Getting a reference to an item of an array of unique_ptr<>?

Question

I have the following

class Panel {
public:
  std::array<std::array<std::unique_ptr<fb::Block>, Panel::Y>, Panel::X> blocks;
};

And i have a member function like this:

void Panel::processMove() {
  if (move == nullptr) {
    return;
  }
  MoveType type = move->type;
  switch (type) {
    case MoveType::BLOCK_SWITCH: {
      std::unique_ptr<fb::Block> src = blocks[cursor.x][cursor.y];
      std::unique_ptr<fb::Block> dst = blocks[cursor.x + 1][cursor.y];

      if ((src == nullptr) && (dst == nullptr)) {
        break;
      }
      ...
    }
  }

I don't know how to get a temporary reference to the pointed item because this code fails to compile with the following error:

call to implicitly-deleted copy constructor of 'std::unique_ptr<fb::Block>'

How am i supposed to get a temporary reference to the pointed item ?

Collin · Accepted Answer · 2013-02-16 20:13:28Z

2

Just get it as a reference:

std::unique_ptr<fb::Block>& src = blocks[cursor.x][cursor.y]
std::unique_ptr<fb::Block>& dst = blocks[cursor.x + 1][cursor.y]

In fact, since you're using C++11, you can shorten this quite a bit using auto

auto& src = blocks[cursor.x][cursor.y]
auto& dst = blocks[cursor.x + 1][cursor.y]

Or, if you know that all the unique pointers hold a pointer to a valid object, you can get a reference to the object itself, going around the pointer entirely (thanks @ChrisHartman)

fb::Block& src = *(blocks[cursor.x][cursor.y]);
fb::Block& dst = *(blocks[cursor.x + 1][cursor.y]);

//C++11 auto:
auto& src = *(blocks[cursor.x][cursor.y])
auto& dst = *(blocks[cursor.x + 1][cursor.y])

edited Feb 16, 2013 at 20:13

answered Feb 16, 2013 at 19:05

Collin

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5 Comments

Chris Hartman Over a year ago

Or you might just want a reference to the item itself, in which case something like fb::Block& srcBlock = *(blocks[cursor.x][cursor.y]); or just auto& srcBlock = *(blocks[cursor.x][cursor.y]); would be appropriate.

bquenin Over a year ago

How a reference to the object itself would work since the object can be null and I thought references couldn't be null ?

Chris Hartman Over a year ago

Seems you have some confusion between pointers and references. The pointer could be null. Then you can't derefence it, in which case the reference to object solution will cause undefined behavior (ie. crash) at run time. But you may know that none of the pointers are null, in which case they always point at an object, and the latter solution would work.

bquenin Over a year ago

Ok, thanks for the clarification. I suspected something like a crash without this predicate. Actually I don't know if the pointers are null or not. So I guess the first solution best suits my needs.

Drew Dormann Over a year ago

@bquenin your comment is valid. Trying to get a reference to the object itself will throw an exception when the pointer doesn't own an object.

Drew Dormann · Accepted Answer · 2013-02-16 19:08:12Z

0

std::unique_ptr can be evaluated as a bool to check if it points to an object.

Change this:

  std::unique_ptr<fb::Block> src = blocks[cursor.x][cursor.y];
  std::unique_ptr<fb::Block> dst = blocks[cursor.x + 1][cursor.y];

  if ((src == nullptr) && (dst == nullptr)) {
    break;
  }

to this:

  std::unique_ptr<fb::Block> &src = blocks[cursor.x][cursor.y];
  std::unique_ptr<fb::Block> &dst = blocks[cursor.x + 1][cursor.y];

  if ( !src && !dst )) {
    break;
  }

answered Feb 16, 2013 at 19:08

Drew Dormann

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2 Comments

bquenin Over a year ago

I tried, but I have another compilation error now: non-const lvalue reference to type 'std::unique_ptr<fb::Block>' cannot bind to a value of unrelated type 'fb::Block'

Lightness Races in Orbit Over a year ago

This isn't the place for going through all your errors one at a time. Find yourself a chatroom or a real-world mentor!

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