Wexiwa Wexiwa - 1 year ago 83
C++ Question

Pointer to template parameter type

I am trying to make my own container, which is just a wrapper around a

that keeps track of the insert order with a

Here is my (simplified) class :

template <class tKey, class tValue>
class OrderedMap
typedef size_t tSize;
inline bool contains(const tKey& key) const { return map_.count(key) > 0; }
inline tValue& at(const tSize& index) { return vector_[index]; }
inline tValue& operator [](const tKey& key)
if(!contains(key)) { throw std::out_of_range("Unknown key"); }
return *map_[key];
inline void push_back(const tKey& key, const tValue& value)
map_[key] = &vector_[size()-1];
inline void set(const tKey& key, const tValue& value)
if(!contains(key)) push_back(key, value);
else *map_[key] = value;

std::vector<tValue> vector_;
std::unordered_map<tKey, tValue*> map_;

The thing is when I try to execute this code sample :

OrderedMap<std::string, std::vector<std::string> > myContainer;
myContainer.set("1", {"11", "12"});
myContainer.set("2", {"21", "22"});
auto myValues = myContainer["1"];

Something wrong is happening when I try to access the data. In this example it triggers an exception telling me the vector is too long when the program tries to copy the data into
, but somewhere else in my code it ends in a read access violation error when dereferencing the pointrer inside
operator []

Clearly I made a mistake somewhere, but I can't find, so what's wrong with my code ?
Am I missing something about template paramters and referencing them ?

edit: I am on Windows compiling with MSVC 12.0.

Answer Source

It is normally a very bad idea to store pointers/references/iterators to elements in a growing vector. In

inline void push_back(const tKey& key, const tValue& value)
    map_[key] = &vector_[size()-1];

You add the element into the vector and then you store a pointer to that element in the map. The problem with this is if you keep adding items to the vector, the vector is going to need to grow which means allocating new memory. This means that the elements in the vector are no longer where they used to be which means all of the pointers in the map are now dangling(point to garbage).

The simple fix would be to store the element in both places. You could also create a std::shared_ptr and store the shared_ptr in both containers.

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