When Should I Use std::copy() Instead of std::memcpy()?

Performance Comparison Between std::memcpy() and std::copy()

While std::memcpy() has traditionally been considered faster for copying raw memory, recent developments in optimizations by compilers suggest that std::copy() may offer performance advantages in certain scenarios.

Benchmark Results

Extensive testing has shown that std::copy() can outperform std::memcpy() in the following situations:

• Sequential Data Layout: When the source and destination arrays are laid out sequentially in memory, std::copy() can take advantage of more precise type information and alignment optimizations.
• Trivially Copyable Types: For simple data types with built-in copy operations, std::copy() can inline these operations without incurring the overhead of function calls.

Potential Performance Gain

In the tests conducted, std::copy() demonstrated a consistent performance increase of approximately 3% over std::memcpy() for SHA-2 hashing operations. However, for MD5 hashing, the results were less consistent and generally showed a slight performance drop for std::copy().

Explanation

The observed performance gain with std::copy() can be attributed to:

• Inline Optimization: Modern compilers aggressively inline std::copy() calls for performance reasons.
• Type Preservation: std::copy() maintains the type information of the elements being copied, enabling optimizations based on data alignment and memory layout.

Conclusion

Based on the benchmark results, it is generally recommended to use std::copy() instead of std::memcpy() for scenarios involving sequential data copies and trivially copyable types. While std::memcpy() remains a valid option for specific cases where raw memory manipulation is necessary, std::copy() offers a more flexible and often faster alternative for most tasks.

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