Question

The following simple program segfaults on my mac (Lion) running gcc 4.2.1:

#include <iostream>

using namespace std;
struct A{
  friend std::ostream& operator << (std::ostream& os, const A& a) {
    os << 3 << endl;
  }
};

template <typename T>
T f() { return T();}

int f() { return 2;}


int main() {
  cout << f() << endl;

  A a= f<A>();
  cout << a << endl;
}

When I run the program, I get:

 ./a.out
2
3
Segmentation fault: 11

When I do a stacktrace, I get:

(gdb) run

Starting program: a.out 
unable to read unknown load command 0x24
unable to read unknown load command 0x26
2
3

Program received signal SIGSEGV, Segmentation fault.
0x00007fff8b84fa49 in ?? ()
(gdb) bt
#0  0x00007fff8b84fa49 in ?? ()
#1  0x00007fff665c1ae8 in ?? ()
#2  0x0000000000000000 in ?? ()

The backtrace has no useful information (anyone know why?). This works
fine in linux.

Answer 1

Let’s make it clearer what happens

(cout << a) << endl;

You forgot a return in your operator<<.