std::unique_ptr not releasing resources properly in a custom deleter scenario in C++17
I'm working with an scenario where a `std::unique_ptr` with a custom deleter is not releasing resources as expected. I have a class that manages a resource (let's say a file handle), and I'm using a custom deleter to ensure that the handle is properly closed when the `std::unique_ptr` goes out of scope. However, I'm observing that the handle remains open after the unique pointer is supposed to be destroyed. Here's a simplified version of my code: ```cpp #include <iostream> #include <memory> class FileHandle { public: FileHandle(const std::string& filename) : file_(fopen(filename.c_str(), "r")) { if (!file_) throw std::runtime_error("Failed to open file."); } ~FileHandle() { if (file_) fclose(file_); } private: FILE* file_; }; void customDeleter(FileHandle* handle) { std::cout << "Deleting FileHandle" << std::endl; delete handle; } void useFile(const std::string& filename) { std::unique_ptr<FileHandle, decltype(&customDeleter)> filePtr(new FileHandle(filename), customDeleter); // some operations with filePtr } int main() { useFile("test.txt"); // FileHandle should be destroyed here return 0; } ``` When I run this code, I see the message from the custom deleter, but the file handle remains open, and when I check the file descriptor count, I see that it has not been closed properly. I’ve verified that the `FileHandle` destructor is being called, but it doesn't seem to be freeing the resources as expected. I also tried changing the deleter to use a lambda function, but the question continues. Is there a specific scenario with how I'm using `std::unique_ptr` in this scenario? Am I missing something in the destructor that prevents proper resource management? Any insights on this would be greatly appreciated. My development environment is Windows. I'm coming from a different tech stack and learning C++. I'm using C++ 3.11 in this project.