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Can I access a C++11 std::map entry while inserting/erasing from another thread?

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半阙折子戏
半阙折子戏 2021-01-12 15:16

Can I access (without locking) an std::map entry while another thread inserts/erases entrys?

example pseudo C++:

typedef struct {
   int value;
   in
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  • 爱一瞬间的悲伤
    爱一瞬间的悲伤 (楼主)
    2021-01-12 15:50

    No no no no no!

    map::erase modifies the links between the map entries and that messes with the search algorithm used in map::at. You can use the elements during an erase, but you can not execute the searching algorithm itself!

    I've created an illustration program. On my machine this program sometimes print OK, sometimes throws a map exception - that means that the search is gone wrong. Increasing the number of reading threads make the exception appear more often.

    #include 
#include 
#include 
#include 

std::map m;

// this thread uses map::at to access elements
void foo()
{
  for (int i = 0; i < 1000000; ++i) {
    int lindex = 10000 + i % 1000;
    int l = m.at(lindex);
    assert(l == lindex);
    int hindex = 90000 + i % 1000;
    int h = m.at(hindex);
    assert(h == hindex);
  }
  std::cout << "OK" << std::endl;
}

// this thread uses map::erase to delete elements
void bar()
{
  for (int i = 20000; i < 80000; ++i) {
    m.erase(i);
  }
}

int main()
{
  for (int i = 0; i < 100000; ++i) {
    m[i] = i;
  }

  std::thread read1(foo);
  std::thread read2(foo);
  std::thread erase(bar);

  read1.join();
  read2.join();
  erase.join();

  return 0;
}

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