VarPtr

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: 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: 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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