Why is there not any warning on a declaration without initialization in a for loop?

I tried to compile the following code using g++ (gcc version 4.8.2 (Debian 4.8.2-1)), with -Wall flag (adding the -Wextra flag does not change anything for me).

#include <iostream> using namespace std ; int main() { int i ; cout << i << endl ; } 

It gave this warning:

test.cpp: In function ‘int main()’: test.cpp:7:13: warning: ‘i’ is used uninitialized in this function [-Wuninitialized] cout << i << endl ; 

But the following code does not yield any warning:

#include <iostream> using namespace std ; int main() { for(int i ; i < 10 ; i++) { cout << i << endl ; } } 

I did further tests.

The following yields the warning:

#include <iostream> using namespace std ; int main() { int i ; while(i<10) { cout << i << endl ; } } 

But the following does not:

#include <iostream> using namespace std ; int main() { int i ; while(i<10) { cout << i << endl ; i++ ; } } 

In the above program, if I replace the while by an if, then I have a warning.

Is there some explanation to this? Why can the compiler recognize the problem in some cases and not in others, although they seem very close?