Hi,
In Day 346 around the 29:00 mark, Casey writes this line in PLATFORM_DEALLOCATE_MEMORY, casting a platform_memory_block pointer to a win32_memory_block pointer to deallocate it:
The relevant structs are defined as below:
I think I understand how pseudo-polymorphism in C works, making use of the fact that the pointer to a struct is also a pointer to the first element in that struct, and if that element is also struct, then also a pointer to the first element in that struct and so on. Casey mentions this as well when he rearranges the win32_memory_block struct like the above configuration.
But I don't understand how the actual code he wrote is an example of this? The platform_memory_block struct does not have any knowledge of the win32_memory_block, so why does this code work?
I guess I am missing something obvious -- what is it?
In Day 346 around the 29:00 mark, Casey writes this line in PLATFORM_DEALLOCATE_MEMORY, casting a platform_memory_block pointer to a win32_memory_block pointer to deallocate it:
1 2 3 4 5 6 | #define PLATFORM_DEALLOCATE_MEMORY(name) void name(platform_memory_block *Block) typedef PLATFORM_DEALLOCATE_MEMORY(platform_deallocate_memory); ... win32_memory_block *Win32Block = ((win32_memory_block *)Block); |
The relevant structs are defined as below:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | struct platform_memory_block
{
u64 Flags;
u64 Size;
u8 *Base;
umm Used;
platform_memory_block *ArenaPrev;
};
struct win32_memory_block
{
platform_memory_block Block;
win32_memory_block *Next;
win32_memory_block *Prev;
u64 LoopingFlags;
};
|
I think I understand how pseudo-polymorphism in C works, making use of the fact that the pointer to a struct is also a pointer to the first element in that struct, and if that element is also struct, then also a pointer to the first element in that struct and so on. Casey mentions this as well when he rearranges the win32_memory_block struct like the above configuration.
But I don't understand how the actual code he wrote is an example of this? The platform_memory_block struct does not have any knowledge of the win32_memory_block, so why does this code work?
I guess I am missing something obvious -- what is it?
Edited by idc112
on
The Block parameter is a pointer to a memory location where a platform_memory_block type exists, but the memory initialized at that location is actually a win32_memory_block.
Which is fine because platform_memory_block is the type of the first member of win32_memory_block, and so it is the first thing at that location, so you can cast freely between the two types.
This is basically the same as using inheritance.
Which is fine because platform_memory_block is the type of the first member of win32_memory_block, and so it is the first thing at that location, so you can cast freely between the two types.
This is basically the same as using inheritance.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | struct platform_memory_block
{
u64 Flags;
u64 Size;
u8 *Base;
umm Used;
platform_memory_block *ArenaPrev;
};
struct win32_memory_block : platform_memory_block
{
//platform_memory_block Block;
win32_memory_block *Next;
win32_memory_block *Prev;
u64 LoopingFlags;
};
|
William,
You're right, I guess I was a victim of coding along without pausing to understand what was really going on with the allocations in the first place. I did in fact overlook that the original allocations were win32_memory_blocks, that platform_memory_blocks were never allocated by themselves. Overnight sleep and your explanation makes it clear. :-)
Thanks!
You're right, I guess I was a victim of coding along without pausing to understand what was really going on with the allocations in the first place. I did in fact overlook that the original allocations were win32_memory_blocks, that platform_memory_blocks were never allocated by themselves. Overnight sleep and your explanation makes it clear. :-)
Thanks!
Edited by idc112
on