Member Avatar for kv79

Hi to all ,just a small question.
When i read in the book for Win32 API tur. in c++,
this: "
cbSize
The size of the structure. "
I asked my self ,what sturcture?
I tought we useing class.

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Good points from and ; here are focused, practical notes that go beyond the basic example.

Think of cbSize as a version/length tag that tells the API how many bytes of the structure were supplied. That allows the OS to evolve structures safely: if cbSize is smaller, the API knows not to touch newer fields. Because of that role, exact byte count and binary layout matter more than the source-language keyword (struct vs class).

Common pitfalls and best practices: always initialize the structure memory (zero it) before use, and make sure cbSize matches the exact Windows structure type declared in the headers — not the size of any C++ wrapper or derived type. Do not change packing for Windows headers, and be mindful of 32-bit vs 64-bit differences (pointer sizes affect sizeof). When writing C++ wrappers, keep the native C struct as a plain member and populate its cbSize from inside the wrapper rather than exposing a wrapped object directly to Win32 calls.

Troubleshooting tips: if a Win32 call fails or silently ignores fields, check GetLastError() for ERROR_INVALID_PARAMETER and verify sizeof of the exact header-declared struct against the docs. Some Win32 structs (for example, NOTIFYICONDATA or WNDCLASSEX) intentionally changed size across OS versions; their cbSize is how the API detects which version the caller expects.

A few clarifications tied to the thread: gave the usage pattern; ’s reminder about language choice is apt — Win32 expects C-style layouts. In C++ a plain data-only struct and class with identical data members have the same layout, but adding constructors, virtual functions or inheritance can change it and break binary compatibility with the API.

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Member Avatar for kv79

After pack of moments,my most logical conclude.
The class is extended(embodiment) structure ,so the writer is knew about that and He have a free to write this.
Am i right?

Member Avatar for Ancient Dragon

cbSize is a member of a structure that you set to the structure size before calling win32 api functions with it. For example:

struct foo
{
    unsigned int cbSize;
    // blabla
};

struct foo MyStruct;
MyStruct.cbSize = sizeof(struct foo);
Member Avatar for kv79

>>But why they put a option with struct when they have a class?Where is advance?

win32 api is mostly written in C, not C++, so that they can be called by almost any computer language. The same functions are called by VB, assembly, C#, pascal, etc. This would not be possible if Microsoft had written the api functions in C++.

Member Avatar for Narue

>But why they put a option with struct when they have a class?
Why not? How do you know a class was even an option?

Member Avatar for kv79

Oh yes ,i mixed with a c++ clas.
right?

Member Avatar for Ancient Dragon

>>i mixed with a c++ clas. right?
Yup.

Member Avatar for kv79

Thank you again for reslove my confuseing.

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