CODEWITHSUNDEEP
c++g++memory-layout
zaheer
zaheer

Reputation: 141

Print layout of C++ object with g++ compiler

Is there a way to print the layout of a C++ object using the g++ compiler or any other means. A simplified example (assuming int takes 4 bytes)

class A{
  int a;
};

class B:public A{
  int b;
}

so the output would be 

A-
0      4
+  a   +

B-
0      4      8
+ A.a  +  b   +

It would be useful to understand the layout of objects (in my case virtual machine code).

Thanks in advance.

Regards, Zaheer

Upvotes: 14

Views: 4925

Answers (3)

fredoverflow
fredoverflow

Reputation: 263320

Looking at the man pages, -fdump-class-hierarchy maybe?

Note that since GCC 8, the -fdump-class-hierarchy option has been replaced with -fdump-lang-class.

Upvotes: 12

Employed Russian
Employed Russian

Reputation: 213879

The information you seek is needed by debuggers and is emitted for them when you compile with -g. On ELF/DWARF platforms (such as Linux), you can see what's there by executing:

g++ -g -c foo.cc
readelf -w foo.o

On other platforms, objdump -g foo.o may work.

For ELF/DWARF, pahole looks like a good place to start.

Upvotes: 4

Philipp
Philipp

Reputation: 49850

C++ doesn't have introspection. Once your code is compiled, every piece of information about classes is lost except for what typeid and std::type_info can give you.

Upvotes: 1

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