How to extract the size of a type from a binary without running or using special tools

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I want to extract the size of certain types from an object file / library

  • without running the binary
  • without special tools
  • for any toolchain (GNU, MSVC, IAR)

I'd like to follow the approach presented here, but in a more generic form.

Ideally it would work like this:


// Some file.cpp
class MyClass {
    // lots of members
};

#include "SizeInfo.h"
const SizeInfo<MyClass, "MyIdentifier"> info;

I would put SizeInfo anywhere I am interested in the size of a type. I might be interested in multiple types per translation unit. The solution should put the string SizeOf MyIdentifier:1234 into the resulting object file so that I can extract both identifier and size using a simple tool like grep. It is expected that the variable is thrown out of the resulting executable or a shared library because it's not used anywhere.

I am using boost in my project, so if that would simplify the implementation, I am all for it.

templates c++
2 Answers

This gives the size of a type in bytes:

sizeof(MyClass)
user_81503180

I think it should be possible to simply get the size of the type with a regular sizeof and then convert the number to a string as explained in this post via variadic templates.

Then you can export a string variable from your object file and grep for it on the outside.

It should work something like this: SizeInfo.h:

namespace detail
{
    template<unsigned... digits>
    struct to_chars { static const char value[]; };

    template<unsigned... digits>
    constexpr char to_chars<digits...>::value[] = {('0' + digits)..., 0};

    template<unsigned rem, unsigned... digits>
    struct explode : explode<rem / 10, rem % 10, digits...> {};

    template<unsigned... digits>
    struct explode<0, digits...> : to_chars<digits...> {};
}

template<unsigned num>
struct num_to_string : detail::explode<num> {};

I downloaded the header file static_string.hpp from this excellent tutorial on static strings.

Some file.cpp:

class MyClass {
    // lots of members
};

#include "static_string.hpp"
namespace sstr = ak_toolkit::static_str;

#include "Sizeinfo.h"
constexpr auto MyClassSize = "MyIdentifier: " + sstr::literal(num_to_string<sizeof(MyClass)>::value);

After compiling with gcc -c Somefile.cpp I can validate the following string inside the object file: MyIdentifier: 8.

user_44977040
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