Custom Deleters for Smart Pointers (C APIs, File Handles)0%

Custom Deleters for Smart Pointers (C APIs, File Handles)

Advanced12 min read•Updated: 2026-10-22

🛠️ Custom Deleters for Smart Pointers

Introduction

By default, smart pointers call delete (or delete[]) upon destruction. However, when interfacing with legacy C libraries (POSIX sockets, C file pointers FILE*, OpenSSL context handles, Win32 handles), cleanup requires custom routines like fclose() or free(). Modern C++ allows binding custom deleters to smart pointers.

📁 Wrapping `FILE*` in `std::unique_ptr`

1#include <iostream>
2#include <memory>
3#include <cstdio>
4 
5// Custom Deleter functor for C file handles
6struct FileCloser {
7    void operator()(std::FILE* fp) const {
8        if (fp) {
9            std::fclose(fp);
10            std::cout << "[RAII]: C File handle closed successfully via fclose().\n";
11        }
12    }
13};
14 
15int main() {
16    // Syntax: std::unique_ptr<Type, Deleter>
17    std::unique_ptr<std::FILE, FileCloser> filePtr(std::fopen("demo.txt", "w"));
18 
19    if (filePtr) {
20        std::fputs("Testing Modern C++ Custom Deleter.\n", filePtr.get());
21        std::cout << "Wrote line to file.\n";
22    }
23 
24    // When filePtr leaves scope, FileCloser::operator() runs fclose() automatically!
25    return 0;
26}

⚡ Lambda as Custom Deleter in `std::shared_ptr`

Unlike unique_ptr (where the deleter type is part of the template signature), std::shared_ptr performs type erasure on deleters, allowing clean inline lambda deleters:

1std::shared_ptr<std::FILE> safeFile(std::fopen("log.txt", "w"), [](std::FILE* f) {
2    if (f) std::fclose(f);
3});

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std::weak_ptr: Breaking Circular References and Observer Patterns	Function Templates and Class Templates

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Function Templates and Class Templates

Custom Deleters for Smart Pointers (C APIs, File Handles)

Advanced12 min read•Updated: 2026-10-22

Study Materials

🛠️ Custom Deleters for Smart Pointers

Introduction

📁 Wrapping `FILE*` in `std::unique_ptr`

1#include <iostream>
2#include <memory>
3#include <cstdio>
4 
5// Custom Deleter functor for C file handles
6struct FileCloser {
7    void operator()(std::FILE* fp) const {
8        if (fp) {
9            std::fclose(fp);
10            std::cout << "[RAII]: C File handle closed successfully via fclose().\n";
11        }
12    }
13};
14 
15int main() {
16    // Syntax: std::unique_ptr<Type, Deleter>
17    std::unique_ptr<std::FILE, FileCloser> filePtr(std::fopen("demo.txt", "w"));
18 
19    if (filePtr) {
20        std::fputs("Testing Modern C++ Custom Deleter.\n", filePtr.get());
21        std::cout << "Wrote line to file.\n";
22    }
23 
24    // When filePtr leaves scope, FileCloser::operator() runs fclose() automatically!
25    return 0;
26}

⚡ Lambda as Custom Deleter in `std::shared_ptr`

Unlike unique_ptr (where the deleter type is part of the template signature), std::shared_ptr performs type erasure on deleters, allowing clean inline lambda deleters:

1std::shared_ptr<std::FILE> safeFile(std::fopen("log.txt", "w"), [](std::FILE* f) {
2    if (f) std::fclose(f);
3});

Next Lesson

Function Templates and Class Templates

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
std::weak_ptr: Breaking Circular References and Observer Patterns	Function Templates and Class Templates

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next

🛠️ Custom Deleters for Smart Pointers

Introduction

📁 Wrapping FILE* in std::unique_ptr

⚡ Lambda as Custom Deleter in std::shared_ptr

Function Templates and Class Templates

Related Lessons

Practice Quiz

Loading Page Content...

🛠️ Custom Deleters for Smart Pointers

Introduction

📁 Wrapping FILE* in std::unique_ptr

⚡ Lambda as Custom Deleter in std::shared_ptr

Function Templates and Class Templates

Related Lessons

Practice Quiz

📁 Wrapping `FILE*` in `std::unique_ptr`

⚡ Lambda as Custom Deleter in `std::shared_ptr`

📁 Wrapping `FILE*` in `std::unique_ptr`

⚡ Lambda as Custom Deleter in `std::shared_ptr`