C++ std::vector: Stop Making These Performance Mistakes!

The STD Vector in C++ is a powerful and widely used tool. However, it's crucial to understand how to use it effectively to avoid performance pitfalls. This article outlines common mistakes and provides guidelines for improving STD Vector usage, focusing on performance optimization.

Understanding STD Vector

The STD Vector is a dynamically resizable container of elements stored contiguously in memory. It's part of the Standard Template Library (STL) and offers the flexibility of resizing, unlike traditional arrays. While convenient, this dynamic nature can introduce performance considerations. It's not named after Linear Algebra vector, but just by some guy, who now probably regrets naming it that.

Performance as the Key Metric

The primary focus here is on performance. While the STL provides a general-purpose implementation, it might not always be the most optimized solution for performance-critical applications. In situations where performance is paramount, consider custom data structures or alternatives like EASTL (EA's STL), optimized for game development. The standard template library is not necessarily optimized for speed or real-time applications like games. However, strategic usage can significantly improve its performance.

Step 1: Don't Use STD Vector (If Possible)

This might sound counterintuitive, but the first step is to evaluate whether STD Vector is the right choice. Ask yourself if its dynamic resizing capabilities are truly needed. The simple array is the most used data structure in programming.

Stack vs. Heap Allocation

A key distinction lies in memory allocation. STD Vector uses the Heap, while STD Array uses the Stack. Stack allocation is generally faster than Heap allocation. Therefore, if the size of your data structure is fixed, always prefer STD Array over STD Vector.

For example, if a function returns a fixed number of elements (e.g., eight colors), using an STD Array is more efficient than a Vector. The priority should be to not use Vector if you can avoid it and use STD Array instead.

Step 2: Memory Allocation and Resizing

Understand how STD Vector manages memory. When a Vector's capacity is exceeded, it needs to reallocate memory, which can be an expensive operation.

Understanding Vector Growth

Every compiler has its own C++ Library with its own implementation. When a vector needs to resize, compilers may grow in different ways.

• MSVC (Visual Studio): Generally grows by 50%.

• GCC: Typically doubles the size.

• Clang: Similar to MSVC.

Doubling leads to fewer allocations at the cost of potentially over-allocating memory.

Pre-allocating Memory

If you know the approximate number of elements a Vector will hold, use the reserve() function to pre-allocate memory. This minimizes reallocations and improves performance. Alternatively, set the size in the Vector's constructor or by using the resize() function.

• reserve(size): Allocates memory for size elements but doesn't create them.

• resize(size): Allocates memory and constructs size default-constructed elements.

Step 3: Avoiding Copies and Moves

Copies and moves can significantly impact performance, especially with complex objects.

Copying Vectors

Avoid unnecessary copying of Vectors. When passing Vectors as function parameters, use const references (const std::vector<T>&) to prevent copying. Copying all the elements into a new vector takes time and allocates new memory.

In-Place Construction with emplace_back()

Instead of constructing an object and then copying or moving it into the Vector using push_back(), use emplace_back(). This constructs the object directly within the Vector's memory, avoiding intermediate copies or moves. Instead of giving the Vector the actual object, give it the parameters of the object you want to construct.

Move Semantics

If copying cannot be avoided, ensure your objects have efficient move constructors to minimize the overhead. Move semantics allow transferring ownership of resources rather than creating entirely new copies. This will be especially useful if your struct or class stores a vector of its own because you would just move the actual pointer rather than copying all the elements. The standard template library will have this sorted for things like strings that you might want to move instead of copy.

Summary of Best Practices

1. Choose the Right Data Structure: If the size is fixed, prefer std::array over std::vector. This means zero allocations since they are just going to be stack allocations.
2. Pre-allocate Memory: Use reserve() or resize() to minimize reallocations. Try and manage the size or capacity yourself.
3. Avoid Copies: Use const references for function parameters and emplace_back() for in-place construction.
4. Utilize Move Semantics: Implement move constructors for efficient resource transfer.

By following these guidelines, you can significantly improve the performance of your C++ code when using STD Vectors.