Question

I’m writing a tiny kernel with c++11 and have two instances with the same type which have to be constructed before any other static objects are created.

The code I wrote is as follows:

// test.hpp
class test {
  // blahblah...
};

// test.cpp
typedef char fake_inst[sizeof(test)] __attribute__((aligned(alignof(test))));

fake_inst inst1;
fake_inst inst2;

// main.cpp
extern test inst1;
extern test inst2;

int kmain() {
    // copy data section

    // initialize bss section

    new (&inst1) test();
    new (&inst2) test();

    // call constructors in .init_array

    // kernel stuffs
}

It builds and works as expected without no warning messages, but not with LTO.

I get tons of warning messages complaining the type matching and I wonder if there’s a workaround since it confuses me to find the other ‘real’ warning or error messages.

Any suggestion?

Answer 1

Perhaps like this?

// ... .h
template<typename T>
union FakeUnion {
  FakeUnion() {}
  ~FakeUnion() {}

  T inst;
};

extern FakeUnion<test> inst1_;
extern FakeUnion<test> inst2_;
static constexpr test& inst1 = inst1_.inst;
static constexpr test& inst2 = inst2_.inst;
// ... .h end

// ... .cpp
FakeUnion<test> inst1_;
FakeUnion<test> inst2_;
// ... .cpp end

Within main, you can then say new (&inst1) test;. It should not give warnings about type inconsistency violations anymore now, because unlike in your code, this code does not contain variables that have different types in different files.