👀 Views: 78 💬 Answers: 1 📅 Created: 2025-07-16

c++17 allocator stdvector cpp

I'm stuck on something that should probably be simple. I'm working with an odd scenario when using a custom allocator with `std::vector` in C++17. The question arises when I try to allocate a large number of elements. When I create a vector with my custom allocator and request a size of over 1,000,000 elements, I get a crash with an behavior message indicating a bad allocation. Here’s a simplified version of my custom allocator: ```cpp #include <memory> #include <iostream> template <typename T> class CustomAllocator { public: using value_type = T; CustomAllocator() noexcept {} template <typename U> CustomAllocator(const CustomAllocator<U>&) noexcept {} T* allocate(std::size_t n) { if (n > std::numeric_limits<std::size_t>::max() / sizeof(T)) { throw std::bad_alloc(); } if (auto p = std::malloc(n * sizeof(T))) { return static_cast<T*>(p); } throw std::bad_alloc(); } void deallocate(T* p, std::size_t) noexcept { std::free(p); } }; int main() { std::vector<int, CustomAllocator<int>> myVector; try { myVector.reserve(1000000); } catch (const std::bad_alloc& e) { std::cerr << "Allocation failed: " << e.what() << '\n'; } return 0; } ``` I expected the vector to allocate memory for the requested elements, but instead, I receive an allocation failure with the message: "Allocation failed: std::bad_alloc". I’ve ensured that my allocator checks for overflow before allocating, so I’m quite confused about why this is happening. I also tested the same code with the default allocator, and it works perfectly. Are there any specific intricacies or common pitfalls with custom allocators in C++17 that I might be missing? Any advice would be greatly appreciated! I'm working on a service that needs to handle this. This is for a microservice running on Windows 11. Has anyone else encountered this?