In the evolving landscape of modern C++, efficient and safe memory management remains a cornerstone of robust software development. Gone are the days when raw pointers and manual new/delete were the only tools in a developer’s arsenal, often leading to common pitfalls like memory leaks and dangling pointers. Today, smart pointers, particularly std::unique_ptr, offer a powerful solution for managing dynamically allocated resources with RAII (Resource Acquisition Is Initialization) principles. However, a common point of confusion for many developers revolves around the optimal way to create an std::unique_ptr: should one use new directly or leverage the factory function std::make_unique? Understanding the subtle yet significant differences between std::make_unique and std::unique_ptr with new is crucial for writing exception-safe, performant, and maintainable C++ code. This article delves deep into these distinctions, providing clear insights and best practices for modern C++ development.

Understanding std::unique_ptr and Direct new Allocation

std::unique_ptr is a smart pointer that owns and manages another object through a pointer and disposes of that object when the unique_ptr itself goes out of scope. It strictly enforces exclusive ownership, meaning only one unique_ptr can point to a given resource at any time. This powerful ownership semantics prevents common errors like double-freeing or managing memory through multiple, conflicting pointers.

Traditionally, to create an std::unique_ptr, developers would often use the new operator directly within its constructor. For instance, creating a unique pointer to an integer would look like std::unique_ptr<int> ptr(new int(10));. While seemingly straightforward, this approach can introduce subtle vulnerabilities, particularly concerning exception safety. The direct use of new separates the allocation of memory from the construction of the smart pointer, creating a tiny window where an exception can lead to a memory leak.

Consider a function call like f(std::unique_ptr<A>(new A()), std::unique_ptr<B>(new B()));. The order of evaluation for function arguments is not guaranteed. The compiler might allocate memory for A, then for B, and then call the constructors for unique_ptr<A> and unique_ptr<B>. If, for example, the allocation for B throws an exception after new A() has succeeded but before std::unique_ptr<A> has been constructed, the memory allocated for A would be leaked, as no smart pointer would take ownership of it. This specific scenario highlights a critical disadvantage of using new directly with std::unique_ptr.

Introducing std::make_unique: The Factory Function

Introduced in C++11 and standardized in C++14, std::make_unique is a utility function designed to simplify and improve the safety of creating std::unique_ptr instances. It acts as a factory function, handling both the memory allocation and the object construction within a single, atomic operation. Its primary purpose is to provide a safer and more convenient way to construct unique_ptr objects, mitigating the exception safety concerns inherent in direct new usage.

Using std::make_unique is straightforward: instead of std::unique_ptr<MyClass> ptr(new MyClass(args));, you simply write auto ptr = std::make_unique<MyClass>(args);. The return type is automatically deduced, making the code more concise and less prone to type-related errors. This single-step operation ensures that if an exception occurs during object construction, the memory allocation is correctly handled, preventing leaks. This adherence to RAII principles is one of the strongest arguments for its adoption.

For instance, if we revisit the problematic function call, using std::make_unique ensures safety: f(std::make_unique<A>(), std::make_unique<B>());. In this case, each make_unique call is a complete expression. If an exception occurs, the allocated memory is immediately managed and cleaned up by the temporary unique_ptr object before the exception propagates. This fundamental design choice makes std::make_unique the preferred method for creating unique pointers in modern C++.

Core Differences and Key Question & Answer :

Does std::make_unique have any efficiency benefits like std::make_shared?

Compared to manually constructing std::unique_ptr:

std::make_unique<int>(1); // vs std::unique_ptr<int>(new int(1)); 

The motivation behind make_unique is primarily two-fold:

• make_unique is safe for creating temporaries, whereas with explicit use of new you have to remember the rule about not using unnamed temporaries.

  foo(make_unique<T>(), make_unique<U>()); // exception safe foo(unique_ptr<T>(new T()), unique_ptr<U>(new U())); // unsafe* 
  

• The addition of make_unique finally means we can tell people to ’never’ use new rather than the previous rule to “’never’ use new except when you make a unique_ptr”.

There’s also a third reason:

• make_unique does not require redundant type usage. unique_ptr<T>(new T()) -> make_unique<T>()

None of the reasons involve improving runtime efficiency the way using make_shared does (due to avoiding a second allocation, at the cost of potentially higher peak memory usage).

* It is expected that C++17 will include a rule change that means that this is no longer unsafe. See C++ committee papers P0400R0 and P0145R3.