c++ - g++ does not show a 'unused' warning

Question

I have a little piece of code in C++:

#include <iostream>
#include <iterator>
#include <string>

using namespace std;

int main() {

    int i=0;
    istream_iterator<string> EOS;
    double x;

    return 0;
}

Now i compile it with my g++ (GCC) 4.4.4

g++ -W -Wall -pedantic test.cc -o test

And get:

test.cc: In function 'int main()':
test.cc:9: warning: unused variable 'i'
test.cc:11: warning: unused variable 'x'

Why there is no warning for unused EOS?

Answer 1

It is not a primitive value, so its constructor and/or destructor might have desired side effects.

To illustrate that this happens in practice: I use a class to time sections of code, that looks roughly like this:

class Timed {
    double start;
    public:
        Timed() { start = now(); }
        ~Timed() { std::cout << (now() - start) << '
'; }
}

So to measure how long a function takes, I simply do:

void slow() {
    Timed t;
    // heavy operation here...
}

The variable t never gets used, but it's still important to the behaviour of the code.