C++23 introduces two new standard library function templates`std::out_ptr` and `std::inout_ptr`, along with their corresponding types `std::out_ptr_t` and `std::inout_ptr_t`, aimed at enhancing the readability and safety of code interacting with C-style APIs. This article will provide a technical analysis of the design motivations, functional details, usage scenarios, and implementation mechanisms of these two new features, ensuring accurate and thorough explanations for developers.

1. Design Motivation

Background of the Problem

In modern C++, smart pointers (such as std::unique_ptr and std::shared_ptr) are the primary tools for managing dynamic resources, providing automatic memory management and type safety. However, many legacy systems or third-party libraries still use C-style APIs, which often pass output parameters through pointers to pointers (such as int**). For example:

void c_api(int** p) {
    *p = new int(42);
}

Calling such APIs requires manually passing the address of the pointer:

int* p = nullptr;
c_api(&p);
// Use p
delete p;

This approach has the following issues:

Poor Readability: &p only expresses low-level address operations, making the intent unclear.
Prone to Errors: Manual memory management can easily lead to leaks or double deletions.
Incompatible with Smart Pointers: Directly passing the raw pointer of a smart pointer can disrupt its lifetime management.

std::out_ptr and std::inout_ptr encapsulate pointer-to-pointer operations, providing a safer and more intuitive interface, especially suitable for interacting with C-style APIs.

Goals

Improve Readability: Clearly express the intent of “output” or “input-output” through semantically meaningful function templates.
Automated Management: Collaborate with smart pointers to ensure resources are correctly reset or released.
Generality: Support standard smart pointers and user-defined smart pointers.

2. Functionality and Interface

std::out_ptr and std::inout_ptr are defined in the <memory> header file, returning temporary objects std::out_ptr_t and std::inout_ptr_t for interacting with C-style APIs. Below are their core functionalities and differences.

2.1 `std::out_ptr`

Usage: For output parameters of C-style APIs, where the API returns newly allocated resources through pointers to pointers.
Behavior:

On construction: Accepts a smart pointer (such as std::unique_ptr or std::shared_ptr), generating a pointer to its raw pointer (T**).
After calling the API: The API modifies the original pointer to point to the new resource.
On destruction: Calls the smart pointer’s reset(), transferring the new resource returned by the API to the smart pointer for management.

Applicable Scenarios: When the API allocates new resources, the caller is responsible for management.

Example

#include <iostream>
#include <memory>

void c_api(int** p) {
    *p = new int(42);
}

int main() {
    std::unique_ptr<int> pi = std::make_unique<int>(51);
    c_api(std::out_ptr(pi));
    std::cout << *pi << '\n'; // Output: 42
}

std::out_ptr(pi) generates int**, which is passed to c_api.
c_api sets *p to the newly allocated int (value 42).
std::out_ptr_t calls pi.reset(new_ptr) on destruction, updating pi to manage the new resource, while the old resource (51) is automatically released.

2.2 `std::inout_ptr`

Usage: For input-output parameters of C-style APIs, where the API may release old resources and return new ones.
Behavior:

On construction: Calls the smart pointer’s release(), relinquishing ownership of the raw pointer and generating a pointer to the raw pointer (T**).
After calling the API: The API may delete the old pointer and set the new resource.
On destruction: Calls reset(), transferring the new resource returned by the API to the smart pointer for management.

Applicable Scenarios: When the API may modify or replace the resource pointed to by the pointer and may be responsible for releasing the old resource.

Example

#include <iostream>
#include <memory>

void c_api(int** p) {
    delete *p; // Release old resource
    *p = new int(42);
}

int main() {
    std::unique_ptr<int> pi = std::make_unique<int>(51);
    c_api(std::inout_ptr(pi));
    std::cout << *pi << '\n'; // Output: 42
}

std::inout_ptr(pi) calls pi.release(), causing pi to relinquish control of the old resource and generate int**.
c_api releases the old resource (51) and allocates a new resource (42).
std::inout_ptr_t calls pi.reset(new_ptr) on destruction, allowing pi to manage the new resource.

