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k1k4ss0
Asked: 2020-03-24 11:57:20 +0800 CST 2020-03-24 11:57:20 +0800 CST

A constructor (not the default) can be called from new

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I am making a hash table, in which each cell has several keys, what I have done is an object called cell in which the different keys of the table are stored as an array. Inside the table object, in turn, I have a vector of cells. The problem is that I have this code:

In the hash table object

#define Tablon
#include <vector>

template <class T >

class Tabla{

private:
std::vector<T> vCelda;
int size;

public:

Tabla(int n_celdas,int s_celda/*,int f_disp,int f_exp*/):

    vCelda(n_celdas,celda(s_celda))
    {
    }

in the cell object

template <class T>
class celda{

    private:
        int  nClaves;
        T* claves;
        int libres;
    public:

        celda(int nC ){

            nClaves=nC;
            claves = new T [nClaves];
            libres=nClaves; 
        }

vCelda=new celda<T> [n_celdas] (s_celda) ;

When I compile it to see if it's ok it gives me the following error:

In file included from main.cpp:3:0:
tabla.hpp: In constructor ‘Tabla<T>::Tabla(int, int)’:
tabla.hpp:20:26: error: missing template arguments before ‘(’ token
     vCelda(n_celdas,celda(s_celda))

If I put the following line:

vCelda(n_celdas,celda<T>(s_celda))

The following error is thrown:

 In file included from main.cpp:3:0:
 tabla.hpp: In instantiation of ‘Tabla<T>::Tabla(int, int) [with T = DNI]’:
 main.cpp:39:37:   required from here
 tabla.hpp:20:38: error: no matching function for call to ‘std::vector<DNI, std::allocator<DNI> >::vector(int&, celda<DNI>)’
      vCelda(n_celdas,celda<T>(s_celda))
                                  ^

I have tried removing the parameter in parentheses, but obviously it gives another error, since there is no empty default constructor. I'm wondering if C++ contains any predefined ways to perform these types of operations.

c++

1 Answers

  1. Best Answer

    As the error says, you cannot use constructors with arguments in the operator new[].

    You can use initialization between braces:

    vCelda = new celda<T> [n_celdas] { s_celda };

    But this option is quite limited...it will only initialize the exact number of elements you specify... or use the literal value0 . But of course, in that case, you could directly use the default constructor.

    Since you're using dynamic memory anyway, your best bet is to use a std::vector< >:

    #include <vector>

template <class T > class Tabla {
  std::vector< celda< T > > vCelda;

public:
  Tabla( int n_celdas, int s_celda /*, int f_disp, int f_exp*/ ) :
    vCelda( n_celdas, celda( s_celda ) ) {
  }
};

    You initialize it directly with the desired size, and as a second argument you pass it a reference constto the value you want to introduce, with which said value is copied to all the elements of thestd::vector< > .

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