uninitialised variable but NO error

[geek.arnuld]

this is the programme: [1]

#include <iostream> // std::cout

int main()
{
int sum;
std::cout << sum << "\n";
}


when i compile it using gcc 4.1.2, i get this:

----------------------------------------------------------------------------
[unix@arch cpp]$ g++ first.cpp -o first
[unix@arch cpp]$ ./first
sum is: -1208774992
[unix@arch cpp]$
-----------------------------------------------------------------------------


i think the programme is wrong BUT why this does not give any error
like "uninitialised variable"?





[1] http://home.no.net/dubjai/win32cpptut/html/w32cpptut_01_02_02.html

 

[Mike Wahler]

this is the programme: [1]

#include <iostream> // std::cout

int main()
{
int sum;
std::cout << sum << "\n";
}


when i compile it using gcc 4.1.2, i get this:

----------------------------------------------------------------------------
[unix@arch cpp]$ g++ first.cpp -o first
[unix@arch cpp]$ ./first
sum is: -1208774992
[unix@arch cpp]$

It does not because your code does not violate any language rules.
However, when you execute it, it will produce 'undefined
behavior' (even if it 'appears to work OK').

like "uninitialised variable"?

Some compilers can and do issue a 'warning' about this,
but there's no language requirement that one must.

Also, if your compiler can warn about this, it's possible
that you'll need to configure it to do so. Check your
documentation.

-Mike

 

[Rolf Magnus]

this is the programme: [1]

#include <iostream> // std::cout

int main()
{
int sum;
std::cout << sum << "\n";
}


when i compile it using gcc 4.1.2, i get this:

----------------------------------------------------------------------------
[unix@arch cpp]$ g++ first.cpp -o first
[unix@arch cpp]$ ./first
sum is: -1208774992
[unix@arch cpp]$
-----------------------------------------------------------------------------


i think the programme is wrong BUT why this does not give any error
like "uninitialised variable"?

Because it doesn't violate the syntax rules of C++. It just invokes
undefined behavior. GCC can issue a warning in such a case, but only if you
enable optimizations (because only then it will do a more extensive code
flow analysis). To get a warning, add "-Wuninitialized -O" to the command
line options of g++.
As a minimum for the warning flags, you should add to the g++ command
line: "-ansi -pedantic -Wall -Wextra" (-Wall includes -Wuninitialized).