Rather than *decrementing* `rbegin`, you should increment it, as shown here:&lt;sup&gt;1&lt;/sup&gt;

    double penultimate = *++foo.rbegin();

as `rbegin()` returns a *reverse iterator*, so `++` is the operator to move backwards in the container. Note that I&#39;ve also dropped the superfluous parentheses: that&#39;s not to everyone&#39;s taste.

Currently the behaviour of your program is *undefined* since you are actually moving to `end()`, and you are not allowed to dereference that. The arbitrary nature of the output is a manifestation of that undefined behaviour.

&lt;hr&gt;

&lt;sup&gt;1&lt;/sup&gt;Do retain the minimum size check that you currently have.

The clearest way, in my mind, is to use the construct designed for this purpose (C++11):

    double penultimate = *std::prev(foo.end(), 2)

I would just do `*--(--foo.end())`; no need for reverse iterators. It&#39;s less confusing too.

I was wondering there is a way I can align &lt;Text&gt; vertically in React Native. I&#39;m trying to position &lt;Text&gt; on the bottom but I don&#39;t think I can find a good way to do it.

If anyone knows how to solve this issue, please let me know.

Text vertical align in react native (using nativebase)

I&#39;m moving a project from `project.json` to the new-style csproj format, and it includes a class derived from [`DbParameterCollection`][1]. In my real project I&#39;m using multi-targeting, but for the purposes of this question we only need to care about `net45`.

The compiler is telling me that I have to override three properties that I didn&#39;t have to before:

- [`IsFixedSize`][2]
- [`IsReadOnly`][3]
- [`IsSynchronized`][4]

If you follow those documentation links (which are for .NET 4.5) you&#39;ll see that all the properties are *virtual* - not abstract. If I build the code just by calling `csc`, all is well... it&#39;s only when using the .NET Core SDK that I run into the issue.

Here&#39;s sample code to reproduce the problem:

Project file:

    &lt;Project Sdk=&quot;Microsoft.NET.Sdk&quot;&gt;    
      &lt;PropertyGroup&gt;
        &lt;TargetFramework&gt;net45&lt;/TargetFramework&gt;
      &lt;/PropertyGroup&gt;    
    &lt;/Project&gt;

C# code:

    using System;
    using System.Collections;
    using System.Data.Common;
    
    public class DummyParameterCollection : DbParameterCollection
    {
        public override int Count =&gt; 0;
        public override object SyncRoot =&gt; null;
        public override void Remove(object value) {}
        public override void RemoveAt(int index) {}
        public override void RemoveAt(string parameterName) {}
        public override int Add(object value) =&gt; 0;
        public override void Insert(int index, object value) {}
        public override void AddRange(Array values) {}
        public override void Clear() {}
        public override bool Contains(object value) =&gt; false;
        public override bool Contains(string value) =&gt; false;
        public override void CopyTo(Array array, int index) {}
        public override int IndexOf(object value) =&gt; -1;
        public override int IndexOf(string parameterName) =&gt; -1;
        protected override DbParameter GetParameter(int index) =&gt; null;
        protected override DbParameter GetParameter(string parameterName) =&gt; null;
        protected override void SetParameter(int index, DbParameter value) {}
        protected override void SetParameter(string parameterName, DbParameter value) {}
        public override IEnumerator GetEnumerator() =&gt; null;
    }

Errors:

&gt; DummyParameterCollection.cs(5,14): error CS0534: &#39;DummyParameterCollection&#39; does not implement inherited abstract member &#39;DbParameterCollection.IsSynchronized.get&#39; [c:\Users\skeet\Test\ParameterCollection\ParameterCollection.csproj]&lt;br&gt;
&gt; DummyParameterCollection.cs(5,14): error CS0534: &#39;DummyParameterCollection&#39; does not implement inherited abstract member &#39;DbParameterCollection.IsFixedSize.get&#39; [c:\Users\skeet\Test\ParameterCollection\ParameterCollection.csproj]&lt;br&gt;
&gt; DummyParameterCollection.cs(5,14): error CS0534: &#39;DummyParameterCollection&#39; does not implement inherited abstract member &#39;DbParameterCollection.IsReadOnly.get&#39; [c:\Users\skeet\Test\ParameterCollection\ParameterCollection.csproj]


I believe I know the immediate cause of the problem, but not the reasons *why* it&#39;s like this, or the best workaround.

