# Why undefined function compiled (C++)?

When I read through below example in [cpprefernce](https://en.cppreference.com/w/cpp/language/abstract_class)

struct Abstract
{
    virtual void f() = 0;  // pure virtual
}; // "Abstract" is abstract
 
struct Concrete : Abstract
{
    void f() override {}   // non-pure virtual
    virtual void g();      // non-pure virtual
}; // "Concrete" is non-abstract
 
struct Abstract2 : Concrete
{
    void g() override = 0; // pure virtual overrider
}; // "Abstract2" is abstract
 
int main()
{
    // Abstract a;   // Error: abstract class
    Concrete b;      // OK
    Abstract& a = b; // OK to reference abstract base
    a.f();           // virtual dispatch to Concrete::f()
    // Abstract2 a2; // Error: abstract class (final overrider of g() is pure)
}

my first thought was: How is this program compiled? since `Concrete::g` is not defined. But when I click `Run` button, it did exit normally.

Then, I copy whole example and write a simple cmake file to compile on my machine. Of course, it won't compile.

`CMakeLists.txt`

cmake_minimum_required(VERSION 3.10)

project(main)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
set(CMAKE_CXX_STANDARD 17)

add_executable(main ${CMAKE_CURRENT_SOURCE_DIR}/src/main.cc)

The error showed:

/usr/bin/ld: CMakeFiles/main.dir/src/main.cc.o: warning: relocation against `_ZTV8Concrete' in read-only section `.text'
/usr/bin/ld: CMakeFiles/main.dir/src/main.cc.o: in function `main':
/home/sw/GitHub/dap/src/main.cc:53: undefined reference to `vtable for Concrete'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status

This definitely indicates that the virtual `g()` is undefined.

So how did the online compiler work without error?

I checked the online compile argument. It shows:

g++ -std=c++23  -O2 -Wall -Wextra -pedantic -pthread -pedantic-errors main.cpp -lm  -latomic  && ./a.out

Ok, there got to be some magic here.

## The Answer

TLDR, the optimization `-O2` flag did the trick.

According to latest GCC [doc](https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Optimize-Options.html):

> `-O1`
> Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
> With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
> ...
> `-O2`
> Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code.
> -O2 turns on all optimization flags specified by -O1.

Godbolt example: [link](https://godbolt.org/z/GfGGKenW9) (you can try to remove `-O2` flag and see what happened)

However, I couldn't find what specific flag remove the dead code or prevent the error. Maybe someone can tell me... :joy:

---

*So why cppreference provide such an example? I know they want to show `Abstract2` is not constructable, but this confuse me at the first time. (~~or maybe just the skill issue lol~~)*