C++26's std::is_within_lifetime: Checking Object Lifetime During Constant Evaluation
By
ibobev
Sesame, salt, and substance. A flagship bake.
Summary
The article explains C++26's new std::is_within_lifetime function, which checks whether a pointer points to an object within its lifetime during constant evaluation. The primary use case is determining which union member is active at compile time, solving a specific problem in implementing space-efficient optional types. The function is consteval-only, takes a pointer parameter, and addresses a fundamental lifetime checking issue rather than being union-specific. The author discusses the design rationale, practical applications, and current lack of compiler support for this C++26 feature.
Key quotes
· 5 pulledC++26 adds bool std::is_within_lifetime(const T* p) to the <type_traits> header. This function checks whether p points to an object that is currently within its lifetime during constant evaluation.
The most common use case is checking which member of a union is currently active.
It's consteval only - std::is_within_lifetime is consteval, meaning it can only be used during compile-time. You cannot call it at runtime.
The committee chose to solve the problem at a more fundamental level. Instead of adding a union-specific check, they provided a general mechanism to query object lifetime.
C++26's std::is_within_lifetime is a focused addition that solves a real problem in constant evaluation: checking which union member is active without invoking undefined behavior.
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