Why is "unused variable" warning not reported for all variables? [duplicate]

Question

This question already has answers here:

g++ does not show a 'unused' warning (3 answers)

Closed 8 years ago.

I have this code:

// initializer lists
#include <iostream>
#include <vector>

int main()
{
    int values[] { 1, 2, 3 };

    std::vector<int> v { 4, 5, 6 };

    std::vector<std::string> cities {
        "London", "New York", "Paris", "Tokio"
    };

    return 0;
}

However the gcc compiler gives me unused variable warning only for values array. Why v and cities is not reported?

Answer 1

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

Classical example: a Timer object which measures the time between its construction and destruction: https://stackoverflow.com/a/5302868/1938163