Dynamic memory use in class member

Question

Hello:

I'm getting some odd behavior in one of my class members and it is truly throwing me for a loop but I'm certainly not seeing the issue (long week!)

void MyFakeStringClass::readStream( iostream& nInputStream )
{
    // Hold the string size
    UINT32 size = 0;

    // Read the size from the stream
    nInputStream.read( reinterpret_cast< char* >( &size ), sizeof( UINT32 ) );

    // Create a new buffer
    char* buffer = new char[ size ];

    // Read the stream
    nInputStream.read( buffer, size );

    // Save to class member
    value = string( buffer );

    // Clean up
    delete[] buffer;
    buffer = NULL;
}

This issue arises when I use two or more MyFakeStringClass's. buffer is somehow still containing data from previous calls to MyFakeStringClass::readStream.

Say for example I read in two strings 'HelloWorld', 'Test' the resulting MyFakeStringClass objects contain 'HelloWorld', and 'TestoWorld' ( should be 'HelloWorld', 'Test' ).

The second time the function is accessed buffer is still containing 'old' memory. How is this possible as it is locally scoped and deleted? I've confirmed that buffer is being somehow shared with a debugger.

Answer 1

You need to create the string using:

value = string( buffer, size );

If you do not specify the size, it will assume buffer is a null-terminated string. Since there is no null terminator, it reads past the end of the data, and gives you previous contents of the memory.

Answer 2

It does have the right size, but if you call new and delete then new again you will get the same value because the memory manager doesn't initialize anything for you. If you are using it as a null terminated string you need to initialize it to zero yourself.

Answer 3

new char does not initialize the memory. It will be filled with whatever random data was left in that memory from the previous use.

If there were no other memory allocations between one two calls to readStream, it's very likely you'll get a buffer starting at the same address.

That said, you have an error that buffer is not 0 terminated and the constructor you are using assumes it is. You are luck that you haven't had a lot of other problems.

You can use the length specified constructor:

value = string( buffer, size );

Or if you needed to use buffer for something that absolutely required a 0 terminated buffer, you could also change your code to this:

// Create a new buffer
char* buffer = new char[ size + 1];

// Read the stream
nInputStream.read( buffer, size );

// Add the 0 termination to the end of the string
buffer[size] = '\0';