a function definition cannot appear inside another function, but a function declaration can. Inside the function bool clusterLabelling(), after bool inTback(const tIndex_t index), the only thing that can follow is the semicolon (which would make that line a function declaration).
Thanks vlad and cubbi, but vlad's solution looks a little strange to me yet. I am going to declare the function inside the function clusterLabeling() and define it after it. Because I need to use datas from clusterLabeling().
But it seems my idea don't work, compilation error message shows all the
Honestly, if you want to do it that way then vlad's solution is good. Otherwise, you could pass the vector(s) you are referring to as parameters to that function.