gistfile1.txt

// some header file
    // Allow applications to add configuration settings if HPX_MAIN is set          
    std::vector<std::string> user_main_config(                         
        std::vector<std::string> const& config);                                    
                                                                                    
    // Make sure our configuration information is injected into the startup         
    // procedure                                                                    
    struct register_user_main_config                                   
    {                                                                               
        register_user_main_config();
        
        static std::vector<std::string> (*prev_user_main_config_function)(                   
            std::vector<std::string> const&) = nullptr;
    };                                                                              
                                                                                    
    extern register_user_main_config cfg;

// some source file
                                                                                      
    std::vector<std::string> user_main_config(                                    
        std::vector<std::string> const& config)                                   
    {                                                                             
        std::vector<std::string> cfg(config);                                     
                                                                                      
        // rx jobs should run on at least two cores to avoid deadlocks            
        cfg.push_back("hpx.os_threads=2");                                        
                                                                                      
        // disable the aliasing of HPX's command line options                     
        // (this allows for the short options as defined by Catch)                
        cfg.push_back("hpx.commandline.aliasing=0");                              
                                                                                      
        // If there was another config function registered, call it               
        if (register_user_main_config::prev_user_main_config_function)                                       
            return register_user_main_config::prev_user_main_config_function(cfg);                           
                                                                                      
        return cfg;                                                               
    }                                                                             
                                                                                      
    // make sure user_main_config is registered                                   
    register_user_main_config cfg;                                                
                                                                                      
    register_user_main_config::register_user_main_config()                        
    {                                                                             
        prev_user_main_config_function =                                          
            hpx_startup::user_main_config_function;                               
        hpx_startup::user_main_config_function = &user_main_config;               
    }