C++ Access float pointer inside a class -> Access violation

Question

Visual C++, Microsoft Visual Studio 2010 Express:

Access from this malloc works:

float* block  = (float *)_aligned_malloc(32 * sizeof(float), CACHE_ALIGNMENT);
block[0] = (float)30;  // I work fine.

But when it is inside this class, it does not work:

class Test
{
    private:
        //static const int numberOfElements = 1024;

    public:
        float* block1;
        float* block2;

    // Constructor
        Test::Test(int nElements)
        {
            float* block1 = (float *)_aligned_malloc(nElements * sizeof(float), CACHE_ALIGNMENT);
            float* block2 = (float *)_aligned_malloc(nElements * sizeof(float), CACHE_ALIGNMENT);
        }
    // Destructor
        Test::~Test(void) 
        {
            _aligned_free(block1);
            _aligned_free(block2);
        }
};

...

Test testClass = Test(32); 
testClass.block1[0] = (float)30;  // Access violation!

...

Or if declared as a pointer, the same thing (this is how I first tried it):

Test* testClass = new Test(32);
testClass.block1[0] = (float)30;  // Access violation!

What am I doing wrong in terms of accessing the values when the float* is define inside a class? Is the problem something else?

@sad_man, not sure if God's that concerned with the issue at hand. — Michael Krelin - hacker, Jan 24 '12 at 11:18
If you really must try to manage memory yourself, always remember the [Rule of Three](http://stackoverflow.com/questions/4172722/what-is-the-rule-of-three). Although that's only one of the errors here. — Mike Seymour, Jan 24 '12 at 11:32

score 4 · Accepted Answer · answered Jan 24 '12 at 11:12

4

You assign to the variables local to constructor, not member variables. Try something like

    Test::Test(int nElements)
    {
        block1 = (float *)_aligned_malloc(nElements * sizeof(float), CACHE_ALIGNMENT);
        block2 = (float *)_aligned_malloc(nElements * sizeof(float), CACHE_ALIGNMENT);
    }

and check the values to make sure allocation succeeded.

answered Jan 24 '12 at 11:12

Michael Krelin - hacker

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@PlasmaHH true, there should be a warning about shadowing member. – Michael Krelin - hacker Jan 24 '12 at 11:24
Or more broad (depending on the compiler) "set but not used variable" – PlasmaHH Jan 24 '12 at 11:27
@PlasmaHH, both apply indeed. – Michael Krelin - hacker Jan 24 '12 at 11:28
Yes you are correct. I fixed this previously but for some reason missed it now. Thank you. – user1166780 Jan 24 '12 at 11:33

score 0 · Answer 2 · answered Jan 24 '12 at 11:26

0

The declarations of the variables block1 and block2 local to the constructor shadow the member variables block1 and block2.

With g++ you can get a warning for this with -Wshadow option. With Visual Studio I'm afraid there is no option to warn when a variable shadows another one.

answered Jan 24 '12 at 11:26

ouah

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Yes you are also correct, I did not have warning level 4 set (warning of unused variables). Thank you. – user1166780 Jan 24 '12 at 11:38

C++ Access float pointer inside a class -> Access violation

2 Answers2