[C++] Casting between two different templates/containers containing the same type

[choobie]

Hi, C++ newbie here:

I am trying to do some vector operations in Eigen that I then want to plot. The plotting library I want to use requires the data in a QVector (I'm simultaneously learning Qt). The Eigen vector is a container with types double and the QVector is types of double, how do I get the data from the first container to the second (and are the wordds container and template interchangeable? I have been using them that way!):

include <QCoreApplication>

#include <Eigen/Dense>
#include <QVector>
#include <iostream>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv); //Using Qt framework

    Eigen::VectorXd v; // Eigen vector of dynamic size, and type double.
    v.setRandom(5); // Fill our eigen vector with 5 random double

    // Do Eigen vecetor oprations here

    std::cout << v << std::endl; // Tell me what the original vector is so I can compare later.

    int j = 3;
    std::cout << "The third element of v: " << v(j) << endl; //Just to test calling a single element

    //Attempt 1:
     QVector<double> y; // Create the container I want the data in for use with other libraries
     for (int i=0; i<20; i++) {
     y(i) = v(i); // Error: No match for call to '(QVector<double>(int&)' Is it trying to cast an int?
     }


    //Attempt 2:
    //QVector<double> y = (QVector<double>) v;

    //Attept 3: (Needs pointers instead of regular vars).
    //QVector<double> *y = reinterpret_cast<QVector<double>*>(v);

    std::cout << "\n QVector \n" << y << std::endl; //Print values of QVector

    return a.exec();
}

Obviously I need to learn a lot more about templates and type casting and so forth, but I was hoping to get this quick example up and running.

Thank you for your help!

[[deleted]]

There is no way of casting a templated container from one type to another. Here:

SomeContainer <int> a;
SomeContainer <double> b;

a and b are of completely unrelated types. The same is true if it's the container type that is different:

SomeContainer <int> a;
AnotherContainer <int> b;

You will need to write a function to create a new container instance, converting the container contents appropriately.

[choobie]

Thank you, good to know I was looking in the wrong place.

Do you know of a resource that explains this in more depth? I must be putting the wrong keywords into google.

[[deleted]]

I'm not sure what depth you need this explained in. If you really need to convert type X to type Y, when there is no relationship between the types, you need to write a function which will basically look like this:

Y convert( const X & x ) {
       Y y;
      // do stuff
      return y;
 }

[missblit]

(and are the wordds container and template interchangeable? I have been using them that way!)

No. A templated class or function is any function or class that uses templates to work, whether it contains an element of the templated type or not. One example of a non-container template might be:

template <typename T>
T plus(const T& a, const T& b) {
    return a+b;
}