Using managed arrays

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Introduction

Managed C++ allows you to declare and use Managed .NET arrays which means you don't have to do memory allocation of your own and garbage collection will take care of memory de-allocation. Managed .NET arrays are basically objects of the System::Array class and thus you have the additional benefit that you can call methods of the System::Array class on your Managed .NET arrays. Both single and multi-dimensional arrays are supported though the syntax is slightly different from that used with old-style unmanaged arrays.

Single dimensional arrays

Arrays of a Reference  type

String* StrArray[] = new String* [5];
for(int i=0;i<5;i++) 
    StrArray[i] = i.ToString();
for(int i=0;i<5;i++) 
    Console::Write("{0} ",StrArray[i]);

Here the syntax is not much too different from that in unmanaged code, except that the array we create will be managed and thus garbage collected. Just as in unmanaged arrays, the array index is zero based.

Arrays of a Value type

int NumArray __gc[] = new int __gc[5];
for(int i=0;i<5;i++)
    NumArray[i]=i;
for(int i=0;i<5;i++)
    Console::Write("{0} ",__box(NumArray[i]));

For value types you have to explicitly use the __gc keyword when declaring arrays. If we do not use the __gc keyword the new operator will create an unmanaged array. But of course, the array members are still native value types and thus we need to box them into managed types using the __box keyword when passing them to Console::Write.

Multi-dimensional arrays

String* MultiString[,] = new String*[2,3];
for(int i=0;i<2;i++)
    for(int j=0;j<3;j++)
        MultiString[i,j] = String::Format("{0} * {1} = {2};",
            __box(i),__box(j),__box(i*j));
for(int i=0;i<2;i++)
{
    for(int j=0;j<3;j++)
        Console::Write("{0} ",MultiString[i,j]); 
    Console::WriteLine();
} 

Whoa! What's that, eh? Looks really weird, huh? I thought so too when I first came across the Managed C++ style of declaring Managed .NET arrays. You now have just one pair of brackets and each of the dimensional indexes are separated by commas. Unlike in unmanaged arrays, a 2-dimensional array is NOT an array of arrays. It is simply a 2-dimensional array.

Jagged arrays

Jagged arrays are multi-dimensional arrays that are non-rectangular. For example, you can have a 2-dimensional array where each of the single dimensional sub-arrays are of a different dimension. Unfortunately you cannot have jagged arrays in Managed C++ because there is currently no support for jagged arrays. What might piss you off even more is the fact that C# supports jagged arrays. But then you can always simulate jagged arrays which is what I did. It's not the same thing really, and I do hope they add jagged array support in the next release of VC++ .NET.

Jagged array Simulation

Array* Multi[] = new Array*[2];
Multi[0] = new String*[4];
Multi[1] = new String*[2];
for(int i=0;i<2;i++)
    for(int j=0; j<Multi[i]->Length;j++)
        Multi[i]->SetValue(String::Format("{0} * {1} = {2};",
            __box(i),__box(j),__box(i*j)),j);
for(int i=0;i<2;i++)
{
    for(int j=0; j<Multi[i]->Length;j++)
        Console::Write("{0} ",Multi[i]->GetValue(j));
    Console::WriteLine();
} 

What I did was to create an array of System::Array objects and then I initialized each of those System::Array objects to an array of String objects. And we access an object as Multi[i]->GetValue(j) where i and j are the dimensional indexes. I know that some of you are groaning right now, but I couldn't find any alternative solution. I have posted this in the VC++ .NET newsgroups too but haven't got any responses.

Arrays as Function arguments

Array of Managed types

static void MakeArrayUpper(String* TwoDArray[,])
{
    for(int i=0;i <= TwoDArray->GetUpperBound(0); i++)
        for(int j=0;j <= TwoDArray->GetUpperBound(1); j++)
            TwoDArray[i,j]=TwoDArray[i,j]->ToUpper(); 
}

Well that was rather straightforward, wasn't it? I have used GetUpperBound which is a member method of the System::Array class on our array. As mentioned earlier and as I will explain later down below, we can use any System::Array method on our arrays.

Array of value types

static void DoubleIntArray(int int_array __gc[])
{
    for(int i=0; i< int_array->Length; i++)
        int_array[i] *= 2;
}

Well this is just about similar to the previous version except that we have used the __gc keyword here. One very interesting issue here is that arrays of value types are passed by reference. This might seem odd at first glance, but remember that the array itself is a System::Array object and is thus a managed type which means it's always passed by reference.

Returning arrays from functions

static String* CreatePopulateDummyArray() [,]
{
    String* Small[,] = new String* [2,2];
    Small[0,0] = "Colin Davies";
    Small[0,1] = "James T Johnson";
    Small[1,0] = "Kannan Kalyanaraman";
    Small[1,1] = "Roger Wright";
    return Small;
}

LOL. I can imagine the incredulous look on some of your faces. For whatever reasons they might have had, Microsoft have decided to make it this way and so be it. After all this is C++ and it's not ever expected to make things easier for anyone. Well, so whenever we want to return a managed array, we suffix our function name with our array's dimensional order. Well, when you consider that in unmanaged C++ there was no way to actually return an array except by returning a pointer to it's first element, I must say that this is a great improvement. Now you can actually do something like this.

