Question

I have a VS2008 C++ program where I’m wrapping a C API for use in a C++ program. The C API is expecting an array of TABLE_ENTRY values as shown below.

Other than copying the data from each of the MyClass structures in to a new TABLE_ENTRY structure in MyClassCollection::GetTable(), is there a way to get the functionality I’m looking for?

Thanks,
PaulH

struct TABLE_ENTRY {
    const char* description;
    DWORD value;
};

class MyClass
{
public:
    MyClass( const char* desc, DWORD value ) : 
        description( desc ),
        some_value( 1 )
    {
    };

    TABLE_ENTRY* GetTable()
    {
        entry_.description = description.c_str();
        entry_.value = some_value;
        return &entry_;
    };

    TABLE_ENTRY entry_;
    std::string description;
    DWORD some_value;
};

class MyClassCollection
{
public:
    TABLE_ENTRY* GetTable()
    {
        return collection_.front()->GetTable();
    };

    void Add( MyClass* my_class )
    {
        collection_.push_back( my_class );
    }
private:
    std::vector< MyClass* > collection_;
};

int _tmain( int argc, _TCHAR* argv[] )
{
    MyClass class1( "class1", 1 );
    MyClass class2( "class2", 2 );

    MyClassCollection collection;
    collection.Add( &class1 );
    collection.Add( &class2 );

    TABLE_ENTRY* table = collection.GetTable();

    // table is to be used by the C API. Therefore, these next
    // calls should function as shown.
    TABLE_ENTRY entry1 = table[ 0 ]; // should be class1's table (works)
    TABLE_ENTRY entry2 = table[ 1 ]; // should be class2's table (full of junk)

    return 0;
}

Answer 1

I’d go for copying to a vector<TABLE_ENTRY> and pass &entries[0] to the C API.

And, I would not store the TABLE_ENTRYs in your C++ class. I’d only make them just as you call the API, and then throw them away. That’s because the TABLE_ENTRY duplicates the object you copy from, and it is storing a direct char* pointer to a string who’s memory is managed by a std::string. If you modify the source string (and cause reallocation), you have a dangling pointer.