Question

我有std::unordered_map我用存储区大小100初始化了。当我更改max_load_factor时，代码在访问bucket_size()时给出了段错误。我在linux上使用g ++作为编译器。我想增加负载系数，以便元素发生碰撞。

1＆GT;为什么我会遇到段错？ 2 - ;为max_load_factor设置unordered_map的正确方法是什么？据我所知，std::unordered_map的构造函数不接受加载因子作为参数。

没有设置max_load_factor的代码，没有问题

// unordered_map::bucket_size
#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

int main ()
{
  std::unordered_map<int, std::string> mymap(10);
  unsigned nbuckets = mymap.bucket_count();
  std::cout << "mymap has " << nbuckets << " buckets:\n";
  std::cout << "mymap load factor " << mymap.max_load_factor() << endl;

  for (unsigned i=0; i<nbuckets; ++i) {
    std::cout << "bucket #" << i << " has " << mymap.bucket_size(i) << " elements.\n";
  }
  return 0;
}

输出

$ g++ -std=c++11  map.cpp  && ./a.out
mymap has 11 buckets:
mymap load factor 1
bucket #0 has 0 elements.
bucket #1 has 0 elements.
bucket #2 has 0 elements.
bucket #3 has 0 elements.
bucket #4 has 0 elements.
bucket #5 has 0 elements.
bucket #6 has 0 elements.
bucket #7 has 0 elements.
bucket #8 has 0 elements.
bucket #9 has 0 elements.
bucket #10 has 0 elements.

现在，一旦我介绍了更改max_load_factor的代码，我就会遇到段错误。

#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

int main ()
{
  std::unordered_map<int, std::string> mymap(10);
  unsigned nbuckets = mymap.bucket_count();
  std::cout << "mymap has " << nbuckets << " buckets:\n";
  mymap.max_load_factor(10);
  std::cout << "mymap load factor " << mymap.max_load_factor() << endl;

  for (unsigned i=0; i<nbuckets; ++i) {
    std::cout << "bucket #" << i << " has " << mymap.bucket_size(i) << " elements.\n";
  }
  return 0;
}

输出

$ g++ -std=c++11  map.cpp  && ./a.out
mymap has 11 buckets:
mymap load factor 10
bucket #0 has 0 elements.
bucket #1 has 0 elements.
bucket #2 has 0 elements.
Segmentation fault

Answer 1

我怀疑在max_load_factor发生变化后，无法保证容器是否会保持相同的桶数。如果是这种情况，您可能会迭代无效数量的存储桶，因为nbuckets是在代码中先前定义的。

更改max_load_factor（）导致std :: unordered_map

1 个答案: