problem with initialization by default in class Assignment operator and vector in c++

Question

Hi i am having problems storing default instances of a class in a vector, when i use no default constructor everything go fine.

this is the class structure:

#ifndef NEURO_H
#define NEURO_H

#include<iostream>
#include<fstream>
#include<vector>

using namespace std;

class neurona
{
private:
    double v_r; //potencial de reposo
    double v_u; //potencial umbral
    double v_d; //potencial alcanzado al disparar

public:
    neurona(double=-65.,double=-55.,double=35.) ;
    ~neurona()          ;
    neurona(const neurona &)    ;
    neurona & operator = (const neurona &);

};


neurona::neurona(double r,double u,double d):v_r(r),v_u(u),v_d(d)
{}


neurona::~neurona()
{}

neurona::neurona(const neurona &z)
{
    v_r=z.v_r;
    v_u=z.v_u;
    v_d=z.v_d;
}

neurona & neurona::operator = (const neurona & z)
{
    if(this!=&z)
    {
    v_r=z.v_r;
    v_u=z.v_u;
    v_d=z.v_d;
    }
    return *this;
}
 #endif

and this is the main

   int main()
    {
    neurona n1()        ;
    neurona n2(3.,3.,3.)    ;
    neurona n3=n2   ;

    vector<neurona> v_neu(3)    ;

v_neu.push_back(n1);

//v_neu[0]=n1;
//v_neu[1]=neurona2;
//v_neu[2]=neurona3;

  cout<<"okidoki"<<endl;
}

i get the following error

neurona.cc: In function ‘int main()’: neurona.cc:15:11: error: no match for ‘operator=’ (operand types are ‘__gnu_cxx::__alloc_traits >::value_type {aka neurona}’ and ‘neurona()’) v_neu[0]=n1; In file included from neurona.cc:1:0: neurona.h:41:11: note: candidate: neurona& neurona::operator=(const neurona&) neurona & neurona::operator = (const neurona & z) ^~~~~~~ neurona.h:41:11: note: candidate: neurona& neurona::operator=(const neurona&) neurona & neurona::operator = (const neurona & z) ^~~~~~~

if instead of using n1 in the push_back() use n2 everything works fine, the same with n3.