C copy set to vector

Question

I need to copy std  set to std  vector   std  set  lt double gt  input  input insert 5   input insert 6    std  vector  lt double gt  output  std  copy input begin    input end    output begin       Error  Vector iterator not dereferencable   Where is the problem

User · Answer

You haven t reserved enough space in your vector object to hold the contents of your set   std  vector lt double gt  output input size     std  copy input begin    input end    output begin

User · Answer

You need to use a back inserter   std  copy input begin    input end    std  back inserter output      std  copy doesn t add elements to the container into which you are inserting   it can t  it only has an iterator into the container   Because of this  if you pass an output iterator directly to std  copy  you must make sure it points to a range that is at least large enough to hold the input range   std  back inserter creates an output iterator that calls push back on a container for each element  so each element is inserted into the container   Alternatively  you could have created a sufficient number of elements in the std  vector to hold the range being copied   std  vector lt double gt  output input size     std  copy input begin    input end    output begin       Or  you could use the std  vector range constructor   std  vector lt double gt  output input begin    input end

User · Answer

std  copy cannot be used to insert into an empty container  To do that  you need to use an insert iterator like so   std  set lt double gt  input  input insert 5   input insert 6    std  vector lt double gt  output  std  copy input begin    input end    inserter output  output begin

User · Answer

here s another alternative using vector  assign   theVector assign theSet begin    theSet end

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set lt T gt  s         vector lt T gt  v  v assign s begin    s end

User · Answer

Just use the constructor for the vector that takes iterators   std  set lt T gt  s          std  vector v  s begin    s end        Assumes you just want the content of s in v  and there s nothing in v prior to copying the data to it

User · Answer

I think the most efficient way is to preallocate and then emplace elements   template  lt typename T gt  std  vector lt T gt  VectorFromSet const std  set lt T gt  amp  from        std  vector lt T gt  to      to reserve from size          for  auto const amp  value   from          to emplace back value        return to      That way we will only invoke copy constructor for every element as opposed to calling default constructor first and then copy assignment operator for other solutions listed above  More clarifications below    back inserter may be used but it will invoke push back   on the vector  https   en cppreference com w cpp iterator back insert iterator   emplace back   is more efficient because it avoids creating a temporary when using push back    It is not a problem with trivially constructed types but will be a performance implication for non-trivially constructed types  e g  std  string   We need to avoid constructing a vector with the size argument which causes all elements default constructed  for nothing   Like with solution using std  copy    for instance  And  finally  vector  assign   method or the constructor taking the iterator range are not good options because they will invoke std  distance    to know number of elements  on set iterators  This will cause unwanted additional iteration through the all set elements because the set is Binary Search Tree data structure and it does not implement random access iterators    Hope that helps

[c++] C++, copy set to vector

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