[dorianh]

I always assumed that in C, a given structure would always have the same memory layout, no matter what the compiler is (as long as the compilers target the same architecture of course).

I always assumed that in C++, the memory layout could change a lot between compilers (the location of the pointer to the vtable for example). Do you know if it's true, and if the layout do change, could you give an example?

[saagarjha]

> I always assumed that in C, a given structure would always have the same memory layout, no matter what the compiler is (as long as the compilers target the same architecture of course).

Usually, but not always. In most cases there is one efficient way to pack the structure and still maintain member alignment, but there's not requirement that the amount of padding looks like this.

> I always assumed that in C++, the memory layout could change a lot between compilers (the location of the pointer to the vtable for example). Do you know if it's true, and if the layout do change, could you give an example?

Yes, once you have a non-POD type the memory layout can be fairly arbitrary as you cannot really inspect it and compilers are free to lay it out as they wish.

[ChrisSD]

Most platforms have have system level C APIs that are exposed to userspace. When using these APIs it's necessary to layout structures as they expect. Therefore all compilers on the same platform (OS+arch) would usually produce the same layout for structs so that they are compatible.

However this isn't universally true. Some platforms might not have a well defined C ABI.