Arrays vs. Vectors in C : When Does Performance Really Matter?

Performance Differences between Arrays and Vectors in C

When working with C data structures, many developers may have encountered the recommendation to avoid using arrays, even as a fundamental data structure in the language. This article explores the performance implications of this suggestion by comparing native C arrays with standard library containers, specifically std::vectors.

One issue with C arrays is when using them dynamically, as this requires manual memory management. Keeping track of size, deleting and managing the arrays can add unnecessary overhead to the code. Additionally, using arrays on the stack is discouraged due to the lack of range checking and loss of size information when passed as pointers. For these cases, it's better to use std::array, which encapsulates a C array in a class, providing size and iteration capabilities.

However, when comparing std::vectors to native C arrays, the performance gap is negligible. A closer look at the generated assembly code for basic indexing, dereferencing, and increment operations reveals that these actions on vectors are essentially identical to those on arrays. These operations involve memory access and pointer manipulation, which are performed in the same way for both data structures.

It's worth noting that allocating arrays with new and non-class objects or classes without a user-defined constructor can offer some performance advantages over std::vectors. This is because std::vectors initialize all elements to default values (such as 0 for integers) on construction. If elements don't require initial values, using arrays directly may be slightly more efficient. However, in most situations, the performance differences are minimal.

Ultimately, the choice between arrays and std::vectors depends on the specific requirements of the application. If dynamic memory allocation and manual housekeeping are necessary, std::vectors offer a more convenient and safe approach. For static arrays with no need for dynamic features, native C arrays can still be an efficient option, particularly with non-class objects and classes without user-defined constructors that don't require initial values.

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