对 STL 容器的安全并行只读访问

I pretend that their doesn't exist any faulty implementation and every implementation of the C++11 standard is correct.

示例:

// concurrent thread access to a stl container
// g++ -std=gnu++11 -o p_read p_read.cpp -pthread -Wall -pedantic && ./p_read
#include <iostream>
#include <iomanip>
#include <string>
#include <unistd.h>

#include <thread>
#include <mutex>

#include <map>

#include <cstdlib>
#include <ctime>
using namespace std;

// new in C++11
using str_map = map<string, string>;

// thread is new in C++11
// to_string() is new in C++11

mutex m;
const unsigned int MAP_SIZE = 10000;

void fill_map(str_map& store) {
    int key_nr;
    string mapped_value;
    string key;

    while (store.size() < MAP_SIZE) {
        // 0 - 9999
        key_nr = rand() % MAP_SIZE;

        // convert number to string
        mapped_value = to_string(key_nr);
        key = "key_" + mapped_value;

        pair<string, string> value(key, mapped_value);
        store.insert(value);
    }
}

void print_map(const str_map& store) {
    str_map::const_iterator it = store.begin();

    while (it != store.end()) {
        pair<string, string> value = *it;
        cout << left << setw(10) << value.first << right << setw(5) << value.second << "
";
        it++;   
    }
}

void search_map(const str_map& store, int thread_nr) {
    m.lock();
    cout << "thread(" << thread_nr << ") launched
";
    m.unlock();

    // use a straight search or poke around random
    bool straight = false;
    if ((thread_nr % 2) == 0) {
        straight = true;
    }

    int key_nr;
    string mapped_value;
    string key;
    str_map::const_iterator it;

    string first;
    string second;

    for (unsigned int i = 0; i < MAP_SIZE; i++) {

        if (straight) {
            key_nr = i;
        } else {
            // 0 - 9999, rand is not thread-safe, nrand48 is an alternative             
            m.lock();
            key_nr = rand() % MAP_SIZE;
            m.unlock();
        }

        // convert number to string
        mapped_value = to_string(key_nr);
        key = "key_" + mapped_value;

        it = store.find(key);

        // check result
        if (it != store.end()) {
            // pair
            first = it->first;
            second = it->second;

            // m.lock();
            // cout << "thread(" << thread_nr << ") " << key << ": "
            //      << right << setw(10) << first << setw(5) << second << "
"; 
            // m.unlock();

            // check mismatch
            if (key != first || mapped_value != second) {
                m.lock();
                cerr << key << ": " << first << second << "
"
                     << "Mismatch in thread(" << thread_nr << ")!
";
                exit(1);

                // never reached
                m.unlock();
            }
        } else {
            m.lock();
            cerr << "Warning: key(" << key << ") not found in thread("
                 << thread_nr << ")
";
            exit(1);

            // never reached
            m.unlock();
        }
    }
}

int main() {
    clock_t start, end;
    start = clock();

    str_map store;
    srand(0);

    fill_map(store);
    cout << "fill_map finished
";

    // print_map(store);
    // cout << "print_map finished
";

    // copy for check
    str_map copy_store = store;

    // launch threads
    thread t[10];
    for (int i = 0; i < 10; i++) {
        t[i] = thread(search_map, store, i);
    }

    // wait for finish
    for (int i = 0; i < 10; i++) {
        t[i].join();
    }
    cout << "search_map threads finished
";

    if (store == copy_store) {
        cout << "equal
";
    } else {
        cout << "not equal
";
    }


    end = clock();
    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "
";
    cout << "CPU-TIME START " << start << "
";
    cout << "CPU-TIME END " << end << "
";
    cout << "CPU-TIME END - START " << end - start << "
";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "
";

    return 0;
}

这段代码可以用 GCC 4.7 编译并在我的机器上运行良好.