It looks like the .NET Core SDK (and VS2017 when it&#39;s loaded this project) uses the reference assemblies. If I open `C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Data.dll` in Reflector, *that* shows the properties as being abstract as well. Whereas if I open `c:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Data.dll`, that shows the properties as being virtual.

I can work around this by overriding the properties and just returning `false` from all of them - but is that the best way of handling this situation? Beyond that, is there any good reason why the reference assemblies don&#39;t match the real assemblies (and documentation) in this case? I would expect the reference assemblies to be autogenerated, so it&#39;s odd for *some* things to be incorrect like this...


  [1]: https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbparametercollection?view=netframework-4.5
  [2]: https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbparametercollection.isfixedsize?view=netframework-4.5#System_Data_Common_DbParameterCollection_IsFixedSize
  [3]: https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbparametercollection.isreadonly?view=netframework-4.5#System_Data_Common_DbParameterCollection_IsReadOnly
  [4]: https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbparametercollection.issynchronized?view=netframework-4.5#System_Data_Common_DbParameterCollection_IsSynchronized


Why are three properties in DbParameterCollection abstract in reference assemblies but virtual otherwise?

I have a `std::list&lt;double&gt; foo;`

I&#39;m using 

    if (foo.size() &gt;= 2){
    	double penultimate = *(--foo.rbegin());
    }

but this always gives me an arbitrary value of `penultimate`.

What am I doing wrong?


How can I get the penultimate element in a list?

I have a <code>std::list&#x3C;double> foo;</code>
I'm using
<pre><code class="hljs language-ini">if (foo.size() >= 2){
	double penultimate = *(--foo.rbegin());
}
</code></pre>
but this always gives me an arbitrary value of <code>penultimate</code>.
What am I doing wrong?

I am trying to build a solution on Visual Studio Community 2017, but I keep getting the error &quot;**Cannot open include file: &#39;stdio.h&#39;** &quot;.

I&#39;ve read through several similar questions, but I still can&#39;t fix this problem. It looks like the **stdio.h** file is called in the **stdafx.h** file. Below are more details. Any suggestions?

**System details:**

 * Windows 10
 * Visual Studio Community 2017 v.15.2 (26430.6)
-- Installed Desktop Development with C++  ([Screenshot: Installation list][1])

----------

**Step 1:** I wrote the famous [Hello, World!][2] program in C++.

    #include &quot;stdafx.h&quot;
    #include &lt;iostream&gt;

    using namespace std;

    int main()
    {
        cout &lt;&lt; &quot;Hello, World!&quot; &lt;&lt; endl;
        return 0;
    }

**Step 2:** I clicked on *Build* &amp;rarr; *Build Solution*.

**Problem:** ***&#39;stdio.h&#39;**: No such file or directory*. Full Error:

    1&gt;------ Build started: Project: HelloWorld, Configuration: Debug Win32 ------
    1&gt;stdafx.cpp
    1&gt;c:\users\dahiana mini\desktop\learncpp\helloworld\helloworld\stdafx.h(10):
        fatal error C1083: Cannot open include file: &#39;stdio.h&#39;: No such file or directory
    1&gt;Done building project &quot;HelloWorld.vcxproj&quot; -- FAILED.
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========


----------

**Troubleshooting details and things I&#39;ve tried:**

 1. *Configuration Properties** &amp;rarr; *VC++ Directories*
`Include Directories  $(VC_IncludePath);$(WindowsSDK_IncludePath);`
 2. [Screenshot: Solution Explorer (files in the project)][3]
 3. Code in **stdafx.cpp** file:

