Creation of a template object C++ - c++

Question

How do I call a template constructor whith objects of another class?
Example:
#include <iostream>
using namespace std;
class RGB{
private:
int _R;
int _G;
int _B;
public:
RGB(int a, int b, int c){
_R=a;
_G=b;
_B=c;
}
};
class Gray{
int _G;
public:
Gray(int x){
_G=x;
}
};
template <class T1, class T2>
class Pixel{
public:
Pixel(T1 i, T2 j, int a, int b){
foo=i;
bar=j;
x=a;
y=b;
}
void setPixel(T1 a, T2 b){
foo=a;
bar=b;
}
void getPixel();
private:
int x;
int y;
T1 foo;
T2 bar;
};
int main(){
Gray g(3);
RGB rgb(4, 5, 6);
Pixel<Gray,RGB> pi(g, rgb, 10,27);
return 0;
}
I can use the constructor of the class only with the default constructors of the two classes Bar1 and Bar2 but not with the two objects of the classes that I have created before(in this case bar1 and bar2).
Errors are:
main.cpp: In instantiation of 'Pixel::Pixel(T1, T2, int, int) [with T1 = Gray; T2 = RGB]':
main.cpp:62:37: required from here
main.cpp:35:37: error: no matching function for call to 'Gray::Gray()'
main.cpp:35:37: note: candidates are:
main.cpp:26:5: note: Gray::Gray(int)
main.cpp:26:5: note: candidate expects 1 argument, 0 provided
main.cpp:23:7: note: Gray::Gray(const Gray&)
main.cpp:23:7: note: candidate expects 1 argument, 0 provided
main.cpp:35:37: error: no matching function for call to 'RGB::RGB()'
main.cpp:35:37: note: candidates are:
main.cpp:16:9: note: RGB::RGB(int, int, int)
main.cpp:16:9: note: candidate expects 3 arguments, 0 provided
main.cpp:9:7: note: RGB::RGB(const RGB&)
main.cpp:9:7: note: candidate expects 1 argument, 0 provided
The errors go away if I add default values to the RGB and Gray constructors.
Sorry for the mess, I'm new here...

Answer 1

You need to learn about initialization lists:
Pixel(T1 i, T2 j, int a, int b) : x(a), y(b), foo(i), bar(j) {
}
foo=i;, etc. in the body of the Pixel constructor is assigning to an already existing object. When your Pixel constructor is called, it trys to call the default constructor for its subobjects, Gray and RGB, however, both those types do not have default constructors generated since you provide a user-defined constructor. The solution is to use initialization lists to initialize the subobjects in your Pixel class.
Also, note that subobjects of the class are constructed in the order they are declared inside the class definition, so you should always initialize them in the same order as they are declared in your constructor initialization list.