RootGraphicsEnabler.cxx

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// ROOT libraries
#include "TROOT.h"
#include "TApplication.h"
#include "TGClient.h"
#include "TSystem.h"
#include "TVirtualX.h"
#include "TGX11.h" // this header currently triggers a compiler error,
                   // that we had to disable via compiler flag -Wno-variadic-macros

// C/C++ standard libraries
#include <iostream>
#include <string>
#include <stdexcept> // std::runtime_error
#include <cstdlib> // getenv()


namespace { // local namespace

  struct RootGraphicsEnablerClass {

    RootGraphicsEnablerClass() { EnableRootGraphics(); }
    
    RootGraphicsEnablerClass(std::ostream& out) { EnableRootGraphics(&out); } 
  
    static void EnableRootGraphics(std::ostream* out = nullptr);
    
  }; // RootGraphicEnablerClass
  
  // static instance that is here only to ensure the initialization function is called;
  // this is currently verbose
  RootGraphicsEnablerClass RootGraphicsEnabler{
#if not defined(NDEBUG) // output only if debug is not suppressed
    std::cout
#endif
    };
  
  
  void RootGraphicsEnablerClass::EnableRootGraphics
    (std::ostream* out /* = nullptr */)
  {
    
    if (out) (*out) << "RootGraphicsEnablerClass hacking its way forth." << std::endl;
    
    //======================================================================
    // Setup the root environment for a program started with no arguments
    //======================================================================
    
    if (out) (*out) << "  ==> get the current TApplication (and make sure gROOT is valid)" << std::endl;
    TApplication* app = ROOT::GetROOT()->GetApplication();

    // ROOT::GetROOT() should initialize gROOT.    
    if (!gROOT)
      throw std::runtime_error("RootGraphicsEnabler: no ROOT global pointer"); 
    
    if (!app) {
      if (out) (*out) << "  ==> create a TApplication" << std::endl;
      int    argc = 0;
      char** argv = nullptr;
      new TApplication("TApplicationFromRootGraphicsEnabler", &argc, argv);
    } // if no application
    
    if (out) (*out) << "  ==> set batch mode off (now it is " << (gROOT->IsBatch()? "on": "already off") << std::endl;
    gROOT->SetBatch(kFALSE);
    
    if (!gClient) {
      if (out) (*out) << "  ==> creating a TGClient" << std::endl;
      
      if (out) (*out) << "      ==> loading graphics library (X11)" << std::endl;
      int res = gSystem->Load("libGX11.so");
      if (out) {
        switch (res) {
          case  0: break; // successfully loaded
          case  1: (*out) << "          (it was already)" << std::endl; break;
          case -1: (*out) << "          ERROR: not found!" << std::endl; break;
          case -2: (*out) << "          ERROR: version mismatch!" << std::endl; break;
          default: (*out) << "          ERROR: undocumented (code=" << res << ")" << std::endl; break;
        } // switch
      }
      
      if (out) (*out) << "      ==> creating TVirtualX" << std::endl;
      gVirtualX = new TGX11("X11", "X11 session");
      
      std::string const DISPLAY = getenv("DISPLAY");
      
      if (out) (*out) << "      ==> creating the TGClient (DISPLAY='" << DISPLAY << "')" << std::endl;
      new TGClient(DISPLAY.c_str());
    } // if no graphics client

    if (out) (*out) << "RootGraphicsEnablerClass hacking compleled." << std::endl;
    
  } // RootGraphicsEnabler::EnableRootGraphics()

} // local namespace