How to store objects without copy or move constructor in std::vector?

Question

To improve efficiency of std::vector<T>, it's underlying array needs to be pre-allocated and sometimes re-allocated. That, however, requires the creation and later moving of objects of type T with a copy ctor or move ctor.

The problem that I am having is that T cannot be copied or moved because it contains objects that cannot be copied or moved (such as atomic and mutex). (And, yes, I am implementing a simple thread pool.)

I would like to avoid using pointers because:

1. I do not need a level of indirection and so I do not want one.
2. (Pointers are less efficient and increase complexity. Using pointers increases memory fragmentation and decreases data locality which can (but not necessarily must) cause a noticeable performance impact. Not so important, but still worth consideration.)

Is there a way to avoid a level of indirection here?

UPDATE: I fixed some incorrect assumptions and re-phrased the question, based on feedback in comments and answers.

Answer 1

For the start, std::mutex can not be copied or moved, therefore you are forced to use some kind of indirection.

Since you want to store mutex in a vector, and not copy it, I would use std::unique_ptr.

vector<unique_ptr<T>> does not allow certain vector operations (such as for_each)

I am not sure I understand that sentence. It is perfectly possible to do range for :

std::vector< std::unique_ptr< int > > v; // fill in the vector for ( auto & it : v )   std::cout << *it << std::endl; 

or to use std algorithms :

#include <iostream> #include <typeinfo> #include <vector> #include <memory> #include <algorithm>   int main() {     std::vector< std::unique_ptr< int > > v;     v.emplace_back( new int( 3 ) );     v.emplace_back( new int( 5 ) );     std::for_each( v.begin(), v.end(), []( const std::unique_ptr< int > & it ){ std::cout << *it << std::endl; } ); } 

Answer 2

That requires however the creation of objects of type T with a copy ctor.

That is not entirely right, as of C++11, if you use the constructor of std::vector which will default construct a number of elements, then you don't need to have a copy or move constructor.

As such, if no threads are added or deleted from your pool, you can then just do:

int num = 23; std::vector<std::mutex> vec(num); 

If you want to add or delete things dynamically, then you have to use an indirection.

1. Use std::vector + std::unique_ptr as already proposed
2. Use a std::deque, that allows you to neatly use it with range based for loops or std-algorithms and avoids all indirections. (Which only allows additions)
3. Use a std::list/forward_list this solution is similar to number one, however it has the additional benefit of easier usage with range based for and algorithms. It's probably the best if you are only accessing the elements sequentially as there is no support for random-access.

Like this:

std::deque<std::mutex> deq; deq.emplace_back(); deq.emplace_back();  for(auto& m : deq) {     m.lock(); } 

As a final note, std::thread is of course moveable, so you can use std::vector + std::vector::emplace_back with it.