S-expression lexer in C++11

lexer.cpp

#include <iostream>
#include <sstream>

namespace ya {

void each_token(std::istream& input, void (*proc)(const std::string& token))
{
    std::stringstream buffer;
    auto send = [&buffer, &proc]() {
	if (0 < buffer.tellp()) {
	    proc(buffer.str());
	    buffer.str("");
	    return true;
	}
	return false;
    };
    while (!input.eof()) {
	char c = input.get();
	switch (c) {
	case EOF:
	case ' ':
	case '\r':
	case '\n':
	case '\t':
	case '\v':
	    send();
	    break;
	case '(':
	case ')':
	case '[':
	case ']':
	case '\'':
	case '`':
	    if (send()) {
		input.unget();
	    } else {
		buffer << c;
		send();
	    }
	    break;
	case ',':
	    if (send()) {
		input.unget();
	    } else {
		buffer << c;
		if ('@' == input.peek()) {
		    buffer << (char) input.get();
		}
		send();
	    }
	    break;
	case '\"':
	    if (send()) {
		input.unget();
	    } else {
		buffer << c;
		bool escaped = false;
		while (!input.eof()) {
		    char c = input.get();
		    buffer << c;
		    if (!escaped && c == '"') {
			send();
			break;
		    } else if (c == '\\') {
			escaped = true;
		    } else {
			escaped = false;
		    }
		}
	    }
	    break;
	default:
	    buffer << c;
	    break;
	}
    }
    send();
}

}

int main()
{
    std::stringstream input("`(a ,b ,@(c) \"d e\")");
    ya::each_token(input, [](const std::string& token) {
	    std::cout << token << std::endl;
	});
    return 0;
}