Layout of compiled objects

Question

Is there a way—much like viewing the result of preprocessing with gcc -E—to see what my objects look like once compiled into object files?

I am talking about GCC, but a solution including MSVC would be fine.

Caligula

Good point John. The constructor will be code, and like all code belongs to the class, not individual objects. Hence you won't find it in the object layout. – Bats

You can use g++ -S file.cpp to get assembler output in file.s. Is this what you want? – Wanderlust

Answer 1

For Visual C++:

I finally managed to dig up the (well-hidden!) undocumented compiler flags that MSVC++ supports using information from here and here. Here they are:

/d1reportSingleClassLayoutXXX
/d1reportAllClassLayout

(replace XXX with the class name)

Disenchant

Note for others who may land into this quest: This switch will output directly to build output window. So add it in .cpp properties 'Command Line > Additional options'. If your desired class/struct did not show up in the list (despite it is declared in header file), I declared a new class in cpp and make it inherit from that class, that did the job for me. – Heiress

Answer 2

For GCC compiled executables, checkout Pahole. It will show you how the compiler laid out your structs/classes and whether or not they have "holes" in them. Holes are padding due to memory alignment rules.

Dished

the pahole link is dead – Humiliation

@Humiliation I fixed up the link. – Dished

related Find holes in C structs due to alignment – Levesque

Answer 3

Object files contain binary data - the only higher level that most compilers can output is assembler, so if you can't read that you are out of luck. However, take a look at this question for more info in this area.

Symonds

Answer 4

You can inspect the layout of binaries and their contents using map files. Use /MAP for VC and -Map or --print-map for gcc.

Actinism

Answer 5

Your question is a little confusing.

If you want to see the result of preprocessing with MSVC, you can use /E, /P/, or /EP.

There's an undocumented option in MSVC to show the data layout of structures and classes. I'm having trouble finding it right now.

Electroacoustics

>There's an undocumented option in MSVC to show the data layout of structures and classes. I'm having trouble finding it right now. this is exactly what i am looking for! – Caligula

I finally managed to dig up those switches: /d1reportSingleClassLayoutXXX and /d1reportAllClassLayout. – Disenchant

Answer 6

A constructor is just another function (unless it's in-lined). Object files contain a lot of info for the linker; so you should be able to find the function in the .a file (the function names will be mangled though).

Medarda

Answer 7

If you are asking about seeing the memory layout of an object's field (as other answers seems to hint) then there is a visual studio extension that does just that: Struct Layout by Ramon Viladomat (also available via Github)

This will add a contextual item when you right click on a struct definition, that will open a memory layout visualizer window.

The display looks like this:

It has it's quirks, and I've been struggling to set it up correctly on some large projects, but when it works it's extremely useful.

There is also an open feature request ticket on the MS developer community forums to add the feature to the editor by default. Please upvote it if you want that to get traction.

Paragraphia