c++ initialize vector with pointers to class template

Question

i try to init std vector with pointer to tamplate class
using c++11 and g++
Like this and it fail:

template <typename T>
struct Column  
{
    Column( T data)
    {             
        this->data = data;
    }

    T data;
    
}; 

int main(int argv,char** argc)
{
  std::vector<std::vector<Column*>> csv;
  
}

This i need to i can init Column with diffrent types like this :

 Column<std::string>* tmpString = new Column<std::string>(each);
 csv[0].push_back(tmpString);    

or 

 Column<int>* tmpInt = new Column<int>(each);
 csv[0].push_back(tmpString);

is there any way to do this ? or maybe better way ?

It's impossible. Column<std::string> and Column<int> are distinct types. You can implement smth like variant. — 3CEZVQ
– 3CEZVQ, Commented Jan 23, 2023 at 6:58
It sounds like you are looking for something like std::vector<std::any> or maybe std::vector<std::variant<std::string, int>> Anyway std::any and std::variant are the classes you should look into — john
– john, Commented Jan 23, 2023 at 6:59
@273K this is what i try to do to get vector of tamplats as i dont know the type — user63898
– user63898, Commented Jan 23, 2023 at 7:02
OK, since you are on C++11 you could perhaps look at the boost library, which has C++11 compatible versions of any and variant, — john
– john, Commented Jan 23, 2023 at 7:03

Pepijn Kramer · Accepted Answer · 2023-01-23 07:07:56Z

2

It is not possible to do it directly, you will need an abstract baseclass/interface to do something like that.

Also don't use naked new/delete and prefer std::unique_ptr so your instances get cleaned up when vector goes out of scope.

Demo : https://onlinegdb.com/lIhmM_RC3z

Example :

#include <iostream>
#include <string>
#include <vector>
#include <memory>

struct ColumnItf
{
    virtual void visit() const = 0;
};

template<typename type_t>
class Column :
    public ColumnItf
{
public:

    explicit Column(const type_t& data) :
        m_data{ data }
    {
    }

    void visit() const override
    {
        std::cout << m_data << "\n";
    }

private:
    type_t m_data;
};

int main()
{
    std::vector<std::unique_ptr<ColumnItf>> columns;
    
    columns.emplace_back(std::make_unique<Column<int>>(1));
    columns.emplace_back(std::make_unique<Column<std::string>>("Hello"));
    
    for (const auto& itf : columns)
    {
        itf->visit();
    }

    return 0;
}

answered Jan 23, 2023 at 7:07

Pepijn Kramer

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6 Comments

user63898 Over a year ago

There is problem with this , how can i get the m_data and know its type when i Query it in aloop , for example : itf->getData() what shell it return in the base class ? pretend we have this method in base

Pepijn Kramer Over a year ago

No that can't work. You can only have one type per std::vector. Looking at your question again. What you might need is to first create a struct containing members for each row in you csv file. And then make a std::vector of that. e.g struct row { std::string name; int age; } and then make a std::vector<row>. So maybe your question should have been how to read multipe values (of diffent types) from multiple columns of a csv file and store them in a vector.

user63898 Over a year ago

what do you mean can you write shourt example ? but then i will loss the ability to query the matrix like get me col from csv[0][1] for example

user63898 Over a year ago

is there no option to hold in container tamplated structure ? in c++

Pepijn Kramer Over a year ago

There is no way a std::vector can hold more then one type.

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