为什么 C++ STL 映射是慢 Java 的映射类？

Question

My input file is 2GB and in this file every line is a word. I need to write a program to do wordcount. I use Java and C++ do to the same task but result is surprising: C++ is too slow! My code are as follows:

C++:

int main() {

    struct timespec ts, te;
    double cost;
    clock_gettime(CLOCK_REALTIME, &ts);

    map<string, int> map;    
    ifstream fin("inputfile.txt");
    string word;
    while(getline(fin, word)) {
        ++map[word];
    }

    clock_gettime(CLOCK_REALTIME, &te);    
    cost = te.tv_sec - ts.tv_sec + (double)(te.tv_nsec-ts.tv_nsec)/NANO;
    printf("cost: %-15.10f s\n", cost);

    return 0;
}

Output: cost: 257.62 s

Java:

public static void main(String[] args) throws Exception {

    long startTime = System.currentTimeMillis();
    Map<String, Integer> map = new HashMap<String, Integer>();
    FileReader reader = new FileReader("inputfile.txt");
    BufferedReader br = new BufferedReader(reader);

    String str = null;
    while((str = br.readLine()) != null) {
        Integer count = map.get(str);
        map.put(str, count == null ? 1 : count + 1);
    }

    long endTime = System.currentTimeMillis();
    System.out.println("cost : " + (endTime - startTime)/1000 + "s");
}

Output: cost: 124 s

I delete the code inside the while, just read the file but do not do anything, the result is the same. Java cost: 32s, C++ cost: 38s. This gap I can accept. My environment is Ubuntu Linux 13.04 and C++ use -O2 optimation. Why does the STL perform poorly?

Answer 1

The C++std::map is an ordered data-structure usually implemented as a tree. A fairer comparison would be betweenjava.util.HashMap and std::unordered_map orjava.util.TreeMap and std::map .