👀 Views: 37 💬 Answers: 1 📅 Created: 2025-06-14

c++ memory-management std-vector custom-allocator C++

I've been struggling with this for a few days now and could really use some help. I am experiencing an unexpected memory leak in my application that's using a custom allocator with `std::vector`. The leak occurs when I try to resize the vector after populating it with a large number of elements. Here’s the relevant part of my code: ```cpp #include <vector> #include <iostream> #include <memory> template <typename T> class MyAllocator { public: using value_type = T; MyAllocator() = default; template <class U> MyAllocator(const MyAllocator<U>&) {} T* allocate(std::size_t n) { if (auto p = static_cast<T*>(std::malloc(n * sizeof(T)))) { return p; } throw std::bad_alloc(); } void deallocate(T* p, std::size_t) noexcept { std::free(p); } }; int main() { std::vector<int, MyAllocator<int>> vec; vec.reserve(10000); // Reserve space for (int i = 0; i < 10000; ++i) { vec.push_back(i); } vec.resize(5000); // Resizing the vector return 0; } ``` I noticed that when I run this code, my application’s memory usage keeps increasing, and it never seems to recover the memory after the vector is resized. I ran the application through Valgrind, and it reported: ``` ==12345== 40 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==12345== at 0x4C2B1B0: operator new[](unsigned long) ==12345== by 0x4C2A8B2: std::vector<int, MyAllocator<int> >::reserve(unsigned long) ``` After resizing, I thought that the custom allocator should return the memory to the pool, but it seems like that isn’t happening. I’ve tried overriding both `allocate` and `deallocate` in my custom allocator and verified that `deallocate` is being called, but the leak persists. Is there something I might be missing in the implementation of my allocator? Are there any specific pitfalls when using `std::vector` with a custom allocator in C++17? Any insights would be greatly appreciated! For context: I'm using C++ on Windows.