How do I loop over a boost MPL list of non-default constructed classes?

How do I loop over a boost MPL list of non-default constructed classes?

By : user2956953
Date : November 22 2020, 03:03 PM
it helps some times I've solved this by adding a type wrapper struct, like the one seen in Boost.Hana. I added this to the great idea found in this comment that mentions using boost::mpl::transform to add this wrapper type to each item in the list automatically.
The following details my solution:
code :
template <typename T>
struct type_
  using type = T;

template <typename T>
struct add_type_wrapper
  using type = type_<T>;
struct type_printer
  template <typename T>
  void operator()(T)
    using type_t = typename T::type;
using boost::mpl::_1;
using wrapped_list = boost::mpl::transform<type_list, add_type_wrapper<_1>>::type;

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Non-default constructed boost::proto terminal

Non-default constructed boost::proto terminal

By : Arthur Anderson
Date : March 29 2020, 07:55 AM
I wish this help you I'm trying to define a very limited parser combinator library using boost::proto and was wondering if it's by any means possible to define a non-default constructed proto terminal. , You can make the name sym a function that returns a terminal:
code :
proto::terminal<symbol>::type sym(std::string const& s)
{ return { symbol(s) }; }
Cannot copy object constructed from default-constructed array

Cannot copy object constructed from default-constructed array

By : Anurag Upadhyay
Date : March 29 2020, 07:55 AM
around this issue This is the most vexing parse:
code :
Derived c(std::array<double, 3>());
main.cpp: In instantiation of ‘Derived::Derived(Args&& ...) [with Args = {Derived (&)(std::array<double, 3ul> (*)())}]’:
Derived c{std::array<double, 3>{}};
Issue printing array list constructed of defined classes

Issue printing array list constructed of defined classes

By : Whitney
Date : March 29 2020, 07:55 AM
hope this fix your issue You have declared row ArrayList type as local to the constructor. Declare it in your class context: which is probably you are wanting. override the toString() method and provide an implementation in the Row class with proper format you want it to see as String. You will probably need to override toString() method and provide an implementation for the Cell class too. For a List of specific element, i prefer to declare the list with the element name as prefix and list as suffix: suppose instead of ArrayListrow, declare it as ArrayListcellList
For example:
code :
class Cell {
    // your other code

    public void String toString() {
        return "shipment: " + shipment + "; cost: " + cost;

class Row{
    ArrayList<Cell> cellList = new ArrayList<Cell>();

    public Row(Integer[] warehouse) {

        for(int value: warehouse) 
            cellList.add(new Cell(value, 0));


    public void String toString() {
        return cellList.toString();

public Cell(int x, int y){

        cost = x;
        shipment = y;

How can a default-constructed object of a non-copyable class be emplaced into a boost::container::map?

How can a default-constructed object of a non-copyable class be emplaced into a boost::container::map?

By : Jonathan Lastrilla
Date : March 29 2020, 07:55 AM
this one helps. Out of interest I played with this
As an important datapoint, I knew that std::map (in C++11) supports piecewise construction of it's value pairs:
code :
std::map<int, A> stdMap;
boost::container::map<int, A>::value_type p { 
static_assert(std::is_same<decltype(p), std::remove_reference<decltype(*myMap.begin())>::type>::value, "Nonstandard pair");
C# Get a list of all runtime constructed classes of a generic class

C# Get a list of all runtime constructed classes of a generic class

By : Nabeel Adnan
Date : March 29 2020, 07:55 AM
I wish this helpful for you @David Mårtensson: Your answer gave me an idea. I could make a static list of types in any non-generic class, and register each constructed class as it was constructed (when T is known). i.e.
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