iosfwd.cpp

// test_class.hpp
#include <iosfwd>

class TestClass {
public:
    TestClass() = default;
    void hello_to_stream(std::stringstream& ss) const noexcept;
};


// test_class.cpp
#include "test_class.hpp"
#include <sstream>

void TestClass::hello_to_stream(std::stringstream& ss) const noexcept {
    ss << "HELLO";
}