Pub/Sub Broker in C++ but it's not threaded

broker.hpp

#include <vector>
#include <map>

namespace postal {
  
  template< class T >
    class broker {
      public:
      
        typedef std::vector< std::function< void ( T ) > > FuncVector;
      
        broker() {}
      
        void subscribe( std::string channel, std::function< void ( T )> func ) {
          if( _exists( channel ) ) {
            
            m_channels[ channel ].push_back( func );
            
          } else {
            
            FuncVector func_vec = { func };
            auto pair = std::make_pair( channel , func_vec );
            m_channels.insert( pair );
            
          }
        }
      
        void publish( std::vector< std::string > channels, T message ) {
          for( auto chan :  channels ) {
            publish( chan, message );
          }
        }
      
        void publish( std::string channel, T message ) {
          if( _exists( channel ) ) {
            for( auto& func : m_channels[ channel ] ) {
              func( message );
            }
          }
        }
      
        void publish_all( T message ) {
          for( auto chan : m_channels ) {
            for( auto func : chan.second ) {
              func( message );
            }
          }
        }
      
      private:
        bool _exists( std::string channel ) {
          return ( m_channels.count( channel ) > 0 ) ? true : false;
        }
      
        std::map< std::string, FuncVector > m_channels;
    };
  
}

main.cpp

#include "broker.h"

//
// CHANNELS
//
#define BLOG_HITS "blog/hits"
#define HOME_PAGE_HITS "home/hits"


//
// TESTER
//
int main(int argc, const char * argv[]) {

  postal::broker<int> broker;
  
  broker.subscribe( BLOG_HITS, []( int message ) {
    
    std::cout << "BLOG: " << message << std::endl;
    
  });
  
  broker.publish( HOME_PAGE_HITS, 9999999);
  
  broker.subscribe( HOME_PAGE_HITS, []( int message ) {
    
    std::cout << "HOME PAGE: " << message << std::endl;
    
  });
  
  broker.publish( BLOG_HITS, 34);
  
  broker.publish( std::vector< std::string >({ BLOG_HITS, BLOG_HITS, BLOG_HITS }), 55 );
  
  broker.publish_all( 22 );

}

zz_output

BLOG: 34
BLOG: 55
BLOG: 55
BLOG: 55
BLOG: 22
HOME PAGE: 22