        // stdafx.cpp : source file that includes just the standard includes
        // HelloWorld.pch will be the pre-compiled header
        // stdafx.obj will contain the pre-compiled type information

        #include &quot;stdafx.h&quot;

        // TODO: reference any additional headers you need in STDAFX.H
        // and not in this file

 4. Code in **stdafx.h** file:

        // stdafx.h : include file for standard system include files,
        // or project specific include files that are used frequently, but
        // are changed infrequently

        #pragma once

        #include &quot;targetver.h&quot;
        #include &lt;stdio.h&gt;
        #include &lt;tchar.h&gt;

    NOTE: the `#include` for `&lt;stdio.h&gt;` and `&lt;tchar.h&gt;` both have the red squiggle line underneath, and says &quot;cannot open source file&quot;.

    Tried: I tried removing the last two lines, but then I got more errors.

 5. Tried: Since many suggested that stdafx.h is not required, I tried removing just the first line, `#include &quot;stdafx.h&quot;`. But in order for this to work I had to do a little more. **See the answer below.**

  [1]: https://i.stack.imgur.com/V9coq.png
  [2]: https://en.wikipedia.org/wiki/%22Hello,_World!%22_program
  [3]: https://i.stack.imgur.com/z1Lgi.png








Cannot open include file: &#39;stdio.h&#39; - Visual Studio Community 2017 - C++ Error

One of my most beloved/evil inventions I&#39;ve had the fortune to come across is the [constexpr counter](http://b.atch.se/posts/constexpr-counter/), aka stateful metaprogramming. As mentioned in the post, it seems to be legal under C++14, and I&#39;m wondering has anything changed with C++17?

The following is an implementation largely based on the post

    template &lt;int N&gt;
    struct flag
    {
        friend constexpr int adl_flag(flag&lt;N&gt;);
        constexpr operator int() { return N; }
    };

    template &lt;int N&gt;
    struct write
    {
        friend constexpr int adl_flag(flag&lt;N&gt;) { return N; }
        static constexpr int value = N;
    };

    template &lt;int N, int = adl_flag(flag&lt;N&gt;{})&gt;
    constexpr int read(int, flag&lt;N&gt;, int R = read(0, flag&lt;N + 1&gt;{}))
    {
        return R;
    }

    template &lt;int N&gt;
    constexpr int read(float, flag&lt;N&gt;)
    {
        return N;
    }

    template &lt;int N = 0&gt;
    constexpr int counter(int R = write&lt;read(0, flag&lt;0&gt;{}) + N&gt;::value)
    {
        return R;
    }

And we [use it](https://godbolt.org/g/xAygS2) as

    static_assert(counter() != counter(), &quot;Your compiler is mad at you&quot;); 
    
    template&lt;int = counter()&gt;
    struct S {};
    
    static_assert(!std::is_same_v&lt;S&lt;&gt;, S&lt;&gt;&gt;, &quot;This is ridiculous&quot;);

This by the way, is a direct contradiction to https://stackoverflow.com/questions/6105994/storing-states-in-c-metaprogramming

Is stateful metaprogramming ill-formed (yet)?

I&#39;ve added AddressSanitizer flag as follow:

    set (CMAKE_CXX_FLAGS &quot;${CMAKE_CXX_FLAGS} -fsanitize=address&quot;)

Everything builds and runs fine when using `Unix Makefiles`.

The problem comes when generating the Xcode project, it just doesn&#39;t want to link because it cannot find the ASan library.

I already found two solutions, but decided not to use them because they cannot be automated using just CMake:

 1. Adding `-Wl,-undefined,dynamic_lookup` to the linked flags, so it skips linking to dynamic libraries. 
 2. Link with `libclang_rt.asan_osx_dynamic.dylib` directly.

