SDL2 Wrapper class complains `Invalid Renderer` - c++

Question

I'm trying to write a C++ wrapper class around some SDL2 classes.
Now I have this working code, which displays a red screen for 5 seconds (as you can see, my wrapper classes are in namespace sdl2cc):
int main(void)
{
if (SDL_Init(SDL_INIT_VIDEO) < 0) return 1;
sdl2cc::Window window{"SDL_RenderClear"s, sdl2cc::Rect{sdl2cc::Point{SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED}, sdl2cc::Dimension{512, 512}}, {}};
sdl2cc::Renderer renderer{window, {}};
renderer.draw_color(sdl2cc::Color{sdl2cc::RGB{255,0,0}, sdl2cc::Alpha{255}});
SDL_RenderClear(renderer.data());
// renderer.clear();
SDL_RenderPresent(renderer.data());
// renderer.present();
SDL_Delay(5000);
SDL_Quit();
}
In the wrapper class of SDL2's SDL_Renderer I have a std::unique_ptr data member renderer_ pointing to an actual SDL_Renderer.
renderer.data() exposes this pointer (return this->renderer_.get();).
I want to get the member functions renderer.clear() and renderer.present() to work. Sadly neither do. This is how they look:
void sdl2cc::Renderer::clear(void)
{
if (SDL_RenderClear(this->data()) < 0)
{
std::cerr << "Couldn't clear rendering target with drawing color:" << ' ' << SDL_GetError() << '\n';
}
}
void sdl2cc::Renderer::present(void)
{
SDL_RenderPresent(this->data());
}
If I just use renderer.clear(), it will print my error message + Invalid renderer.
If I just use renderer.present(), it will show a black screen.
What is wrong?
Why are my own functions and the SDL functions not equivalent?
The problem seems to lie in the function call:
SDL_RenderClear(renderer.data()); // works
// somewhere else:
void sdl2cc::Renderer::clear(SDL_Renderer* r)
{
if (SDL_RenderClear(r) < 0)
{
std::cerr << "Couldn't clear rendering target with drawing color:" << ' ' << SDL_GetError() << '\n';
}
}
renderer.clear(renderer.data()); // doesn't work: Invalid Renderer
But I still don't understand where the problem lies. To me it seems to accomplish the same thing, but somehow one throws an error, the other doesn't.
EDIT:
Another interesting thing, trying to step in at renderer.clear() with lldb goes directly to the next line, without actually stepping in... I don't even.

Answer 1

The problem had to do with multiply linked libraries.
I compiled my own library with the SDL2 libraries and then compiled my executable with my library and the SDL2 libraries.