reference

References to elements of a vector

When using references it's important to note that if the reference is pointing to a place in a vector that that reference may get invalidated if the vector has to reallocate, see this:

  
#include <iostream>
#include <vector>

struct MyStruct {
    int value;
};

int main() {
    std::vector<mystruct> vec;

    // add one element
    vec.push_back({42});

    // get a reference to the first element
    MyStruct& ref = vec[0];
    std::cout << "Before adding more elements, ref.value = " << ref.value << std::endl;

    // add more elements, likely causing reallocation
    for (int i = 0; i < 1000; ++i) {
        vec.push_back({i});
    }

    // the reference may now be invalid!
    std::cout << "After adding more elements, ref.value = " << ref.value << " (undefined behavior!)" << std::endl;

    return 0;
}
  

On my system the result of the above code is this:

  
Before adding more elements, ref.value = 42
After adding more elements, ref.value = -1170156367 (undefined behavior!)