error while programming c++ using header

Question

Here is my code, im trying to make my own string class but im stuck here... my teacher is not good :/ well i got so many errors but i want to know why this error appears "invalid preprocesing directive #ifndef_CADENA_H"

#include <iostream>
#include <cstdlib>
#ifndef_CADENA_H
#define_CADENA_H
    class cadena {
		char* Ptrcad;
		int tam;
		
		public :
			
		cadena(void);
		char* get(void);
		void set(*char**)
		int longitud;
		~ Cadena (void);
	};
	Cadena :: Cadena(){
		Ptrcad = NULL;
		tam = 0;
	}
	char* Cadena :: get(){
		return Ptrcad;
	}
	void Cadena :: set(const char* x){
		while(*(x+tam)!='\0'){
			tam++;
		}
		Ptrcad = new char (tam);
		for(int i=0;i<tam;i++){
			*(Ptrcad +i)= *(x+i);
		}
	} 

int Cadena :: longitud(){
	return tam;
}
	Cadena :: ~ Cadena(){
		delete [] Ptrcad;
		tam =0;
	}

Answer 1

Because of the underscore I believe. It should be #ifndef CADENA_H and #define CADENA_H. Unless you want to keep the underscore in the name (not reccommended in this case), then it should be #ifndef _CADENA_H and #define _CADENA_H