C++ compiler warns uninitialized local variable of int, but not of object.

ex2.cpp

#include <iostream>
#include <string>

void fn() {
    int x;
    std::string s;
    std::cout << x;   // warning: variable 'x' is uninitialized when used here
    std::cout << s;   // (no warnings due to default constructor)
    std::cout << "\n";
}

int main(void) {
    fn();
    return 0;
}

/****

$ g++ -Wall ex2.cpp
ex2.cpp:7:18: warning: variable 'x' is uninitialized when used here
      [-Wuninitialized]
    std::cout << x;   // warning: variable 'x' is uninitialized when used here
                 ^
ex2.cpp:5:10: note: initialize the variable 'x' to silence this warning
    int x;
         ^
          = 0
1 warning generated.

$ ./a.out
32767

$ g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.3.0
Thread model: posix

 ****/