Declaring `vector` in the `std` namespace is **undefined behavior**. So, your code might work, but it also might not, and the compiler is under no obligation to tell you when your attempt won&#39;t work. That&#39;s a gamble, and I don&#39;t know that avoiding the inclusion of a standard C++ header is worth that.

See the following comp.std.c++.moderated discussion:

&lt;a href=&quot;http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/5c12858c636ab833/6cd57590509ad6b4&quot;&gt;forward declaring std::vector. Works, but is it legal and standard compliant?&lt;/a&gt;

I don&#39;t think so because the compiler would have no way of knowing how much space to allocate for the `container_` object. At best you could do:

    std::vector&lt;int&gt; *container_;

and new it in the constructor, since the compiler knows the size of a pointer.

Apart from what the others said, you may find it useful to know that there is a sanctioned way of forward-declaring iostreams and some related templates: The header `&lt;iosfwd&gt;`. It would be useful if the standard had more such headers.



Relatively new to rails and trying to model a very simple family &quot;tree&quot; with a single Person model that has a name, gender, father_id and mother_id (2 parents). Below is basically what I want to do, but obviously I can&#39;t repeat the :children in a has_many (the first gets overwritten).

    class Person &lt; ActiveRecord::Base
      belongs_to :father, :class_name =&gt; &#39;Person&#39;
      belongs_to :mother, :class_name =&gt; &#39;Person&#39;
      has_many :children, :class_name =&gt; &#39;Person&#39;, :foreign_key =&gt; &#39;mother_id&#39;
      has_many :children, :class_name =&gt; &#39;Person&#39;, :foreign_key =&gt; &#39;father_id&#39;
    end

Is there a simple way to use has_many with 2 foreign keys, or maybe change the foreign key based on the object&#39;s gender? Or is there another/better way altogether?

Thanks!

Rails Model has_many with multiple foreign_keys

What is the shortest chunk of C++ you can come up with to safely clean up a `std::vector` or `std::list` of pointers?  (assuming you have to call delete on the pointers?)

    list&lt;Foo*&gt; foo_list;

I&#39;d rather not use Boost or wrap my pointers with smart pointers. 





Cleaning up an STL list/vector of pointers

Is it possible to forward declare an standard container in a header file?  For example, take the following code:

    #include &lt;vector&gt;
    
    class Foo
    {
    private:
        std::vector&lt;int&gt; container_;
        ...
    };

I want to be able to do something like this:

    namespace std
    {
        template &lt;typename T&gt; class vector;
    }
    
    class Foo
    {
    private:
        std::vector&lt;int&gt; container_;
        ...
    };

Can this be done?

Forward declare a standard container?

Is it possible to forward declare an standard container in a header file? For example, take the following code:
<pre><code class="hljs language-arduino">#include &#x3C;vector>

class Foo
{
private:
 std::vector&#x3C;int> container_;
 ...
};
</code></pre>
I want to be able to do something like this:
<pre><code class="hljs language-arduino">namespace std
{
 template &#x3C;typename T> class vector;
}

class Foo
{
private:
 std::vector&#x3C;int> container_;
 ...
};
</code></pre>
Can this be done?

I just ran across the following error (and found the solution online, but it&#39;s not present in Stack Overflow):

&gt; (.gnu.linkonce.[stuff]): undefined
&gt; reference to [method] [object
&gt; file]:(.gnu.linkonce.[stuff]):
&gt; undefined reference to `typeinfo for
&gt; [classname]&#39;

Why might one get one of these &quot;undefined reference to typeinfo&quot; linker errors?

(Bonus points if you can explain what&#39;s going on behind the scenes.)

g++ undefined reference to typeinfo

I keep seeing people say that exceptions are slow, but I never see any proof. So, instead of asking if they are, I will ask how do exceptions work behind the scenes, so I can make decisions of when to use them and whether they are slow.

From what I know, exceptions are the same as doing a return bunch of times, except that it also checks after each return whether it needs to do another one or to stop. How does it check when to stop returning? I guess there is a second stack that holds the type of the exception and a stack location, it then does returns until it gets there. I am also guessing that the only time this second stack is touched is on a throw and on each try/catch. AFAICT implementing a similar behaviour with return codes would take the same amount of time. But this is all just a guess, so I want to know what really happens.

How do exceptions really work?

How do exceptions work (behind the scenes) in c++

I was thinking along the lines of using `typeid()` but I don&#39;t know how to ask if that type is a subclass of another class (which, by the way, is abstract)


How do I check if an object&#39;s type is a particular subclass in C++?

As a [C#][1] developer I&#39;m used to running through constructors:

&lt;!-- language-all: c# --&gt;

    class Test {
        public Test() {
            DoSomething();
        }

        public Test(int count) : this() {
            DoSomethingWithCount(count);
        }

        public Test(int count, string name) : this(count) {
            DoSomethingWithName(name);
        }
    }

Is there a way to do this in C++?

I tried calling the Class name and using the &#39;this&#39; keyword, but both fail.

  [1]: http://en.wikipedia.org/wiki/C_Sharp_%28programming_language%29



Can I call a constructor from another constructor (do constructor chaining) in C++?

Is it better to have all the private members, then all the protected ones, then all the public ones?  Or the reverse?  Or should there be multiple private, protected and public labels so that the operations can be kept separate from the constructors and so on?  What issues should I take into account when making this decision?

