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In this noncompliant code example, a custom container class uses an allocator object to obtain storage for arbitrary element types, but it . While the CopyElements() function is presumed to call copy constructors for elements being moved into the newly-allocated storage, this example fails to explicitly call object constructorsthe default constructor for any additional elements being reserved. If such an element that has been reserved is accessed through the operator[]() function, it results in undefined behavior, depending on the type T.
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#include <memory> template <typename T, typename Alloc = std::allocator<T> >allocator<T>> class Container { T *UnderlyingStorage; size_t NumElements; void CopyElements(T *From, T *To, size_t Count); public: void reserve(size_t Count) { if (Count > NumElements) { Alloc A; T *P = A.allocate(Count); // Throws on failure try { CopyElements(UnderlyingStorage, P, NumElements); } catch (...) { A.deallocate(P, Count); throw; } UnderlyingStorage = P; } NumElements = Count; } T &operator[](size_t Idx) { return UnderlyingStorage[Idx]; } const T &operator[](size_t Idx) const { return UnderlyingStorage[Idx]; } }; |
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In this compliant solution, the newly allocated memory is all elements are properly initialized by explicitly calling object copy or default constructors for T:
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#include <memory>
template <typename T, typename Alloc = std::allocator<T>>
class Container {
T *UnderlyingStorage;
size_t NumElements;
void CopyElements(T *From, T *To, size_t Count);
public:
void reserve(size_t Count) {
if (Count > NumElements) {
Alloc A;
T *P = A.allocate(Count); // Throws on failure
try {
CopyElements(UnderlyingStorage, P, NumElements);
for (size_t i = NumElements; i < Count; ++i) {
A.construct(&P[i]);
}
} catch (...) {
A.deallocate(P, Count);
throw;
}
UnderlyingStorage = P;
}
NumElements = Count;
}
T &operator[](size_t Idx) { return UnderlyingStorage[Idx]; }
const T &operator[](size_t Idx) const { return UnderlyingStorage[Idx]; }
}; |
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