Question about Draw() and DrawClone

ROOT
8

Your histogram hist2 was not a pointer in your original code so it did not survive after the execution (see the references @Wile_E_Coyote sent you). DrawClone creates a Clone of the histogram before drawing it, as a pointer … that’s why it survives. The simplest way to write your macro is:

void hw1(){
    auto c = new TCanvas("c","TCanvas",10,10,800,800);

    int npoints = 50000;
    TRandom3 xgenerator, ygenerator;
    xgenerator.SetSeed(0);
    ygenerator.SetSeed(0);
    auto hist2 = new TH2F("hist2","histogram",600,-3,3,600,-3,3);
    for(int i=0; I<npoints ;i++){
        float x = xgenerator.Uniform(-3,3);
        float y = ygenerator.Uniform(-3,3);
        hist2->Fill(x,y);
    }

    hist2->Draw("COLZ");
}
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