CLI pointers to unmanaged C++ code

Question

I'm working on my first C++/CLI wrapper and am getting very confused wrt pointers. In the particular project I've got, I'm attempting to wrap an unmanaged C++ dll so I can use it in a C# app. Two objectives here: (1) to be able to use the library in my app, and (2) to learn to do this. Many hours of searching on this site and Google have yielded a bunch of answers that come close; I just can't seem to get all the way there. The unmanaged library I'm trying to incorporate is the key-value store "LevelDB" (I know of at least one .NET wrapper out there but, like I said, I'm trying to learn this).

The code below is the beginning of a CLI wrapper class written as if in unmanaged code. I.e. this, functionally, is what I'm trying to accomplish. Not surprisingly, this generates the following compiler error:

cannot convert parameter 3 from 'cli::interior_ptr<Type>' to 'leveldb::DB **'

When I began, I naively thought that I could/should replace the DB* pointer with a pinned pointer, something like

pin_ptr<leveldb::DB*> db

but this yields a similar error and doesn't work. Other combinations with interior pointers and handles (which, if I understand right, are not applicable here because the library is unmanaged) haven't worked either. I'd appreciate any advice pointing me in the right direction. Any references to good posts/articles would be great too but please include a word or two on how to adapt for my case since, after reading tons of them, I seem to be no closer. Thanks!

Example wrapping class:

#pragma once
#include "leveldb/db.h"

using namespace System;

namespace Wrapper
{
    public ref class DBWrapper 
    {
        leveldb::DB* db;

    public:
        DBWrapper()
        {
            leveldb::Options options;
            options.create_if_missing = true;
            leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
        }

        ~DBWrapper() 
        {
            delete db;
        }
    };
}

Maybe inside a ref class, T* doesn't mean "pointer to T"? Just a guess... — Kerrek SB
– Kerrek SB, Commented Aug 18, 2012 at 18:19

James McNellis · Accepted Answer · 2012-08-18 18:37:50Z

4

Use a local variable:

DBWrapper()
{
    leveldb::Options options;
    options.create_if_missing = true;

    leveldb::DB* local_db(nullptr);
    leveldb::Status status =leveldb::DB::Open(options, "/tmp/testdb", &local_db);
    db = local_db;
}

The object under construction (pointed to by this) can be moved by the garbage collector, so a member variable does not have a fixed address.

answered Aug 18, 2012 at 18:37

James McNellis

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1 Comment

Gadzooks34 Over a year ago

Yes. I'm in a world of linker problems now but this answers my question. Obvious in retrospect. Thank you.

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