Question

#include <iostream> using namespace std;  class Foo {  public:   Foo(): initialised(0)  {   cout << 'Foo() gets called AFTER test() ?!' << endl;  };   Foo test()  {   cout << 'initialised= ' << initialised << ' ?! - ';   cout << 'but I expect it to be 0 from the 'initialised(0)' initialiser on Foo()' << endl;   cout << 'this method test() is clearly working on an uninitialised object ?!' << endl;   return Foo();  }   ~Foo()  {};  private:   int initialised;  };   int main() {   //SURE this is bad coding but it compiles and runs  //I want my class to DETECT and THROW an error to prevent this type of coding  //in other words how to catch it at run time and throw 'not initialised' or something   Foo foo=foo.test();  } 

Answer 1

You can’t prevent people from coding poorly, really. It works just like it ‘should’:

1. Allocate memory for Foo (which is the value of the ‘this’ pointer)
2. Going to Foo::test by doing: Foo::test(this), in which,
3. It gets the value by this->initialised, which is random junk, then it
4. Calls Foo’s default constructor (because of return Foo();), then
5. Call Foo’s copy constructor, to copy the right-handed Foo().

Just like it should. You can’t prevent people from not knowing the right way to use C++.

The best you could do is have a magic number:

class A { public:     A(void) :     _magicFlag(1337)     {     }      void some_method(void)     {         assert (_magicFlag == 1337); /* make sure the constructor has been called */     }  private:     unsigned _magicFlag; } 

This ‘works’ because the chances _magicFlag gets allocated where the value is already 1337 is low.

But really, don’t do this.