Try to pass an object to a same template class to copy - c++

Question

I work actually to understand the concept of template and implement a simple one. I manage to almost cases execpt one to code a constructor to copy an instance of same class.
#include <iostream>
// template <typename T>
template <class T>
class vec2 {
private:
static int instance;
T const x;
T const y;
public:
vec2() : x(0), y(0) {
std::cout << "Default constructor" << std::endl;
vec2<T>::instance++;
return;
}
vec2(T const &x, T const &y) : x(x), y(y) {
std::cout << "Parametric constructor" << std::endl;
vec2<T>::instance++;
return;
}
vec2(vec2<T> const & src) {
*this = src;
std::cout << "Copy constructor" << std::endl;
vec2<T>::instance++;
return;
}
~vec2(){
std::cout << "Destructor" << std::endl;
vec2<T>::instance--;
return;
}
vec2 & operator=(vec2 const & rhs) {
this->x = rhs.get_x();
this->y = rhs.get_y();
return *this;
}
// get
static int get_instance() {
return vec2<T>::instance;
}
T get_x() const {
return this->x;
}
T get_y() const {
return this->y;
}
};
template <class T>
std::ostream & operator<<(std::ostream & out, vec2<T> const & rhs) {
out << "[ " << rhs.get_x() << ", " << rhs.get_y() << " ]";
return out;
}
template <class T>
int vec2<T>::instance = 0;
int main() {
vec2<float> a;
vec2<int> b(21, 42);
vec2<float> c(21.21f, 42.42f);
vec2<bool> d(true, false);
vec2<int> e(b);
std::cout << a << std::endl;
std::cout << b << std::endl;
std::cout << c << std::endl;
std::cout << d << std::endl;
std::cout << e << std::endl;
std::cout << "a.get_x(): " << a.get_x() << std::endl;
std::cout << "a.get_y(): " << a.get_y() << std::endl;
std::cout << "b.get_x(): " << b.get_x() << std::endl;
std::cout << "b.get_y(): " << b.get_y() << std::endl;
std::cout << "c.get_x(): " << c.get_x() << std::endl;
std::cout << "c.get_y(): " << c.get_y() << std::endl;
std::cout << "d.get_x(): " << d.get_x() << std::endl;
std::cout << "d.get_y(): " << d.get_y() << std::endl;
return (0);
}
here the error message, but I'm not expert te read it, and i don't understand what i must change in my code. So if anybody I have an idea to help a newbee in C++, that's can be awesome.
clang++ -std=c++11 -Wconversion *.cpp && ./a.out
main.cpp:24:2: error: constructor for 'vec2<int>' must explicitly initialize the
const member 'x'
vec2(vec2<T> const & src) {
^
main.cpp:72:12: note: in instantiation of member function 'vec2<int>::vec2'
requested here
vec2<int> e(b);
^
main.cpp:9:10: note: declared here
T const x;
^
main.cpp:24:2: error: constructor for 'vec2<int>' must explicitly initialize the
const member 'y'
vec2(vec2<T> const & src) {
^
main.cpp:10:10: note: declared here
T const y;
^
main.cpp:38:11: error: cannot assign to non-static data member 'x' with
const-qualified type 'const int'
this->x = rhs.get_x();
~~~~~~~ ^
main.cpp:25:9: note: in instantiation of member function 'vec2<int>::operator='
requested here
*this = src;
^
main.cpp:72:12: note: in instantiation of member function 'vec2<int>::vec2'
requested here
vec2<int> e(b);
^
main.cpp:9:10: note: non-static data member 'x' declared const here
T const x;
~~~~~~~~^
main.cpp:39:11: error: cannot assign to non-static data member 'y' with
const-qualified type 'const int'
this->y = rhs.get_y();
~~~~~~~ ^
main.cpp:10:10: note: non-static data member 'y' declared const here
T const y;
~~~~~~~~^
4 errors generated.

Answer 1

As #Sam Varshavchik said in the comments, the problem is that you don't initialize your const members in your copy-constructor. Here is the correct implementation:
vec2(vec2<T> const & src) : x(src.get_x()), y(src.get_y()) { //<-- initialization of const members
std::cout << "Copy constructor" << std::endl;
vec2<T>::instance++;
return;
}
Also, *this = src; does feel all kinds of wrong.

Answer 2

here my original class code where i make a copy with *this = src and the result work great. So the question why that's work with regular class and not with the template ?
Plus I imagine the behavior with the regular class, the adress stay the same.
And the method to init all member can be complicated if the is a lot members variables in the class ?
main.c
#include "Vec2.hpp"
#include <iostream>
int main() {
Vec2 a;
Vec2 b(21,42);
Vec2 c(a);
std::cout << a << std::endl;
std::cout << "c.get_x(): " << c.get_x() << std::endl;
std::cout << "c.get_y(): " << c.get_y() << std::endl;
std::cout << "b.get_x(): " << b.get_x() << std::endl;
std::cout << "b.get_y(): " << b.get_y() << std::endl;
std::cout << "a.get_x(): " << a.get_x() << std::endl;
std::cout << "a.get_y(): " << a.get_y() << std::endl;
a = b;
std::cout << "a.get_x(): " << a.get_x() << std::endl;
std::cout << "a.get_y(): " << a.get_y() << std::endl;
return 0;
}
Vec2.cpp
#ifndef VEC2_H
# define VEC2_H
#include <iostream>
class Vec2 {
public:
Vec2(); // canonical
Vec2(float const x, float const y);
Vec2(Vec2 const &); // canonical
~Vec2(); // canonical
Vec2 & operator=(Vec2 const & rhs); // canonical
static int get_instance();
float get_x() const ;
float get_y() const ;
private:
static int instance;
float x;
float y;
};
std::ostream & operator<< (std::ostream & out, Vec2 const & rhs);
#endif
vec2.hpp
#include "Vec2.hpp"
#include <iostream>
Vec2::Vec2() : x(0), y(0) {
std::cout << "Default constructor" << std::endl;
Vec2::instance++;
return;
}
Vec2::Vec2(float const x, float const y) : x(x), y(y) {
std::cout << "Parametric constructor" << std::endl;
Vec2::instance++;
return;
}
Vec2::Vec2(Vec2 const & src) {
*this = src;
std::cout << "Copy constructor" << std::endl;
Vec2::instance++;
return;
}
Vec2::~Vec2() {
std::cout << "Destructor" << std::endl;
Vec2::instance--;
return;
}
Vec2 & Vec2::operator=(Vec2 const & rhs) {
this->x = rhs.get_x();
this->y = rhs.get_y();
return *this;
}
int Vec2::get_instance(){
return Vec2::instance;
}
float Vec2::get_x() const {
return this->x;
}
float Vec2::get_y() const {
return this->y;
}
std::ostream & operator<< (std::ostream & out, Vec2 const & rhs) {
out << "[ " << rhs.get_x() << ", " << rhs.get_y() << " ]";
return out;
}
int Vec2::instance = 0;