Pointers

VBA Internals: Decimal Variables and Pointers In Depth

Decimal Variants

The Decimal type is special. It's a structure that takes 14 bytes. Essentially it's a 96-bit (12-byte) signed integer, plus a 1-byte scaling factor and a 1-byte sign. The scaling works just like the Scale attribute of a SQL Decimal, indicating the number of significant base-10 digits that appear after the decimal. Although the scale is a whole byte, valid values are only 0 to 28. Likewise the only valid values for the sign byte are 0 (positive) or 1 (negative).

Note that all other non-Variant built-in types in VBA take 8 or fewer bytes; they all fit within a single register on a 64-bit platform. But 8 bytes isn't enough for a useful exact decimal type. An ANSI SQL Decimal, in contrast, takes up to 17 bytes. The designers of COM Automation could have taken the approach of making the Decimal a pointer to a variable-length structure, just as a BSTR (String in VBA) is a pointer to a variable-length character buffer. But, presumably to avoid the overhead of an additional pointer indirection, they settled for a compromise by limiting a decimal to 14 bytes and allowing it to fit entirely within the free portion of a 16-byte Variant. As a result, the Decimal type exists only as a sub-type of the Variant. VBA reflects this at a language level by prohibiting declaration of variables explicitly as Decimals, and instead only producing Decimal values through the CDec function.

A "pointer to a Decimal" is thus a tricky thing. Technically, the Automation DECIMAL structure is defined as a 16-byte structure, where the first two bytes are "reserved". This typedef is included in the union typedef of the VARIANT structure, and in practice those first two bytes are still populated with the two-byte VARENUM enumeration value.

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VBA Internals: Variant Variables and Pointers in Depth

Pointers and memory for Variant variables

In the Component Object Model (COM) Automation framework, the VARIANT structure provides a wrapper for passing around any type of data, and a suite of manipulation functions facilitate using the VARIANT as a platform-level dynamically-typed variable. I say platform-level because the structures, enumerations, and functions that implement VARIANTs exist at the Windows API level. Any language -- including those that are not dynamically typed -- can use the API to accomplish something like dynamic types.

VBA does provide dynamically typed variables, and calls them Variants, just like the supporting structures in the COM API. When writing VBA code you never have to call the API functions like VarAdd or VarXor. The compiler and runtime do it for you behind the scenes. But when you pop the hood and start directly working with the bits and bytes of Variant variables and pointers it's important to know what you're really dealing with -- namely, a COM VARIANT structure.

The details of the layout of the 16 bytes in the VARIANT structure are covered in detail in What's in a variable. The full code of the memory utility functions used in the examples in this post are included in Scalar Variables and Pointers in Depth.

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VBA Internals: Array Variables and Pointers in Depth

This is the next installment of a series of deep-dives into the structure and implementation of variables in Visual Basic for Applications. For the previous posts, see the following:

In this post, I will cover the details of array variables and pointers. See Scalar Variables and Pointers in Depth for additional background and for the code for the utility functions HexPtr and Mem_ReadHex.

Pointers and memory for array variables

Like strings, arrays in VBA are treated semantically like value types but are implemented as reference types. Also like strings, arrays in VBA are implemented using a COM automation structure. For arrays the supporting COM type is the safe array, which comes with a large group of utility functions.

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VBA Internals: String Variables and Pointers in Depth

This is the next installment of a series of deep-dives into the structure and implementation of variables in Visual Basic for Applications. For the previous posts, see the following:

In this post, I will cover the details of string variables and pointers. See Scalar Variables and Pointers in Depth for additional background and for the code for the utility functions HexPtr and Mem_ReadHex.

Pointers and memory for string variables

Even though string variables are treated semantically as value types, they are reference types by implementation. The contents of a string variable is actually a pointer to another memory location where the actual string characters are stored. With VBA we can either get the address to the variable itself using VarPtr, or we can go straight to the start of the character buffer by using StrPtr. For a variable declared as a String, then, directly reading the memory at the address returned by VarPtr should give you the same pointer value as calling StrPtr.

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VBA Internals: Scalar Variables and Pointers in Depth

In VBA Internals: What’s in a variable, I introduced the types of variables in VBA as categorized by how they are implemented. In VBA Internals: Getting Pointers, I described the functions used to obtain pointers in VBA. Now we'll take the next step: Putting these two ideas together to really get an inside view of variables in VBA.

Memory utility functions

In the examples here, I use two utility functions of my own making:

  • HexPtr is a quick function to format a LongPtr in fixed-width hexadecimal format.
  • Mem_ReadHex reads an arbitrary number of bytes from memory and returns them as a hexadecimal formatted string, with each byte taking two characters.

Example implementations are included at the end of this article.

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VBA Internals: Getting Pointers

Generally speaking you don't use pointers in VBA. There is no language-level syntax to explicitly obtain or dereference a pointer as in C or C++ (such as int* somePtr = &someInt).

Within VBA there is almost no reason at all to use pointers. The VB runtime handles all the implementation details of allocating, using, and reclaiming memory for you. But sometimes you need (or want) to reach out to the power of an API function that actually requires a pointer and you have no choice but to take that plunge. Or maybe (like me) you're just mischevious or curious, and want to dig into all the implementation details that language and runtime designers are constantly telling us programmers to ignore.

Either way, there are in fact ways to obtain and work with pointers in VBA. You can do some powerful things with them. You can also crash your whole program if you don't know what you're doing. Using pointers is a big topic, so in this post I'll just present an overview and a description of the functions used to obtain them.

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