 2016 12:40 PM | Tags: intermediate community

The insides are revealed:

Exploring std::string

by Shahar Mike

From the article:

Every C++ developer knows that std::string represents a sequence of characters in memory. It manages its own memory, and is very intuitive to use. Today we’ll explore std::string as defined by the C++ Standard, and also by looking at 4 major implementations.