2.3 Main Differences

Feature	`<span>std::out_ptr</span>`	`<span>std::inout_ptr</span>`
Construction Behavior	No `<span>release()</span>`, retains ownership of the smart pointer	Calls `<span>release()</span>`, relinquishes ownership of the old resource
Destruction Behavior	Calls `<span>reset()</span>`, manages the new resource	Calls `<span>reset()</span>`, manages the new resource
Applicable API	Outputs new resources, caller manages	Inputs old resources, API may release and return new resources
Double Deletion Risk	Must ensure API does not release old resources	Avoids double deletion through `<span>release()</span>`

3. Technical Details

3.1 Temporary Objects and Lifetimes

std::out_ptr_t and std::inout_ptr_t are temporary objects that exist only during the lifetime of the full expression containing them.
On destruction, they automatically call the smart pointer’s reset(), ensuring correct resource management.
Avoid dangling references: Temporary objects are immediately destroyed after the function call, ensuring pointer updates are safe.

3.2 Smart Pointer Support

Standard Smart Pointers: Supports std::unique_ptr and std::shared_ptr.
User-Defined Smart Pointers: Users can specialize std::out_ptr_t and std::inout_ptr_t to support custom types, needing to implement interfaces similar to reset() and release().
Deleters:

std::unique_ptr‘s deleter is part of the type, and std::out_ptr and std::inout_ptr handle it automatically.
std::shared_ptr‘s deleter is not part of the type, requiring explicit specification when using std::out_ptr:

std::shared_ptr<int> pi = std::make_shared<int>(51);
c_api(std::out_ptr(pi, std::default_delete<int>())); // Must specify deleter

If the deleter is not specified, the compiler will report an error (e.g., static_assert failure), ensuring type safety.

3.3 Interaction with C APIs

Pointer to Pointers: std::out_ptr and std::inout_ptr generate T**, directly compatible with C-style API’s void* or specific type pointer parameters.
Resource Management: Automatically handles resources returned by the API through the smart pointer’s reset() and release(), avoiding manual delete.

4. Usage Scenarios and Advantages

4.1 Applicable Scenarios

Interaction with C-style APIs: Such as OpenGL, Vulkan, C libraries (like libpng and libxml), which often use T** to pass resources.
Migration of Legacy Code: Transitioning from manual memory management to smart pointer management to enhance safety.
Cross-Language Interfaces: Simplifying pointer passing when interacting with C or C-compatible languages (like Rust).

4.2 Advantages

Readability: std::out_ptr(pi) is clearer than <code>&pi.get(), expressing the intent of "output" or "input-output".
Safety: Automatically manages resources, avoiding leaks or double deletions.
Compatibility: Supports both standard and custom smart pointers, adapting to various scenarios.
Modernization: Bridges C-style APIs with modern C++ smart pointers, reducing the burden of manual memory management.

4.3 Considerations

API Behavior:

When using std::out_ptr, ensure the API does not release old resources, or it may lead to double deletion.
When using std::inout_ptr, confirm whether the API is responsible for releasing old resources.

Deleter Matching: std::shared_ptr requires explicit specification of the deleter to ensure consistency with the original smart pointer.
Performance: The creation and destruction of temporary objects introduce slight overhead, but this is usually negligible under modern compiler optimizations.

5. Examples: Practical Applications

5.1 Using `std::out_ptr` with C API

#include <iostream>
#include <memory>

struct Handle {
    int value;
};

void create_handle(Handle** h) {
    *h = new Handle{42};
}

int main() {
    std::unique_ptr<Handle> handle;
    create_handle(std::out_ptr(handle));
    std::cout << handle->value << '\n'; // Output: 42
}

std::out_ptr(handle) generates Handle**, and the API allocates a new Handle.
On destruction, handle takes over the new resource, and the old resource (if any) is released.

5.2 Using `std::inout_ptr` to Replace Resources

#include <iostream>
#include <memory>

struct Handle {
    int value;
};

void update_handle(Handle** h) {
    delete *h; // Release old resource
    *h = new Handle{42};
}

int main() {
    auto handle = std::make_unique<Handle>(Handle{51});
    update_handle(std::inout_ptr(handle));
    std::cout << handle->value << '\n'; // Output: 42
}

std::inout_ptr(handle) relinquishes ownership of the old resource, and the API replaces it with a new resource.
On destruction, handle manages the new resource, with no risk of double deletion.

