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Quick Q: 'Constexpr' vs 'extern const'. Which has priority?

By Adrien Hamelin | Aug 11, 2016 12:52 PM | Tags: intermediate c++11

Quick A: If you need a compile time constant, you cannot use extern.

Recently on SO:

'Constexpr' vs 'extern const'. Which has priority?

Using extern const in the header file only tells the compiler that the variable exists and that it is not modifiable. It doesn't tell the compiler its value which means it's not a compile-time constant anymore. If it's not a compile-time constant then it can't be used for e.g. case or as an array size.

As said by M.M in the comment, either use

const int MAX_NUMBER_OF_ROWS= 99;

or

constexpr int MAX_NUMBER_OF_ROWS= 99;

directly in the header file and it will be a compile-time constant in all translation units that include the header file.

Overload 134 is now available

By Adrien Hamelin | Aug 11, 2016 12:48 PM | Tags: intermediate community

ACCU’s Overload journal of August 2016 is out. It contains the following C++ related articles.

Overload 134

From the journal:

Some Big-Os are Bigger Than Others
Big-O notation is often used to compare algorithms. Sergey Ignatchenko reminds us that asymptotic limits might not be generally applicable. by Sergey Ignatchenko

Kill the Clones
Problems in code can hide in surprising places. Adam Tornhill demonstrates how to detect software clones and uncover hidden dependencies. by Adam Tornhill

Implementing SNAAAKE
Almost everyone knows the game Snake! Thaddaeus Frogley shares a diary of how his implementation grew over time. by Thaddaeus Frogley

C++ Antipatterns
Certain mistakes crop up frequently in C++. Jonathan Wakely offers some pro-tips to help you avoid common errors. by Jonathan Wakely

Testing Propositions
Is testing propositions more important than having examples as exemplars? Russel Winder considers this hypothesis. by Russel Winder

Quick Q: unordered_map element being deleted

By Adrien Hamelin | Aug 10, 2016 01:13 PM | Tags: intermediate

Quick A: Once a value is deleted, iterators pointing to it become invalid.

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unordered_map element being deleted

After you call my_umap.erase(...), your iterator becomes invalid:

cppreference.com says:

References and iterators to the erased elements are invalidated. Other iterators and references are not invalidated.
This means that once the item is erased, the iterators that pointed to it are no longer valid.

You've got a couple of options:

1. Use the iterator to erase, and use the return value of erase()

Since C++11, erasing by iterator will return an iterator pointing to the next item in the map. So you can use this to keep your iterator valid:

auto it = my_umap.begin();

while (it != my_umap.end()) {

    MyStruct& myStruct = it->second;
    const bool deleteEntry = myStruct.ts.IsElapsed(std::chrono::seconds(5));

    if(deleteEntry){
        assert(my_umap.size() >= 1);
        it = my_umap.erase(it);  // <-- Return value should be a valid iterator.
    }
    else{
        ++it;  // Have to manually increment.
    }
}

2. Store your iterators in a list object and erase after iteration.

Alternatively, you can store delete candidates in a list object (e.g. vector and delete them after your initial iteration:

std::vector<MapType::iterator> deleteCandidates;

for(auto it = my_umap.begin(); it != my_umap.end(); ++it){

    MyStruct& myStruct = it->second;
    const bool deleteEntry = myStruct.ts.IsElapsed(std::chrono::seconds(5));

    if(deleteEntry)
        deleteCandidates.push_back(it);
}

for (auto it : deleteCandidates) {
    my_umap.erase(it);
}

As for why your assertion is failing, you're probably encountering undefined behaviour by accessing an invalid iterator, making your for loop believe that the map is still not empty (because invalidIterator != my_umap.end()).

CppCon 2015 Reflection Techniques in C++--Paul Fultz II

By Adrien Hamelin | Aug 10, 2016 01:11 PM | Tags: community boost

Have you registered for CppCon 2016 in September? Don’t delay – Registration is open now.

While we wait for this year’s event, we’re featuring videos of some of the 100+ talks from CppCon 2015 for you to enjoy. Here is today’s feature:

Reflection Techniques in C++

by Paul Fultz II

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

Reflection is a very powerful and useful feature used in many languages to achieve things like serialization, object-relationship mapping, and general data-driven development. C++ doesn't support reflection natively in the language yet. There are proposals to add compile-time reflection to the language, but C++ has survived all this time without direct support for reflection.

