foo.cpp

// Built with: clang++ -O3 -g -std=gnu++11 -stdlib=libc++ foo.cpp -o foo
#include <fstream>
#include <iostream>

#define PRINT_INFO(x) std::cout << "sizeof(" #x ")=" << sizeof(x) << ", offset=" << ((char*)(x*)&zero - (char*)&zero) << '\n'


int main()
{
    std::ifstream zero("/dev/zero");

    PRINT_INFO(std::ifstream);
    PRINT_INFO(std::istream);
    PRINT_INFO(std::ios);
    PRINT_INFO(std::ios_base);

    return zero.get();
}

output.txt

# Built with Xcode 4.6's clang:
themac:~ jorgen$ ./foo
sizeof(std::ifstream)=448, offset=0
sizeof(std::istream)=168, offset=0
sizeof(std::ios)=152, offset=296
sizeof(std::ios_base)=136, offset=296

# Built with Xcode 5.1's clang:
themac:~ jorgen$ ./foo-modern 
sizeof(std::ifstream)=576, offset=0
sizeof(std::istream)=168, offset=0
sizeof(std::ios)=152, offset=424
sizeof(std::ios_base)=136, offset=424