Question

I can’t seem to find any information on how to calculate padding within a struct or a class within C, C++.

In HLSL there are data structures called “Constant Variables” and they are very much like C structs with #pragma pack (4) enabled. Here is a link for more information with regards to Constant Variables in HLSL.

The problem I am having is with trying to create a struct format descriptor. After parsing the HLSL code, a constant descriptor will contain information about the data types contained within a constant variable struct. It will describe the data type of each member variable, its offset and the total size of the struct. The trouble I am having is in determining the final size of a struct due to padding.

If there is an algorithm for this then I should be able to code it up and calculate the actual padded size of any “Constant Variable” in HLSL. The problem is I don’t know what it is and nor do I know where to find it?

Answer 1

My question is asking for the algorithim used to make #pragma pack 4 work. (e.g. does it look at one variable at a time within a struct and use some sort of max size test to determine where to pad or does it analyses the entire struct and somehow shuffle data around.)

The compiler cannot shuffle data around in a C-style struct. C guarantees that the members are allocated in exactly the order they are declared. Any padding must come between members or at the end of the struct.

A C++ class with access specifiers does not have to be compatible with C, so some reordering is allowed.