Templates vs. std::function: When Should You Prioritize Performance Over Flexibility?

Weighing std::function against Templates for Performance Optimization

The introduction of std::function in C 11 has generated conflicting opinions, with concerns raised about its performance limitations. By comparing it to templates in a performance test, it becomes evident that templates vastly outperform std::function.

Templates excel in cases where performance is paramount. They can be effortlessly inlined by the compiler, leading to much faster execution than virtual function calls used in std::function. However, templates have their drawbacks:

• Distribution as Headers: Unlike compiled binaries, templates must be provided as header files, which may not be desirable for closed-source libraries.
• Longer Compilation Times: Templates can increase compilation time if they are not implemented as extern templates.
• Lack of Enforcement for Functor Requirements: Templates lack a mechanism to explicitly define the expected functor requirements, making code readability more challenging.

Given these considerations, it may seem that std::function is the superior choice for passing functors in situations where performance is less critical. However, it serves a distinct purpose and is not intended as a direct replacement for templates.

When to Use std::function vs. Templates

The choice between std::function and templates hinges on the nature of the design challenge. Templates should be favored whenever feasible, primarily because they enable maximum specification of constraints at compile time, reducing the likelihood of errors and ensuring optimal code optimization.

In situations where the type of callable object needs to be determined at runtime, std::function proves more suitable. This is particularly true when dealing with a collection of heterogeneous callbacks that may include functors, functions, or bound functions.

Additionally, std::function and std::bind facilitate the principles of functional programming, allowing functions to be manipulated as objects. While templates can be used for this purpose, their usefulness in such scenarios is often inseparable from runtime identification of combined callable object types.

Focus on Design, Not Comparison

Comparing std::function and templates as if they were interchangeable misses the key distinction in their intended use cases. Instead, developers should prioritize design considerations and select the most appropriate construct based on the specific requirements of each situation. By aligning the choice with the appropriate scenario, developers can leverage the advantages of both std::function and templates to enhance code performance and maintainability.

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