EmailProcessor.h

#include <functional>
#include <string>

class EmailProcessor
{
public:
    void receiveMessage (const std::string& message)
    {
        if ( _handler_func ) 
        {
            _handler_func( message );
        }
        // other processing
    }
    void setHandlerFunc (std::function<void (const std::string&)> handler_func)
    {
        _handler_func = handler_func;
    }

private:
        std::function<void (const std::string&)> _handler_func;
};

main.cpp

EmailProcessor processor;
MessageSizeStore size_store;
processor.setHandlerFunc( 
        [&] (const std::string& message) { size_store.checkMessage( message ); } 
);

MessageSizeStore.h

#include <string>

class MessageSizeStore
{
    public:
    MessageSizeStore () : _max_size( 0 ) {}
    void checkMessage (const std::string& message ) 
    {
        const int size = message.length();
        if ( size > _max_size )
        {
            _max_size = size;
        }
    }
    int getSize ()
    {
        return _max_size;
    }

private:
    int _max_size;
};