So what&#39;s the problem with these two solutions?

 - When using solution #1, I have to manually open the target scheme in Xcode and add `DYLD_INSERT_LIBRARIES` environment variable pointing to `libclang_rt.asan_osx_dynamic.dylib`.
 - When using solution #2, the path for the ASan library varies between computers.

Additionally as another solution, I tried enabling Address Sanitizer flag from the Xcode target scheme but interestingly it didn&#39;t detect the issues I added, so I didn&#39;t list this as a solution because it failed my test.

Any help will be much appreciated.

What&#39;s the proper way to enable AddressSanitizer in CMake that works in Xcode

I recently watched a great talk by Herb Sutter about &quot;Leak Free C++...&quot; at CppCon 2016 where he talked about using smart pointers to implement RAII (Resource acquisition is initialization) - Concepts and how they solve most of the memory leaks issues.

Now I was wondering. If I strictly follow RAII rules, which seems to be a good thing, why would that be any different from having a garbage collector in C++? I know that with RAII the programmer is in full control of when the resources are freed again, but is that in any case beneficial to just having a garbage collector? Would it really be less efficient? I even heard that having a garbage collector can be more efficient, as it can free larger chunks of memory at a time instead of freeing small memory pieces all over the code.

RAII vs. Garbage Collector

Here is the code:

    int main()
    {
        using namespace std; 
        int nights = 1001; 
        int * pt = new int; // allocate space for an int
        *pt = 1001;         // store a value there
         
        cout &lt;&lt; &quot;nights value = &quot;;
        cout &lt;&lt; nights &lt;&lt; &quot;: location &quot; &lt;&lt; &amp;nights &lt;&lt; endl;
        cout &lt;&lt; &quot;int &quot;;
        cout &lt;&lt; &quot;value = &quot; &lt;&lt; *pt &lt;&lt; &quot;: location = &quot; &lt;&lt; pt &lt;&lt; endl;
    
        double * pd = new double; // allocate space for a double
        *pd = 10000001.0; // store a double there
    
        cout &lt;&lt; &quot;double &quot;;
        cout &lt;&lt; &quot;value = &quot; &lt;&lt; *pd &lt;&lt; &quot;: location = &quot; &lt;&lt; pd &lt;&lt; endl; 
        cout &lt;&lt; &quot;location of pointer pd: &quot; &lt;&lt; &amp;pd &lt;&lt; endl;
        cout &lt;&lt; &quot;size of pt = &quot; &lt;&lt; sizeof(pt);
        cout &lt;&lt; &quot;: size of *pt = &quot; &lt;&lt; sizeof(*pt) &lt;&lt; endl;
        cout &lt;&lt; &quot;size of pd = &quot; &lt;&lt; sizeof pd;
        cout &lt;&lt; &quot;: size of *pd = &quot; &lt;&lt; sizeof(*pd) &lt;&lt; endl;
    
        return 0;
    }

Now here is the author&#39;s note about the code:


&gt; Another point to note is that typically **new** uses a different block of memory than do the ordinary variable definitions that we have been using. Both the variable **nights** and **pd** have their values stored in a memory region called the **stack**, whereas the memory allocated by the **new** is in a region called the **heap or free store**.

*************************************************************************

***Initial Question:***

Now my concern is this: the variable **pd** was create by the keyword **new**, so it should be stored in the region called **heap** just like the variable **pt**, since they were both created by the keyword **new**.

Am I missing something here? Thank you very much in advance for your inputs.


***Revised Question/Follow-up based on the hold:*** 

&gt; **This question was put on hold by 5 people** because they couldn&#39;t understand what I was asking. I believe that my question has already been answered but for those who are still not sure about what I was initially asking please read along:
&gt; 
&gt; I was unclear about the author&#39;s explanation about where the variables and their values were stored in memory. Up to the author explanation, I had a belief that any memory created dynamically (or should I say during runtime after compiling) by using the keyword **new** gets stored in the **heap** not the **stack**. 

