FFT in Third Party C++

[ustk]

Why is this failing despite I see this in many places (JUCE examples, HISE nodes, etc...):

juce::dsp::FFT forwardFFT;

Instead, I have to:

using FFT = juce::dsp::FFT;
FFT forwardFFT = FFT(size);

This might explain why my transform doesn't seem to work despite it compiles... It would be cool to have a minimal C++ example of this working as a starting point in the FFT world
I found some chunks of code just to start with FFT but just doing this is already a nightmare

Any info on how one gets the magnitudes and phase, manipulate it in perform the inverse transform in a real time environment is welcome