Asked  7 Months ago    Answers:  5   Viewed   58 times

There is a thread in the comments section in this post about using std::vector::reserve() vs. std::vector::resize().

Here is the original code:

void MyClass::my_method()
{
    my_member.reserve(n_dim);
    for(int k = 0 ; k < n_dim ; k++ )
         my_member[k] = k ;
}

I believe that to write elements in the vector, the correct thing to do is to call std::vector::resize(), not std::vector::reserve().

In fact, the following test code "crashes" in debug builds in VS2010 SP1:

#include <vector>

using namespace std;

int main()
{
    vector<int> v;
    v.reserve(10);
    v[5] = 2;

    return 0;
}

Am I right, or am I wrong? And is VS2010 SP1 right, or is it wrong?

 Answers

54

There are two different methods for a reason:

std::vector::reserve will allocate the memory but will not resize your vector, which will have a logical size the same as it was before.

std::vector::resize will actually modify the size of your vector and will fill any space with objects in their default state. If they are ints, they will all be zero.

After reserve, in your case, you will need a lot of push_backs to write to element 5. If you don't wish to do that then in your case you should use resize.

One thing about reserve: if you then add elements with push_back, until you reach the capacity you have reserved, any existing references, iterators or pointers to data in your vector will remain valid. So if I reserve 1000 and my size is 5, the &vec[4] will remain the same until the vector has 1000 elements. After that, I can call push_back() and it will work, but the stored pointer of &vec[4] earlier may no longer be valid.

Tuesday, June 1, 2021
 
Norgul
answered 7 Months ago
79

This means that vector keeps memory allocated and have to do an in-place construction (=placement new) of objects being pushed into it. Is this a valid assumption?

Yes

Also, does this mean the container is manually invoking the destructor rather than calling delete?

Yes

Are there any other assumptions that I am missing here? Does this mean I can assume that even a custom written new for the object may not be invoked if I chose to write?

Yes. Consider that even in linked lists, the container will not allocate an instance of your type, but rather a templated structure that contains a subobject of the type. For a linked list that will be some complex type containing at least two pointers (both links) and a subobject of your type. The actual type that is allocated is that node, not your type.

Also it makes sense for a list to use a new and delete as we don't need the continuous memory guarantee.

It does, but it does not new/delete objects of your type.

So, is this kind of behavior is what drives how allocators behave?

I don't really understand this part of the question. Allocators are classes that have a set of constraints defined in the standard, that include both the interface (allocate, deallocate...) and semantics (the meaning of == is that memory allocated with one can be deallocated with the other, any other state in the class is irrelevant).

Allocators can be created and passed onto containers for different reasons, including efficiency (if you are only allocating a type of object, then you might be able to implement small block allocators slightly more efficient than malloc --or not, depends on the situation).

Side note on placement new

I have always found interesting that placement new is a term that seems to have two separate meanings. On the one side is the only way of constructing an object in-place. But it seems to also have a complete different meaning: construct this object acquiring memory from a custom allocator.

In fact there is a single meaning of placement new that has nothing to do with constructing in-place. The first is just a case of the second, where the allocator is provided by the implementation (compiler) as defined in 18.4.1.3 and cannot be overloaded. That particular version of the overloaded allocator does absolutely nothing but return the argument (void*) so that the new-expression can pass it into the constructor and construct the object on the memory (not) allocated by the placement new version that was called.