How should I order the members of a C++ class?

I checked out a copy of a C++ application from SourceForge (HoboCopy, if you&#39;re curious) and tried to compile it.

Visual Studio tells me that it can&#39;t find a particular header file. I found the file in the source tree, but where do I need to put it, so that it will be found when compiling?  

Are there special directories?


Where does Visual Studio look for C++ header files?

On a Unix system, where does gcc look for header files?

I spent a little time this morning looking for some system header files, so I thought this would be good information to have here.

Where does gcc look for C and C++ header files?

What are the C++ coding and file organization guidelines you suggest for people who have to deal with lots of interdependent classes spread over several source and header files?

I have this situation in my project and solving class definition related errors crossing over several header files has become quite a headache.

C++ class header files organization

In C# how do I still show the headers of a gridview, even with the data source is empty. 

I am not auto generating the columns as they are all predefined. 

Currently what I am doing is the following.

Get a DataTable back from a stored procedure, then set the DataSource of the gridview, and then call DataBind().

This works fine when I have data, but when no rows are returned then I just get a blank spot where the grid should be.

**Edit: Thanks all for the .NET 4+ property. I asked this back in the .NET 3.5 days. This is much easier now. :)**




GridView - Show headers on empty data source

Someone asserted on SO today that you should never use anonymous namespaces in header files.  Normally this is correct, but I seem to remember once someone told me that one of the standard libraries uses anonymous namespaces in header files to perform some sort of initialization.

Am I remembering correctly?  Can someone fill in the details?

Uses for anonymous namespaces in header files

I want to convert a `std::string` to lowercase. I am aware of the function `tolower()`. However, in the past I have had issues with this function and it is hardly ideal anyway as using it with a `std::string` would require iterating over each character.

Is there an alternative which works 100% of the time?


How to convert an instance of std::string to lower case

I need to iterate over `std::queue`.
www.cplusplus.com says:

&gt; By default, if no container class is specified for a particular queue class, the standard container class template deque is used.

So can I somehow get to the queue&#39;s underlying deque and iterate over it?

std::queue iteration

I like vectors a lot. They&#39;re nifty and fast. But I know this thing called a valarray exists. Why would I use a valarray instead of a vector? I know valarrays have some syntactic sugar, but other than that, when are they useful?

C++ valarray vs. vector

I have to format [`std::string`](http://en.cppreference.com/w/cpp/string/basic_string) with [`sprintf`](http://en.cppreference.com/w/cpp/io/c/fprintf) and send it into file stream. How can I do this?

std::string formatting like sprintf

I need to go through a set and remove elements that meet a predefined criteria.

This is the test code I wrote:

    #include &lt;set&gt;
    #include &lt;algorithm&gt;

    void printElement(int value) {
        std::cout &lt;&lt; value &lt;&lt; &quot; &quot;;
    }

    int main() {
        int initNum[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        std::set&lt;int&gt; numbers(initNum, initNum + 10);
        // print &#39;0 1 2 3 4 5 6 7 8 9&#39;
        std::for_each(numbers.begin(), numbers.end(), printElement);

        std::set&lt;int&gt;::iterator it = numbers.begin();

        // iterate through the set and erase all even numbers
        for (; it != numbers.end(); ++it) {
            int n = *it;
            if (n % 2 == 0) {
                // wouldn&#39;t invalidate the iterator?
                numbers.erase(it);
            }
        }

        // print &#39;1 3 5 7 9&#39;
        std::for_each(numbers.begin(), numbers.end(), printElement);

        return 0;
    }

At first, I thought that erasing an element from the set while iterating through it would invalidate the iterator, and the increment at the for loop would have undefined behavior. Even though, I executed this test code and all went well, and I can&#39;t explain why.

**My question:**
Is this the defined behavior for std sets or is this implementation specific? I am using gcc 4.3.3 on ubuntu 10.04 (32-bit version), by the way.

Thanks!

**Proposed solution:**

Is this a correct way to iterate and erase elements from the set?

    while(it != numbers.end()) {
        int n = *it;
        if (n % 2 == 0) {
            // post-increment operator returns a copy, then increment
            numbers.erase(it++);
        } else {
            // pre-increment operator increments, then return
            ++it;
        }
    }

**Edit: PREFERED SOLUTION**

I came around a solution that seems more elegant to me, even though it does exactly the same.

    while(it != numbers.end()) {
        // copy the current iterator then increment it
        std::set&lt;int&gt;::iterator current = it++;
        int n = *current;
        if (n % 2 == 0) {
            // don&#39;t invalidate iterator it, because it is already
            // pointing to the next element
            numbers.erase(current);
        }
    }

If there are several test conditions inside the while, each one of them must increment the iterator. I like this code better because the iterator is incremented **only in one place**, making the code less error-prone and more readable.

Content Type	Original Author	Original Content on Stackoverflow
Question	Rob	View Question on Stackoverflow
Solution 1 - C++	Rob Kennedy	View Answer on Stackoverflow
Solution 2 - C++	Evan Teran	View Answer on Stackoverflow
Solution 3 - C++	Sebastian Mach	View Answer on Stackoverflow

Forward declare a standard container?

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

Solution 3 - C++

Cleaning up an STL list/vector of pointers

Rails Model has_many with multiple foreign_keys

Attributions