String* Small[,] = Test::CreatePopulateDummyArray();

Calling System::Array methods

Array::Sort(nArray);
for(int i=0;i<3;i++)
    Console::Write("{0} ",__box(nArray[i]));
Console::WriteLine();
Array::Reverse(nArray);
for(int i=0;i<3;i++)
    Console::Write("{0} ",__box(nArray[i]));
Console::WriteLine();

I have used Array::Sort and Array::Reverse on my arrays. Now isn't that handy?

Full Source Listing

#include "stdafx.h"


#using <mscorlib.dll>
#include <tchar.h>


using namespace System;

__gc class Test
{
public:
    static void MakeArrayUpper(String* TwoDArray[,])
    {
        for(int i=0;i <= TwoDArray->GetUpperBound(0); i++)
            for(int j=0;j <= TwoDArray->GetUpperBound(1); j++)
                TwoDArray[i,j]=TwoDArray[i,j]->ToUpper();        
    }
    static String* CreatePopulateDummyArray() [,]
    {
        String* Small[,] = new String* [2,2];
        Small[0,0] = "Colin Davies";
        Small[0,1] = "James T Johnson";
        Small[1,0] = "Kannan Kalyanaraman";
        Small[1,1] = "Roger Wright";
        return Small;
    }
    static void DoubleIntArray(int int_array __gc[])
    {
        for(int i=0; i< int_array->Length; i++)
            int_array[i] *= 2;
    }
};

int _tmain(void)
{    
    //Single dimensional arrays - reference types

    String* StrArray[] = new String* [5];
    for(int i=0;i<5;i++)    
        StrArray[i] = i.ToString();
    for(int i=0;i<5;i++)    
        Console::Write("{0}  ",StrArray[i]);
    Console::WriteLine();

    //Single dimensional arrays - value types

    int NumArray __gc[] = new int __gc[5];
    for(int i=0;i<5;i++)
        NumArray[i]=i;
    for(int i=0;i<5;i++)
        Console::Write("{0}  ",__box(NumArray[i]));
    Console::WriteLine();

    //Multi dimensional arrays

    String* MultiString[,] = new String*[2,3];
    for(int i=0;i<2;i++)
        for(int j=0;j<3;j++)
            MultiString[i,j] = String::Format("{0} * {1} = {2};",
                __box(i),__box(j),__box(i*j));
    for(int i=0;i<2;i++)
    {
        for(int j=0;j<3;j++)
            Console::Write("{0}  ",MultiString[i,j]);    
        Console::WriteLine();
    }    

    //Simulating jagged arrays

    Array* Multi[] = new Array*[2];
    Multi[0] = new String*[4];
    Multi[1] = new String*[2];
    for(int i=0;i<2;i++)
        for(int j=0; j<Multi[i]->Length;j++)
            Multi[i]->SetValue(String::Format("{0} * {1} = {2};",
                __box(i),__box(j),__box(i*j)),j);
    for(int i=0;i<2;i++)
    {
        for(int j=0; j<Multi[i]->Length;j++)
            Console::Write("{0}  ",Multi[i]->GetValue(j));
        Console::WriteLine();
    }        

    //Passing arrays as arguments to functions and 

    //returning arrays back from the function

    String* Small[,] = Test::CreatePopulateDummyArray();

    for(int i=0;i<2;i++)
        for(int j=0;j<2;j++)
            Console::Write("{0}  ",Small[i,j]);
    Console::WriteLine();
    Test::MakeArrayUpper(Small);
    for(int i=0;i<2;i++)
        for(int j=0;j<2;j++)
            Console::Write("{0}  ",Small[i,j]);
    Console::WriteLine();

    //Passing an int array by reference

    int nArray __gc[] = new int __gc[3];
    nArray[0] = 5;
    nArray[1] = 7;
    nArray[2] = 2;
    for(int i=0;i<3;i++)
        Console::Write("{0}  ",__box(nArray[i]));
    Console::WriteLine();
    Test::DoubleIntArray(nArray);
    for(int i=0;i<3;i++)
        Console::Write("{0}  ",__box(nArray[i]));
    Console::WriteLine();

    //Calling Array member methods

    Array::Sort(nArray);
    for(int i=0;i<3;i++)
        Console::Write("{0}  ",__box(nArray[i]));
    Console::WriteLine();
    Array::Reverse(nArray);
    for(int i=0;i<3;i++)
        Console::Write("{0}  ",__box(nArray[i]));
    Console::WriteLine();
    
    return 0;
}