Here's a small test program I wrote:

#include <iostream>
using namespace std;

class A {

    public:
    int val;

    A(int _val=0):val(_val) { }

    A operator+(A &a) { return A(val + a.val); }
    A operator-(A &a) { return A(val - a.val); }

    friend ostream& operator<<(ostream &, A &);

};

ostream& operator<<(ostream &out, A &a) {
    out<<a.val;
    return out;
}

int main() {
    A a(3), b(4), c = b - a;
    cout<<c<<endl; // this works
    cout<<(b-a)<<endl; // this doesn't
    return 0;
}

I can't seem to get why the line marked "this works" works and the one marked "this doesn't" doesn't. When I try to compile the program with the cout<<(b-a); line, here's what I get:

[felix@the-machine C]$ g++ test.cpp 

test.cpp: In function ‘int main()’:
test.cpp:26:13: error: no match for ‘operator<<’ in ‘std::cout << b.A::operator-(((A&)(& a)))’
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:108:7: note: candidates are: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:117:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>, std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:127:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:165:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:169:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:173:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:91:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:180:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:105:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:191:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:200:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:204:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:209:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:213:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:221:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:225:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:119:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
test.cpp:18:11: note:                 std::ostream& operator<<(std::ostream&, A&)
[felix@the-machine C]$

Quite nasty.