C ++中未初始化变量的默认值是什么？

Question

Below, you see a very very simple program :

#include <stdlib.h>
#include <iostream>

using namespace std;

int main()
{
    int r;
    int c;
    int d;

    cout<<"r="<<r<<endl<<"c="<<c<<endl<<"d="<<d;
    return 0;
}

The question is , why in the output the value of c is different from the value of r and d ?

When I change the order of them in printing on the screen, the value change!

#include <stdlib.h>
#include <iostream>

using namespace std;

int main()
{
    int r;
    int c;
    int d;

    cout<<"c="<<c<<endl<<"r="<<r<<endl<<"d="<<d;    

    return 0;
}

Why this time the value of r is different from the others? I just change the order of printing them on the screen!

Answer 1

What is the default value for not initialized variables in C++

The default value is indeterminate for local variables of fundamental types.

The question is , why in the output the value of c is different from the value of r and d?

Because reading uninitialized variable has undefined behaviour.

When I change the order of variables in cout line, the value changes!

If you're surprised by this, then you've not understood yet what undefined behaviour or unspecified value means.

The value of your uninitialized values can be anything and any change to your code, the change of compiler, the change of processor even the altitude of moon* may change the garbage. Or it might not. In this case it did.

* not very likely a feature in most c++ compilers

Answer 2

Values of uninitialized variables declared in a function is undefined.

Global variables will be set to 0 (or equivalent if it's not a number).