#include <cassert>
#include <sstream>
#include <stdexcept>
#include <string>

class Sink {
public:
    template<class T>
    void put(const T& value) {
        std::ostringstream stream;
        stream << value;
        if (!stream) throw std::runtime_error("conversion failed");
        write(stream.str());
    }
    virtual ~Sink() = default;
private:
    virtual void write(const std::string& text) = 0;
};

class Buffer final : public Sink {
    std::string data_;
    void write(const std::string& text) override { data_ += text; }
public:
    const std::string& data() const { return data_; }
};

int main() {
    Buffer buffer;
    Sink& sink = buffer;
    sink.put(12);
    sink.put(" apples");
    assert(buffer.data() == "12 apples");
}