C `noexcept`: When Should You Use It and What Are the Performance Gains?

Noexcept in Everyday Use: When and Why

The noexcept keyword offers promising optimizations for C functions that can guarantee they won't throw exceptions. While recognizing its benefits, you may have questions about its practical application. Here's some clarification.

1. When to Append Noexcept

You don't have to annotate every function with noexcept. Use it when it's clear that the function will never throw. If you're unsure, consider the following scenarios:

• Don't use noexcept: If the function could potentially throw in rare cases (e.g., file operations or complex algorithms).
• Consider noexcept: If the function's implementation doesn't throw and it's unlikely to do so in the future.

2. Performance Benefits

Noexcept primarily impacts performance through allowing certain compiler optimizations. While modern compilers may not take full advantage of noexcept yet, it has several benefits:

• Reduced binary size: By removing exception handling code from functions marked noexcept, the executable code can shrink.
• Faster execution: Avoiding exception handling overhead and potential jumps can result in faster code execution.

Specific Example

Here's an example where adding noexcept can improve performance:

class MyClass {
public:
  MyClass() noexcept = default;
  MyClass(const MyClass& other) noexcept = default;
};

int main() {
  std::vector<MyClass> v;
  for (int i = 0; i < 100000; i++) {
    v.push_back(MyClass());
  }
}

By marking the copy constructor noexcept, the compiler can use more efficient move operations for object construction, resulting in faster execution.

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