The emplace_back() Trap

A not-so-obvious caveat when using emplace_back()

To reproduce the issue i recently came across, here is the smallest self-contained example showing the problem.

struct Sample {
  int value;
  int* alias;

  explicit Sample(int val = 0) : value(val), alias(&value) {}
};

std::ostream& operator<<(std::ostream& os, const Sample& sample) {
  os << "{ value=" << sample.value
     << " , alias=" << sample.alias
     << " , &value=" << &sample.value << " }";
  return os;
}

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

  vec.emplace_back(1);
  std::cout << "after 1st emplace: vec[0] " << vec[0] << "\n";

  // memroy reallocation
  vec.emplace_back(2);
  std::cout << "after 1st emplace: vec[0] " << vec[0] << "\n";

  // WARNING: Dereferencing vec[0].alias here is undefined behavior.
  std::cout << "*alias = " << *(vec[0].alias) << "\n";
}

output:

after 1st emplace: vec[0] { value=1 , alias=0x5a1202d0d2b0 , &value=0x5a1202d0d2b0 }
after 1st emplace: vec[0] { value=1 , alias=0x5a1202d0d2b0 , &value=0x5a1202d0e2e0 }
*alias = -1591726835

The issue is:
When calling emplace_back(), memory will be reallocated to fit more data, and elements will be copied/moved to new addresses. However, float* alias is just copied/moved as a raw address, meaning its value will not be modified.

To fix this issue, we can explicitly override the default copy/move constructors that compilers generate.

struct Sample {
  int value = 0;
  int* alias = &value;

  Sample() = default;
  explicit Sample(int v) : value(v), alias(&value) {}

  // Rebind alias for copies
  Sample(const Sample& rhs) : value(rhs.value), alias(&value) {}
  Sample& operator=(const Sample& rhs) {
    value = rhs.value; 
    alias = &value; 
    return *this;
  }
  // Rebind alias for moves
  Sample(Sample&& rhs) noexcept : value(rhs.value), alias(&value) {}
  Sample& operator=(Sample&& rhs) noexcept {
    value = rhs.value; 
    alias = &value; 
    return *this;
  }
};

std::ostream& operator<<(std::ostream& os, const Sample& sample) {
  os << "{ value=" << sample.value
     << " , alias=" << sample.alias
     << " , &value=" << &sample.value << " }";
  return os;
}

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

  vec.emplace_back(1);
  std::cout << "after 1st emplace: vec[0] " << vec[0] << "\n";

  // memroy reallocation
  vec.emplace_back(2);
  std::cout << "after 1st emplace: vec[0] " << vec[0] << "\n";

  // WARNING: Dereferencing vec[0].alias here is undefined behavior.
  std::cout << "*alias = " << *(vec[0].alias) << "\n";
}

output:

after 1st emplace: vec[0] { value=1 , alias=0x5eee05be82b0 , &value=0x5eee05be82b0 }
after 1st emplace: vec[0] { value=1 , alias=0x5eee05be92e0 , &value=0x5eee05be92e0 }
*alias = 1

It’s not just emplace_back: operations such as push_back, insert, emplace, and resize can also expose the issue if they involve rearranging the container’s data.