C++11 segfault on reference to unordered_map in vector

Question

When i run the following code I get a segfault on the 'find()' call.

#include <vector>
#include <unordered_map>

struct item {
    std::unordered_map<int,int> map;
};

int main(int argc, char** argv) {
   std::vector<item> stack;
   stack.emplace_back();
   std::unordered_map<int,int>& topmap=stack.back().map;
   stack.emplace_back();
   auto ind=topmap.find(5);
   if(ind!=topmap.end()) {
      printf("Found element five in second to top item\n");
   }
}

(compiled with -std=c++11)

However, if the second emplace_back() call is removed, there is no segfault.

Why is this? Am I using reference variables wrong? Does adding another element to the stack invalidate topmap?

score 3 · Accepted Answer · answered Jul 30 '14 at 20:18

3

The most likely explanation is that the second call to emplace_back causes a re-allocation, invalidating topmap. You can easily check this by looking at stack.capacity(). If that changes between calls, there has been a re-allocation.

answered Jul 30 '14 at 20:18

juanchopanza

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2

Woops, this is obviously the explanation. I should use std::list instead if I want my references to remain valid. – Jeremy Salwen Jul 30 '14 at 20:21

score 2 · Answer 2 · answered Jul 30 '14 at 20:22

If you read e.g. this reference, you will see the text

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated."

Adding new items to the end of a vector typically reallocates the data inside the vector and causes this behavior. More specifically, since map is an ordinary instance inside the structure, it will too be reallocated.

score 1 · Answer 3 · answered Jul 30 '14 at 20:21

Does adding another element to the stack invalidate topmap?

Not mandatory, but it can. It depends if reallocation happened when std::vector::emplace_back() is called. std::vector::emplace_back documentation:

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

C++11 segfault on reference to unordered_map in vector

3 Answers3