C++ operator lookup rules / Koenig lookup
- by John Bartholomew
While writing a test suite, I needed to provide an implementation of operator<<(std::ostream&... for Boost unit test to use.
This worked:
namespace theseus { namespace core {
std::ostream& operator<<(std::ostream& ss, const PixelRGB& p) {
return (ss << "PixelRGB(" << (int)p.r << "," << (int)p.g << "," << (int)p.b << ")");
}
}}
This didn't:
std::ostream& operator<<(std::ostream& ss, const theseus::core::PixelRGB& p) {
return (ss << "PixelRGB(" << (int)p.r << "," << (int)p.g << "," << (int)p.b << ")");
}
Apparently, the second wasn't included in the candidate matches when g++ tried to resolve the use of the operator. Why (what rule causes this)?
The code calling operator<< is deep within the Boost unit test framework, but here's the test code:
BOOST_AUTO_TEST_SUITE(core_image)
BOOST_AUTO_TEST_CASE(test_output) {
using namespace theseus::core;
BOOST_TEST_MESSAGE(PixelRGB(5,5,5)); // only compiles with operator<< definition inside theseus::core
std::cout << PixelRGB(5,5,5) << "\n"; // works with either definition
BOOST_CHECK(true); // prevent no-assertion error
}
BOOST_AUTO_TEST_SUITE_END()
For reference, I'm using g++ 4.4 (though for the moment I'm assuming this behaviour is standards-conformant).
