assign_ptr

assign_ptr is a utility for interfacing C++ smart pointers with C-style APIs that allocate resources through output parameters.

Illustration of the problem

// Allocates memory to output parameter 'p' via 'new'
// Caller responsible for freeing the allocated memory via 'delete'
void foo_alloc(int*& p);

How to move the allocated raw pointer into a smart pointer for safety?

Temporary raw pointer

int* p_ = nullptr;
foo_alloc(p_);
std::unique_ptr<int> p(p_);

Doable, but pollutes the scope with a temporary variable.

Temporary raw pointer with limited scope

std::unique_ptr<int> p;
{
    int* p_ = nullptr;
    foo_alloc(p_);
    p.reset(p_);
}

Requires more lines of boilerplate code.

assign_ptr utility function

std::unique_ptr<int> p;
foo_alloc(zpp::assign_ptr(p));

Done.

Caveat

There is one unintuitive edge case to be aware of.

int* p = nullptr;
if (foo_alloc(p) && p != nullptr)
{
    // Do something with `p`
}
delete p;

is not equivalent to

std::unique_ptr<int> p;
if (foo_alloc(zpp::assign_ptr(p)) && p != nullptr)
{
    // Impossible to enter this branch
    // because `p` has not yet been set
}

C++17 offers a solution via if-statement initialization.

std::unique_ptr<int> p;
if (foo_alloc(zpp::assign); p != nullptr)
{
    // Do something with `p`
}

Supported smart pointers

Defined requirements

assign_ptr should work with any class/struct that defines the following:

• element_type type
• reset(element_type*) member function

Tested smart pointers

• std::unique_ptr
• std::shared_ptr
• std::auto_ptr

Compiler support

C++11 (but backporting to C++03 is possible)