Quick Q: Return a local object rvalue reference, right or wrong?

By Adrien Hamelin | Mar 11, 2015 01:54 PM | Tags: c++11 basics

Quick A: Wrong!

Recently on SO:

Return a local object rvalue reference,right or wrong?

I see once return a local object,the compiler will take the return value optimization.(RVO,NRVO).

The part of the Standard blessing the RVO goes on to say that if the conditions for the RVO are met, but compilers choose not to perform copy elision, the object being returned must be treated as an rvalue.

So we just write code like this:

Widget makeWidget()
{
Widget w;
…
return w;//never use std::move(w);
}

I never see somebody write code like this:

Widget&& makeWidget()
{
Widget w;
…
return std::move(w);
}

I know that returns an lvalue reference of local object is always wrong. So, returns an rvalue reference of local object is also wrong?

Quick Q: Calling C++ Application Code from C library?

By Adrien Hamelin | Mar 11, 2015 01:50 PM | Tags: intermediate

Quick A: You need to create a stub.

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Calling C++ Application Code from C library?

Folks, Assuming I have a c++ application / library running which implements say

/* Alloc API */
void* my_alloc(int size) {
    return malloc(sizeof(size));
}

This is not under "extern c".

I have a C dynamic library from which I need to call my_alloc, can I directly call that API?

Like,

int test_my_alloc (int size) {
    int *x;

    x = (int*)my_alloc(size);
    if (x == NULL) {
        return 0;
    } else {
        return 1;
    }
}

Secrets of the Conditional (ternary) Operator -- Alex Kulikov

By Blog Staff | Mar 7, 2015 06:06 PM | Tags: basics

Hot off kukuruku:

Secrets of the Conditional (ternary) Operator

by Alex Kulikov

(Note: Translation of the original Russian-language article here.)

From the article:

Every self-respecting C/C++ programmer knows what the ternary operator is, and most everyone used it at least once in their programs. But do you know all the secrets of the ternary operator? What potential dangers are associated with its use and what features, seemingly not related to its direct purpose, it has? This article gives you the opportunity to test your knowledge and maybe learn something new.

Let's start with a small test...

Quick Q: Why can a constexpr function return a non-constant?

By Adrien Hamelin | Mar 6, 2015 11:21 AM | Tags: c++11 basics

Quick A: Because constexpr means can be evaluated at compile time, not that it will be. And it won’t be if the inputs aren’t compile-time constants.

Recently on SO:

I am confused about a constexpr function?

In C++ Primer, Fifth Edition, §6.5.2:

A constexpr function is defined like any other function but must meet certain restrictions: The return type and the type of each parameter in must be a literal type (§2.4.4, p. 66), and the function body must contain exactly one return statement

but another sentence in this chapter (page 239):

A constexpr function is permitted to return a value that is not a constant

// scale(arg) is a constant expression if arg is a constant expression
constexpr size_t scale(size_t cnt) { return new_sz() * cnt; }

Is it a contradictory summary? I am confused about it.
The return type of scale is literal type?
update: what's the difference between literal type and constant ?

Quick Q: How to write variadic template constructor?

By Adrien Hamelin | Mar 5, 2015 10:44 AM | Tags: c++11 basics

Quick A: use an std::initializer_list.

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Writting variadic template constructor

Recently I asked this question but now I would like to expand it. I wrote the following class:

template <class T>
class X{
public:
    vector<T> v;
    template <class T>
    X(T n) {
        v.push_back(n);
    }
    template <class T, class... T2>
    X(T n, T2... rest) {
        v.push_back(n);
        X(rest...);
    }
};

When creating and object using

X<int> obj(1, 2, 3);  // obj.v containts only 1

Vector only contains the first value, but not others. I've checked and saw that constructor is called 3 times, so I'm probably creating temp objects and filling their vectors with the rest of the arguments. How do I solve this problem?

Quick Q:How to modify a tuple in a vector of tuples c++?

By Adrien Hamelin | Mar 5, 2015 10:40 AM | Tags: c++11 basics

Quick A: Capture the tuple by reference.

Recently on SO:

Modifying a tuple in a vector of tuples c++

I have a vector of tuples vector<tuple<int,int>> vector; and I want to modify one of the tuples it contains.

for (std::tuple<int, int> tup : std::vector)
{
    if (get<0>(tup) == k)
    {
        /* change get<1>(tup) to a new value
         * and have that change shown in the vector
         */
    }
}

I am unsure how to change the value of the tuple and have the change be reflected in the vector. I have tried using

get<1>(tup) = v;

but that doesn't change the value of the tuple that is in the vector. How can I do this? Thanks.

How to write a standard-like algorithm -- Indi

By Marcin Hoppe | Mar 5, 2015 01:56 AM | Tags: basics

Explicit C++ has posted a nice tutorial on how to implement an algorithm in C++.

How to write a standard-like algorithm

by Indi

from the article:

Writing a standard-like algorithm should be one of the key parts of a modern C++ beginner’s course outline. This post will be a whirlwind guide through the steps toward creating a standard-like algorithm. The focus is not on the algorithm itself, but on the process of creating one.

Iterators++, Part 3 -- Eric Niebler

By Felix Petriconi | Mar 4, 2015 12:17 AM | Tags: library advanced

Eric Niebler concludes his series about proxy iterators with:

Iterators++, Part 3

by Eric Niebler

From the article:

This is the forth and final post in a series about proxy iterators, the limitations of the existing STL iterator concept hierarchy, and what could be done about it. The first three posts describe the problems of proxy iterators, the way to swap and move their elements, and how to rigorously define what an Iterator is.

This time around I’ll be focusing on the final problem: how to properly constrain the higher-order algorithms so that they work with proxy iterators.

A conditional copy constructor -- Andrzej Krzemieński

By Felix Petriconi | Mar 3, 2015 01:08 AM | Tags: c++11 advanced

Andrzej writes in his recent blog about an issue library writers have to take care of.

A conditional copy constructor

by Andrzej Krzemieński

From the article:

In this post we will try to define a ‘wrapper’ class template that does or does not have a copy constructor depending on whether the wrapped class has it. This will be a good opportunity to explore in depth a couple of advanced C++ features. Note that this is a rather advanced topic and, unless you are writing or maintaining a generic library, you will probably never need this knowledge.

Vector Hosted Lists--Thomas Young

By Adrien Hamelin | Mar 2, 2015 12:01 AM | Tags: performance intermediate

Want perfomance and speed? Vectors are the solution:

Vector Hosted Lists

by Thomas Young

From the article:

Vectors are great when adding or removing elements at the end of a sequence, but not so hot when deleting elements at arbitrary positions.

If that's a requirement, you might find yourself reaching for a pointer-based list.

Not so fast!

Memory locality is important, contiguous buffers are a really good thing, and a standard vector will often out-perform pointer-based lists even where you perform non-contiguous, list-style modifications such as arbitrary element deletion.

And we can 'host' a list within a vector to get the advantages of a contiguous buffer at the same time as 0(1) complexity for these kinds of manipulations...