Building a std::map with a std::string parameter and an non-movable/copyable parameter

I'm trying to build a simple log system, but I've met a problem with my concept:

class logger_instance
{
    std::string log_file;
    std::string log_type;
    std::ofstream log;

public:
    logger_instance(std::string log_type) : log_type(log_type), log_file("logs\\" + log_type + ".log"), log(log_file)
    {
        operator << ("logger_instance(" + log_type + ") init.\n");
    }

    std::string get_log_type() const
    {
        return log_type;
    }

    std::string get_log_file() const
    {
        return log_file;
    }


    logger_instance& operator << (std::string message)
    {
        time_t current_time;
        time(&current_time);

        log << ctime(&current_time);
        log << " : ";
        log << message;
        log << "\n";
    }
};

class logger_manager
{
    std::map < std::string, logger_instance > loggers;

public:
    void add_logger(std::string logger)
    {
        auto it = loggers.find(logger);

        if (it == loggers.end())
        {
            loggers.emplace(logger, logger);
        }
    }

    logger_instance& operator[](std::string logger)
    {
        return loggers[logger];
    }
};

Compiler : Visual Studio 2013 Update 4

The problem which I'm encountering is that the emplace function doesn't seem to understand that I want the logger_instance object to be built on-the-spot, in the map container. Given the fact that I'm not smarter than a compiler, the problem is in my concept, but I'm unable to find a solution.