how to implement a class that support both default ctor and operator=

[aj3423]

#include <iostream>
#include <string>
using namespace std;

struct create_info {
	string text;
};

struct window {
	create_info c_info;

	window() {}
};

template<typename wnd_t>
struct wnd {
	wnd_t* impl;

	wnd() {
		impl = new wnd_t;
	}
	wnd(create_info& ci) { // plan 1
		impl->c_info = ci;
	}
};

int main() {
	// usage 1
	wnd<window> w1; 
	w1 = create_info(); // need to support this
	cout << w1.impl->c_info.text << endl;
	// usage 2
	wnd<window> w2 = create_info();// also need to support this
	cout << w2.impl->c_info.text << endl;
}

I need the wnd class support both usage 1/2, but the code crashes. From the debugger I see this line
w1 = create_info();
creates a new instance of wnd, and in the constructor impl is used without being initialized.
Anyway to solve this?

Thanks.

[Cubbi]

well, since impl is not initialized, do initialize it before attempting impl->c_info

While you're at it, you could change its type to a better-behaved pointer, since otherwise you have more code to write (destructor, copy ctor, copy assignment at least)

#include <iostream>
#include <string>
#include <memory>
using namespace std;

struct create_info {
    string text;
    create_info() : text("just testing") {}
};

struct window {
    create_info c_info;

    window() {}
};

template<typename wnd_t>
struct wnd {
    std::unique_ptr<wnd_t> impl; // before C++11, use std::auto_ptr

    wnd() : impl(new wnd_t) {
    }

    wnd(const create_info& ci) : impl(new wnd_t)  {   
        impl->c_info = ci;
    }
};

int main() {
    wnd<window> w1;

    w1 = create_info();
    cout << w1.impl->c_info.text << '\n';

    wnd<window> w2 = create_info();
    cout << w2.impl->c_info.text << '\n';
}

[aj3423]

Thanks,
but here's another problem that i forgot to say in the first post, three windows are created

struct window {
	create_info c_info;

	window() {
		cout << "window instance creating" << endl;//this displays three times
	}
};

so I didn't initialize impl in the second constructor.

[Cubbi]

That's because w2 = create_info(); creates and destroys a temporary in this case. You could add an assignment operator that takes a create_info on the right, if you need to avoid that temporary:

   wnd& operator=(const create_info& ci) {
        impl->c_info = ci;
        return *this;
    }
};

Although this is questionable usage: I would make the constructor from create_info explicit and disallow assignment from create_info altogether. Do you have a use for a default-constructed wnd?

[aj3423]

At first I thought the wnd should have default create_info even if not explicitly constructed with a create_info, maybe it's not necessary:)
anyway the operator=() works fine

Thanks Cubbi.