请问什么是strlen elision？它是否属于copy elision及modern C++ idioms？

Hey there! Let's tackle your two questions about C++ optimizations clearly:

1. What's the exact definition of strlen elision? #

Strlen elision is a compiler optimization that cuts out unnecessary calls to the strlen() function (or similar runtime string-length checks) by leveraging compile-time knowledge of a string's length.

Here are some common scenarios where this kicks in:

• When using string literals: If you write std::string my_str("hello world");, the compiler already knows the literal's length (11) at compile time. Instead of making a runtime call to strlen("hello world") to get that value for the string constructor, it directly uses the precomputed length.
• With constexpr or statically known strings: If you're working with strings whose length can be determined during compilation (like constexpr string objects or static character arrays with fixed sizes), the compiler skips the runtime strlen check entirely, replacing it with the known length value.

This optimization reduces runtime overhead by avoiding a function call that would just do work the compiler could have done ahead of time.

2. Is strlen elision a type of copy elision? #

No, they're distinct optimizations—strlen elision is not part of copy elision.

Copy elision focuses on eliminating unnecessary copy or move operations of C++ objects. For example, when you return a local std::string from a function, the compiler can often skip copying that object into the caller's memory space (this is called NRVO, Named Return Value Optimization, a form of copy elision). It's all about reducing object duplication.

Strlen elision, by contrast, is a runtime function call elimination optimization. It's targeted at removing the cost of calculating string lengths at runtime when the compiler already knows the answer. The two optimizations solve different problems and belong to separate categories of compiler optimizations.

内容的提问来源于stack exchange，提问作者user5560811