5.3 Custom Smart Pointers

Assuming a custom smart pointer CustomPtr:

template<typename T>
class CustomPtr {
    T* ptr;
public:
    void reset(T* p) { delete ptr; ptr = p; }
    T* release() { T* tmp = ptr; ptr = nullptr; return tmp; }
    T* get() const { return ptr; }
    // ...
};

Specializing std::out_ptr_t:

namespace std {
    template<typename T>
    struct out_ptr_t<CustomPtr<T>> {
        // Implement similar to std::unique_ptr
    };
}

allows the use of CustomPtr with std::out_ptr.

6. Technical Summary and Recommendations

6.1 Core Conclusions

std::out_ptr: Suitable for output parameters of C-style APIs, automatically transferring new resources to smart pointer management, ensuring the API does not release old resources.
std::inout_ptr: Suitable for input-output parameters, avoiding double deletion through release(), ideal for scenarios where the API replaces resources.
Temporary Objects: std::out_ptr_t and std::inout_ptr_t ensure safe lifetime management.
Deleter Support: std::shared_ptr requires explicit specification of the deleter, while std::unique_ptr handles it automatically.
Generality: Supports both standard and custom smart pointers, adapting to a wide range of scenarios.

6.2 Practical Application Recommendations

Select the Appropriate Pointer:

If the API only outputs new resources, use std::out_ptr.
If the API may release old resources and return new ones, use std::inout_ptr.

Verify API Behavior:

Read the API documentation to confirm resource allocation and release responsibilities.
Test edge cases (such as null pointers, exceptions) to ensure no undefined behavior.

Integrate Smart Pointers:

Prefer using std::unique_ptr to simplify deleter management.
For std::shared_ptr, ensure the deleter is consistent.

Code Readability:

Use std::out_ptr and std::inout_ptr instead of &pi.get() to improve intent expression.
Add comments explaining the API’s resource management behavior.

Cross-Platform Compatibility:

Test compilers that support C++23 (such as GCC 13+, Clang, MSVC).
For environments that do not support C++23, manually implement similar functionality (&pi.get() plus <code>reset()).

Performance Optimization:

The overhead of creating and destroying temporary objects is usually negligible, but measure the actual impact in performance-critical paths.
Combine features like [[assume]] and other C++23 features to further optimize API calls.

Through std::out_ptr and std::inout_ptr, C++23 significantly simplifies interaction with C-style APIs, allowing developers to efficiently manage resources while maintaining the safety and readability of modern C++.

In-Depth Analysis of std::out_ptr and std::inout_ptr in C++23

1. Design Motivation

Background of the Problem

Goals

2. Functionality and Interface

2.1 `<span>std::out_ptr</span>`

Example

2.2 `<span>std::inout_ptr</span>`

Example

2.3 Main Differences

3. Technical Details

3.1 Temporary Objects and Lifetimes

3.2 Smart Pointer Support

3.3 Interaction with C APIs

4. Usage Scenarios and Advantages

4.1 Applicable Scenarios

4.2 Advantages

4.3 Considerations

5. Examples: Practical Applications

5.1 Using `<span>std::out_ptr</span>` with C API

5.2 Using `<span>std::inout_ptr</span>` to Replace Resources

5.3 Custom Smart Pointers

6. Technical Summary and Recommendations

6.1 Core Conclusions

6.2 Practical Application Recommendations

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1. Design Motivation

Background of the Problem

Goals

2. Functionality and Interface

2.1 <span>std::out_ptr</span>

Example

2.2 <span>std::inout_ptr</span>

Example

2.3 Main Differences

3. Technical Details

3.1 Temporary Objects and Lifetimes

3.2 Smart Pointer Support

3.3 Interaction with C APIs

4. Usage Scenarios and Advantages

4.1 Applicable Scenarios

4.2 Advantages

4.3 Considerations

5. Examples: Practical Applications

5.1 Using <span>std::out_ptr</span> with C API

5.2 Using <span>std::inout_ptr</span> to Replace Resources

5.3 Custom Smart Pointers

6. Technical Summary and Recommendations

6.1 Core Conclusions

6.2 Practical Application Recommendations

Related posts

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2.1 `<span>std::out_ptr</span>`

2.2 `<span>std::inout_ptr</span>`

5.1 Using `<span>std::out_ptr</span>` with C API

5.2 Using `<span>std::inout_ptr</span>` to Replace Resources