Question

The first print shows the member value to be false, and the other two prints show it as true. Why does the first output differ from the last two?

#include <vector>
#include <iostream>

using namespace std;

class MyClass
{
public:
  bool value;
  bool stuff;
};

class Container
{
public:
  vector<MyClass> my_classes;
  Container()
  {
    MyClass c;
    cout << c.value << endl;
    my_classes.push_back(c);
  }
};

int main (int argc , char* argv[] )
{
  MyClass mc;
  cout << mc.value << endl;
  Container con;
  cout << con.my_classes[0].value << endl;
  return 0;
}

Answer 1

The two members are primitive (non-class) objects and thus uninitialized. That means that their values will be arbitrary at runtime. You must initialize them in the constructor:

class MyClass {
public:
  bool value;
  bool stuff;
  MyClass(): value(false), stuff(false) { }
};