This talk will discuss the various techniques the can be used to achieve reflection including boost fusion, the visitor patter, and do-it-yourself with some macros and metaprogramming. This talk will discuss how these techniques can be used to implement serialization or object-relational mapping.

CppCon 2015 constexpr: Applications--Scott Schurr

By Adrien Hamelin | Aug 8, 2016 02:03 PM | Tags: performance c++14

Have you registered for CppCon 2016 in September? Don’t delay – Registration is open now.

While we wait for this year’s event, we’re featuring videos of some of the 100+ talks from CppCon 2015 for you to enjoy. Here is today’s feature:

constexpr: Applications

by Scott Schurr

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

I'm excited about constexpr. It's probably my favorite C++11 feature and it's gotten even better with C++14. But when I talk to other developers about constexpr they seem puzzled. What sorts of useful computations can the compiler possibly do before runtime?

I'd like to take this session to explore some of the capabilities that constexpr brings to the table. We'll look at compile-time parsing, floating-point computations, and containers. We'll also talk about motivations for computing these at compile time.

This session builds on the "constexpr: Introduction" talk.

Quick Q: Why doesn't c++ have a specified order for evaluating function arguments?

By Adrien Hamelin | Aug 5, 2016 02:00 PM | Tags: performance intermediate

Quick A: In order to get the best performance.

recently on SO:

Why doesn't c++ have a specified order for evaluating function arguments?

C++ leaves as much as practical up to the implementation of the compiler, especially where optimization opportunities might occur. Before fixing something like this, existing compiler writers are consulted to see if there is a cost.

Some order of evaluation guarantees surrounding overloaded operators and complete-argument rules where added in C++17. But it remains that which argument goes first is left unspecified. In C++17, it is now specified that the expression giving what to call (the code on the left of the ( of the function call) goes before the arguments, and whichever argument is evaluated first is evaluated fully before the next one is started, and in the case of an object method the value of the object is evaluated before the arguments to the method are. (There may be some minor errors in this description: ask a narrow question about it for a more vetted answer).

These where vetted and determined to not cause significant prolems for existing compilers. Reordering of arguments was considered, and discarded, presumably for good reasons. Or even poor reasons, like existing compilers have a default order and it could break (possibly non-compliant) code on that compiler to force them all to do it in one global order.

In short, C++ leaves lots of freedom to compiler writers. This has let compiler writers find optimization opportunities in strange crannies. The kind of optimizations available today are way beyond what the original writers of C++, let alone C++, may have suspected where practical or be considered reasonable.

Quick Q:Why does std::set seem to force the use of a const_iterator?

By Adrien Hamelin | Aug 5, 2016 01:57 PM | Tags: basics

Quick A: A set does not allow the modification of its keys.

Recently on SO:

Why does std::set seem to force the use of a const_iterator?

A set is like a map with no values, only keys. Since those keys are used for a tree that accelerates operations on the set, they cannot change. Thus all elements must be const to keep the constraints of the underlying tree from being broken.

CppCon 2015 C++ Atomics: The Sad Story of memory_order_consume--Paul E. McKenney

By Adrien Hamelin | Aug 5, 2016 01:52 PM | Tags: performance experimental

Have you registered for CppCon 2016 in September? Don’t delay – Registration is open now.

While we wait for this year’s event, we’re featuring videos of some of the 100+ talks from CppCon 2015 for you to enjoy. Here is today’s feature:

C++ Atomics: The Sad Story of memory_order_consume: A Happy Ending At Last?

by Paul E. McKenney

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

One of the big advantages of C++ atomics is that developers can now let the compiler know about the intent behind their multi-threaded synchronization mechanisms. At least they can tell the compiler as long as the synchronization mechanism in question is not RCU. You see, all production compilers promote RCU's memory_order_consume to memory_order_acquire. Although this promotion does ensure correctness, it also ensures the additional overhead of memory-barrier instructions on weakly ordered systems and of needlessly suppressed compiler optimizations on all systems.

All previous attempts to resolve this issue have foundered on either standard-committee reluctance to eviscerate the standard for a special case, compiler-writer reluctance to eviscerate their compilers for a special case, and kernel-developers reluctance to eviscerate their source base for late-to-the-party compiler support.

But now there is a glimmer of hope in the guise of a small set of small patches to the Linux kernel that eliminate the most challenging use cases. Will this hope be realized? Come to this talk to here the story, which by September will hopefully have a happy ending!