C++默認構造器

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

class Example3 {
    string data;
public:
    Example3 (const string& str) : data(str) {}
    Example3 () {}
    const string& content () const {return data;}
};

int main(int argc, char const *argv[])
{
    Example3 foo;
    Example3 bar ("Test");
    cout << "bar's content: " << bar.content() << endl;
    return 0;
}