Unexpected Behavior of std::vector with Custom Allocator in C++17
I've hit a wall trying to I've been banging my head against this for hours. I'm encountering unexpected behavior when using a custom allocator with std::vector in C++17. My custom allocator is designed to track memory allocations, but when initializing a vector with it, I'm observing that the memory allocation count does not seem to match the expected number of allocations. For instance, when I create a vector of 10 integers, I expect the allocation count to increase by 1, but it increments by 2 instead. Here's a simplified version of my allocator: ```cpp #include <iostream> #include <memory> template <typename T> struct TrackingAllocator { using value_type = T; TrackingAllocator() = default; template <typename U> TrackingAllocator(const TrackingAllocator<U>&) {} T* allocate(std::size_t n) { std::cout << "Allocating " << n << " units of " << sizeof(T) << " bytes.\n"; return static_cast<T*>(::operator new(n * sizeof(T))); } void deallocate(T* p, std::size_t n) { std::cout << "Deallocating " << n << " units of " << sizeof(T) << " bytes.\n"; ::operator delete(p); } }; int main() { std::vector<int, TrackingAllocator<int>> vec(10); return 0; } ``` And the output when I run this code is: ``` Allocating 10 units of 4 bytes. Allocating 10 units of 4 bytes. ``` It seems that the `std::vector` is allocating memory twice during construction, which I wasn't expecting. I tried to review the vector's implementation, but I couldn't figure out why this is happening. I also checked that my allocator is correctly handling the allocation and deallocation, yet it appears like the vector is managing its memory differently than anticipated. Is there something in the C++17 specifications or the implementation of `std::vector` that requires special attention when using custom allocators? Any guidance on how to resolve this discrepancy would be appreciated. Additionally, I'm using GCC 11.2.0, and I compiled with `-std=c++17` flag. Thanks in advance for your help! My development environment is Linux. I recently upgraded to C++ LTS. Any help would be greatly appreciated! I'm working in a Windows 10 environment. Any ideas how to fix this?