overloading template<class T> operator>> for vector<T>

[etrusks]

Hi, i want to ask for advice how this one thing could be done.

I want to overload this operator

template<class T>
std::istream& operator>>(std::istream& is, vector<T>& vec)

The problem is - as far as i understand i have to at 1st create
1. T default_value; //using default constructor
2. than is >> default_value;
3. only than vec.pushback(default_value);
4. and repeat 2 and 3 again and again as long as i need.

But what if in some cases T has no default constructor? Than this code T default_value; would not work out.

How could i make this work if i need to store all these values in my vector?


Here is the code :

template<class T>
std::istream& operator>>(std::istream& is, std::vector<T>& vec){

	// { val , val , val , ... , val }

	char comma, brackets;
	
	is >> brackets;
	if (brackets != '{'){
		is.clear(std::ios_base::failbit);
		return is;
	}
	
	is >> brackets;
	if (brackets == '}'){
		// vector will be empty
		return is;
	}
	else is.putback(brackets);

	T temp = T();   // Here compiler gives me error if T has no default constructor

	while (is >> temp){
		vec.push_back(temp);
		is >> comma;
		if (comma == ',') {}
		else if (comma == '}') return is;  //end of vector
		else { 
			is.clear(std::ios_base::failbit);
			return is;  // something went wrong
		}
	}
	is.clear(std::ios_base::failbit);
	return is;

}

[etrusks]

Any ideas?

[fg109]

I think the error is more likely that you don't have a copy constructor. At any rate, why can't you just use T temp;?

[etrusks]

No its still the same problem

Error	1	error C2512: 'S<int>' : no appropriate default constructor available

So the problem is i cant use this cin >> vec for vectors that contain elements without default constructors. But i bet there is a way to fix this. Maybe some clever trick or writing this all function differently?

[fg109]

Are you sure that it's giving you an error for that line? I compiled the code in your post with no problem. I also ran a (simple) test with no problem:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

#include <iostream> #include <vector> template <class T> struct myClass { T val; }; template <class T> std::istream& operator>>(std::istream& is, myClass<T>& mc) { is >> mc.val; return is; } //insert your code here int main() { std::vector<myClass<int> > v; std::cin >> v; for (int i = 0; i < v.size(); ++i) std::cout << v[i].val << ' '; return 0; }

{ 1, 2, 3, 4, 5, 6, 7, 8, 9 } //input 1 2 3 4 5 6 7 8 9 //output

[Disch]

So the problem is i cant use this cin >> vec for vectors that contain elements without default constructors. But i bet there is a way to fix this. Maybe some clever trick or writing this all function differently?

None that I can think of.

The only other alternative I can think of would be to clone/copy construct an existing element in the vector instead of default constructing one -- but that has the same problem if the class doesn't have a copy constructor.

You could side-step this with a 'read' style function instead of a >> overload:

template<typename T>
void read(std::istream& is, std::vector<T>& vec, T obj = T())
{
    //... 
    is >> obj;
    vec.push_back(obj);
    //...
}

Now if the object does not have a default ctor, you can call the function with any ctor:

read( is, vec, MyObject(15,"whatever") );

The downside is that now a copy ctor is mandatory.

And I guess not having >> semantics is also a downside (though not IMO. I freaking hate that iostream overloaded that operator).

[etrusks]

@fg109

Are you sure that it's giving you an error for that line? I compiled the code in your post with no problem. I also ran a (simple) test with no problem:

Isn't compiler generates some kind of default constructor in this case?

template <class T>
struct myClass
{
    T val;
};

In my code there was only this kind of constructor and actually i had copy constructor but as u said I could also have just used T temp instead of T temp = T(). I think if u would try to run this it wouldn't run.

template<typename T>
struct S{
	
	S(T value) : val( value ){}

	T& get();
	const T& get() const;
	void set(T);
	S<T>& operator=(const T&);

private:
	T val;
};

@Disch
This is actually very good idea!!
void read(std::istream& is, std::vector<T>& vec, T obj = T())
I could even return bool to use this function in while(read(cin, ..., ...))


Thanks you guys both a lot, really appreciate!!! :)