&gt; So, it confused me when he wrote that the variable **pd** has is value
&gt; stored in the **stack**, but again how is that possible if the variable
&gt; was create during &quot;runtime&quot; with the keyword **new**, so it should be
&gt; in the **heap**, not the **stack**.

**Please try to use the code above as the reference and in particular the **variables (nights, pd, and pt**) in your answer so that I can understand it from that code&#39;s perspective.

Did the author make a mistake in explaining stack and heap in C++ or am I misreading something?

[C++20][1] introduced the `std::ssize()` free function as below:

```c++
template &lt;class C&gt;
    constexpr auto ssize(const C&amp; c)
        -&gt; std::common_type_t&lt;std::ptrdiff_t,
                              std::make_signed_t&lt;decltype(c.size())&gt;&gt;;
```

A possible implementation seems using `static_cast`, to convert the return value of the `size()` member function of cl*ass C* into its signed counterpart.

Since the `size()` member function of C always returns non-negative values, why would anyone want to store them in signed variables? In case one really wants to, it is a matter of simple `static_cast`. 

Why is `std::ssize()` introduced in C++20?

  [1]: https://en.wikipedia.org/wiki/C%2B%2B20


Why is std::ssize() introduced in C++20?

    std::tie(a, b) = std::minmax(a, b);


I think this is intuitive code. Clean and understandable. Too bad it doesn&#39;t work as intended, as [`std::minmax`](https://en.cppreference.com/w/cpp/algorithm/minmax) templates for `const&amp;`. If therefore the values are swapped inside the `std::pair&lt;const&amp;, const&amp;&gt;` than one assignement will overwrite the other value:

    auto[a, b] = std::make_pair(7, 5);

    std::tie(a, b) = std::minmax(a, b);

    std::cout &lt;&lt; &quot;a: &quot; &lt;&lt; a &lt;&lt; &quot;, b: &quot; &lt;&lt; b &lt;&lt; &#39;\n&#39;;


&gt; a: 5, b: 5


The expected output here is `a: 5, b: 7`.

----------

I think this is important as implementing transform functions to apply a function onto some ranges requires such statements for intuitive lambdas. For example:

    std::vector&lt;int&gt; v{ 0, 1, 0, 2, 0 };
    std::vector&lt;int&gt; u{ 1, 0, 1, 0, 1 };
    
    perform(v.begin(), v.end(), u.begin(), [](auto&amp; a, auto&amp; b){ 
    	std::tie(a, b) = std::minmax(a, b);    
    }); 

    //v would be == {0, 0, 0, 0, 0}
    //u would be == {1, 1, 1, 2, 1}


----------

 One solution I found was constructing an [`std::tuple`](https://en.cppreference.com/w/cpp/utility/tuple) explicitly without any reference qualifiers over the `std::pair&lt;const&amp;, const&amp;&gt;` to enforce a copy:


    std::tie(a, b) = std::tuple&lt;int, int&gt;(std::minmax(a, b)); 

 But this `&lt;int, int&gt;` redundancy seems rather awful, especially when having said`auto&amp; a, auto&amp; b` before. 


----------


Is there a nice, short way to perform this assignement? Could it be that this is the wrong direction and just saying `if (a &gt;= b) { std::swap(a, b); }` would be the best approach here?


Content Type	Original Author	Original Content on Stackoverflow
Question	Paul Logue	View Question on Stackoverflow
Solution 1 - C++	Bathsheba	View Answer on Stackoverflow
Solution 2 - C++	Gregory Currie	View Answer on Stackoverflow
Solution 3 - C++	user541686	View Answer on Stackoverflow

How can I get the penultimate element in a list?

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

Solution 3 - C++

Why are three properties in DbParameterCollection abstract in reference assemblies but virtual otherwise?

Text vertical align in react native (using nativebase)

Attributions