In-depth Comparison of std::make_shared vs. Direct std::shared_ptr Construction in C++: Efficiency, Exception Safety, and Memory Management

In the smart pointer system of C++11 and later, std::shared_ptr serves as a core component for shared ownership models, where its construction method significantly impacts program performance and exception safety. Based on standard implementations, this article compares std::make_shared and direct std::shared_ptr constructor calls, analyzing their differences from underlying mechanisms through a step-by-step approach.

Differences in Heap Allocation Mechanisms

std::shared_ptr manages two key entities: the control block and the managed object. The control block stores metadata such as reference counts and type-erased deleters, while the managed object holds the actual user data. Consider the following example code:

std::shared_ptr<Object> p1 = std::make_shared<Object>("foo");
std::shared_ptr<Object> p2(new Object("foo"));

In the case of std::make_shared, the standard library performs a single heap allocation, reserving space for both the control block and the Object instance in one operation. This is achieved through internal optimizations, reducing overhead from memory allocator calls. In contrast, the construction of p2 involves two steps: first, new Object("foo") triggers a heap allocation to create the object; then, the std::shared_ptr constructor executes another allocation for the control block. Thus, make_shared reduces memory fragmentation risks and may improve cache locality by consolidating allocations.

Considerations for Exception Safety

Prior to C++17, the evaluation order of function arguments was not strictly defined, potentially leading to resource leaks. For example:

void F(const std::shared_ptr<Lhs> &lhs, const std::shared_ptr<Rhs> &rhs);
F(std::shared_ptr<Lhs>(new Lhs("foo")), std::shared_ptr<Rhs>(new Rhs("bar")));

One possible evaluation order is: allocate the Lhs object first, then the Rhs object, followed by constructing the two shared_ptr instances. If the Rhs constructor throws an exception, the memory allocated for Lhs cannot be freed, as it has not yet been passed to a shared_ptr. Using std::make_shared avoids this issue by atomizing allocation and construction:

F(std::make_shared<Lhs>("foo"), std::make_shared<Rhs>("bar"));

Since C++17, argument evaluation order has been standardized to fully execute one argument before proceeding to the next, but make_shared still offers a cleaner exception safety guarantee.

Delayed Effects on Memory Deallocation

The single allocation of std::make_shared introduces a potential drawback: the memory for the control block and managed object is bundled and cannot be freed independently. The lifetime of the control block depends on the counts of both shared_ptr and weak_ptr instances. When weak_ptrs exist, they check the shared_ptr count in the control block to determine object validity, thus keeping the control block alive until the weak_ptr count reaches zero. With make_shared, this means the object memory may be delayed in deallocation until all weak_ptrs are destroyed.

In comparison, the two-step allocation via new and the shared_ptr constructor allows more flexible memory management: object memory can be freed immediately when the shared_ptr count drops to zero, while control block memory is released later when the weak_ptr count reaches zero. This may be preferable in memory-sensitive scenarios.

Practical Recommendations and Summary

In most cases, std::make_shared is preferred due to its efficiency and exception safety. It reduces heap allocation frequency, lowers overhead, and simplifies code. However, developers must weigh the impact of delayed memory release, especially in environments with long-lived weak_ptrs or memory constraints. For applications requiring fine-grained control over memory deallocation, the two-step allocation method may be considered. Understanding these underlying mechanisms aids in optimizing resource management strategies for C++ programs.