Default values for implicit Default constructor in C++

Question

I was going through the C++ Object model when this question came. What are the default values for the data members of a class if the default constructor is invoked?

For example

class A
{
     int x;
     char* s;
     double d;
     string str;       // very high doubt here as string is a wrapper class
     int y[20];
     public :
     void print_values()
     {
         cout<<x<<' '<<s<<' '<<d<<' '<<str<<' '<y[0]<<' '<<y<<endl;
     }
}

int main()
{
    A temp;
    temp.print_values(); // what does this print?
    return 0;
}

All are uninitialized besides the string, so can contain almost anything. — goji
– goji, Commented Oct 1, 2013 at 1:18
You can initialize them all within the default constructor to whatever values you want. — nhgrif
– nhgrif, Commented Oct 1, 2013 at 1:20
@nhgrif I understand that I could do it.But I wanted to know what happens the implicit default constructor by the compiler is invoked. — user1429322
– user1429322, Commented Oct 1, 2013 at 1:24

Adam · Accepted Answer · 2013-10-01 01:27:30Z

1

The value of an un-initialized variable is undefined, no matter where the variable lives.

Undefined does not necessarily mean zero, or anything in particular. For example, in many debug builds the memory is filled with a pattern that can be used to detect invalid memory accesses. These are stripped for release builds where the memory is simply left as it was found.

answered Oct 1, 2013 at 1:27

Adam

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1 Comment

sh03 Over a year ago

In the case of std::string the variable is defined, though. While using any other variable, in that class, may yield undefined behavior, str does not.

aout · Accepted Answer · 2013-10-01 01:22:30Z

1

You can't really predict what's going to be in your memory when you're allocating it. There could be pretty much anything as the memory you're reading has not been set to 0 (or anything else should I say). Most of the time you'll find the values to be 0 for numeric values in little executables.

answered Oct 1, 2013 at 1:22

aout

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