Tuesday, August 17, 2021
 
juananrey
answered 4 Months ago
78

I've found answer for this,

Reference: https://www.mztools.com/articles/2007/MZ2007016.aspx

public string GetProjectTypeGuids(EnvDTE.Project proj)
        {
            string projectTypeGuids = "";
            object service = null;
            Microsoft.VisualStudio.Shell.Interop.IVsSolution solution = null;
            Microsoft.VisualStudio.Shell.Interop.IVsHierarchy hierarchy = null;
            Microsoft.VisualStudio.Shell.Interop.IVsAggregatableProject aggregatableProject = null;
            int result = 0;
            service = GetService(proj.DTE, typeof(Microsoft.VisualStudio.Shell.Interop.IVsSolution));
            solution = (Microsoft.VisualStudio.Shell.Interop.IVsSolution)service;

            result = solution.GetProjectOfUniqueName(proj.UniqueName, out hierarchy);

            if (result == 0)
            {
                aggregatableProject = (Microsoft.VisualStudio.Shell.Interop.IVsAggregatableProject)hierarchy;
                result = aggregatableProject.GetAggregateProjectTypeGuids(out projectTypeGuids);
            }

            return projectTypeGuids;
        }

        public object GetService(object serviceProvider, System.Type type)
        {
            return GetService(serviceProvider, type.GUID);
        }

        public object GetService(object serviceProviderObject, System.Guid guid)
        {
            object service = null;
            Microsoft.VisualStudio.OLE.Interop.IServiceProvider serviceProvider = null;
            IntPtr serviceIntPtr;
            int hr = 0;
            Guid SIDGuid;
            Guid IIDGuid;

            SIDGuid = guid;
            IIDGuid = SIDGuid;
            serviceProvider = (Microsoft.VisualStudio.OLE.Interop.IServiceProvider)serviceProviderObject;
            hr = serviceProvider.QueryService(ref SIDGuid, ref IIDGuid, out serviceIntPtr);

            if (hr != 0)
            {
                System.Runtime.InteropServices.Marshal.ThrowExceptionForHR(hr);
            }
            else if (!serviceIntPtr.Equals(IntPtr.Zero))
            {
                service = System.Runtime.InteropServices.Marshal.GetObjectForIUnknown(serviceIntPtr);
                System.Runtime.InteropServices.Marshal.Release(serviceIntPtr);
            }

            return service;
        }
    }

Its working fine for my requirement.

Thursday, August 19, 2021
 
MByD
answered 4 Months ago
39

Somebody suggested:

Okay, I did a repair install of VS and that added the folders.

does that help?

Wednesday, August 25, 2021
 
Sturla Molden
answered 4 Months ago
72

Make your bool_xxx functions actually functors of a specific kind of type (tag dispatching would be enough). Then override || and && for them such that these operators return a bool_and, or bool_or. Then you can use your bool_ predicates like so:


std::find_if(vect.begin(), vect.end(), bool_x1() || bool_x2() && (bool_x3() || bool_x4() && bool_x5()));

If you're tempted to write a "where" function then you apparently want to do this more than once with a different set of bool_xxx functions. Even if you know that you want a certain type of composition now, you may as well make it as universal as possible. This is how I'd do it.

Edit:

Based on this comment: @Jerry: For example I need to know: id=where(v < 10.0 && v>1.0); and somewhere later I would like to know: id=where(v < fun(v)); you may be better off with boost::lambda:


namespace l = boost::lambda;
std::find_if(vect.begin(), vect.end(), l::_1 < 10.0 && l::_1 > 1.0);
std::find_if(vect.begin(), vect.end(), l::_1 < l::bind(fun, l::_1));

Or, if you hate lambda or aren't allowed to use it...or just want a very slightly nicer syntax (but inability to use functions directly) then just make your own placeholder type and override it to return bool_xxx functors on operators <, >, etc...

Edit2: Here's an untested where that returns a vector of iterators to all objects matching:


template < typename ForwardIter, typename Predicate >
std::vector<ForwardIter> where(ForwardIter beg, ForwardIter end, Predicate pred)
{
  ForwardIter fit = std::find_if(beg,end,pred);
  if (fit == end) return std::vector<ForwardIter>();

  ForwardIter nit = fit; ++nit;
  std::vector<ForwardIter> collection = where(nit,end,pred);
  collection.push_front(fit);
  return collection;
}

It's recursive and could be slow on some implementations but there's one way to do it.

Monday, October 11, 2021
 
AnchovyLegend
answered 2 Months ago
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