C++: How to get pointer to 2 dimensional dynamic array?

Question

I looked at this excellent answer but can't figure out how to apply it to this snipped:

//this is in the .hpp file
std::atomic<int> size = 10;
std::recursive_mutex *locks[2];

//in some function of the class
//it's important that the 2nd array dimension is dynamic
the_lock[0] = new std::recursive_mutex[size]; 
the_lock[1] = new std::recursive_mutex[size];
std::recursive_mutex (*locks_2)[2][size] = &locks;

The assignment gives me

error: cannot convert ‘std::recursive_mutex* (*)[2]’ to ‘std::recursive_mutex (*)
[2][(((sizetype)(((ssizetype)((**here be long type information, since I'm using
templates a lot**, long unsigned int, std::less<long unsigned int>          
>::size.std::atomic<long unsigned 
int>::<anonymous>.std::__atomic_base<_IntTp>::operator 
std::__atomic_base<_IntTp>::__int_type<long unsigned int>()) + -1)) + 1)]’ in 
initialization

How can I obtain a pointer to 'locks'?

Arrrr... please don't call your mutex "lock". A mutex is a mutex, and a lock is a lock. — Kerrek SB
– Kerrek SB, Commented Jul 3, 2013 at 22:10
Sorry for that. I'm trying to implement Java-style pseudo code from a book and am keeping the names to avoid confusion — mort
– mort, Commented Jul 3, 2013 at 22:13
OK -- I think that's saying more about Java than about you :-) (Or at least about the book.) — Kerrek SB
– Kerrek SB, Commented Jul 3, 2013 at 22:13

Kerrek SB · Accepted Answer · 2013-07-03 22:18:08Z

3

The error message is actually giving away the solution for free:

std::recursive_mutex * (*locks_2)[2] = &locks;

edited Jul 3, 2013 at 22:18

answered Jul 3, 2013 at 22:12

Kerrek SB

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3 Comments

mort Over a year ago

This does NOT WORK! ' error: cannot convert ‘std::recursive_mutex* ()[2]’ to ‘std::recursive_mutex ()[2]’ in initialization'

Kerrek SB Over a year ago

@mort: Sorry, my bad, forgot a *. Fixed.

mort Over a year ago

I accepted this answer because it helped figure out the correct type. However, the other answers are correct as well.

user268396 · Accepted Answer · 2013-07-03 22:18:34Z

2

One can use the fact that this sort of thing has been officially declared to be ridiculously hard to get right for no apparent reason that the compiler cannot solve for you. If your compiler supports C++ 2011 that is. Use:

auto my_ptr = &locks; // point to locks, let the compiler worry about types

Requires that you compile your code as C++ 2011 though. (-std=c++11 for GCC).

answered Jul 3, 2013 at 22:18

user268396

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1 Comment

mort Over a year ago

Not a problem, since I already use '-std=c++11' (with clang though)

paul · Accepted Answer · 2013-07-03 22:12:00Z

1

locks is just an array of pointers. So you could use a pointer pointer to point to it.

std::recursive_mutex **locks_2 = locks;

answered Jul 3, 2013 at 22:12

paul

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2 Comments

Kerrek SB Over a year ago

Strictly speaking, that's not a "pointer to locks". At best that's a "pointer to locks[0].

mort Over a year ago

Works and is sufficient for my requirements.

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