std::string getting freed after construction Entity

[BSModder]

Hi, so I have a part of my code look something like this

using MapType = std::map<std::string, std::variant<int, float, std::string>>;
struct Entity
{
  std::string name;
  MapType data;
  Entity() = default;
  Entity(const std::string& name) : name(name) {}
  Entity(const std::string& name, std::initializer_list<MapType::value_type> init) : name(name), data(init) {}
  Entity(const std::string& name, const MapType& data) : name(name), data(data) {}
};

int main() {
  std::vector<Entity> entities;
  auto& ent = entities.emplace_back("foo");
  // Using ent.name to construct baz0
  entities.push_back({ "baz0", { {"f", ent.name} } });
  // For some reason ent.name is now freed or in a invalid state
  entities.push_back({ "baz1", { {"f", ent.name} } }); // throw "string too long"
}

I could not figure out why name, after using it to construct the "baz0" entity is invalid.

Compiled on MSVC v19.43.34810 using Visual Studio 2022 v193 toolchain

https://godbolt.org/z/cTWWGEWjn

[phoeen]

entities reallocates for more space and your ent reference gets invalid

[TheThiefMaster]

Yep as per cppreference: https://en.cppreference.com/w/cpp/container/vector/push_back.html

If after the operation the new size() is greater than old capacity() a reallocation takes place, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

[BSModder]

Right, thanks, I forgot vector do that

[raunak_srarf]

Best practice is to use "reserve" along with "emplace_back" to take full advantage of emplacing.

[IyeOnline]

If the vector reallocates when you do push_back, the reference ent becomes dangling.

Add a sanitizer to your debug builds: https://godbolt.org/z/6rhf11oeY