By Blog Staff | Jul 7, 2026 01:11 PM | Tags: None

SANDOR_DARGO_ROUND.JPGLibrary hardening is one of those features that makes you wonder why it wasn’t standardized sooner. It catches real bugs, at negligible cost, across code you don’t have to change. The three major implementations have been doing their own versions of this for years — C++26 finally makes it portable and consistent.

C++26: Standard library hardening

by Sandor Dargo

From the article:

Undefined behavior (UB) in C++ is one of the hardest categories of bugs to deal with. It can silently corrupt memory, cause crashes far from the actual mistake, or — worst of all — just happens to work on your machine. A significant share of UB in real codebases comes not from exotic language features, but from basic misuse of the standard library: accessing a vector out of bounds, calling front() on an empty container, or dereferencing an empty optional.

C++26 addresses this directly with standard library hardening, introduced via P3471R4.

What is library hardening?

Library hardening converts certain undefined behavior in the standard library into detectable contract violations at runtime. When a hardened precondition is violated, the runtime reacts before any other observable side effect — think of it as adding bounds checking to operations that are UB today.

This is not a new idea. All three major standard library implementations already ship vendor-specific hardening modes. The problem is that these mechanisms are all different, non-portable, and inconsistently specified. P3471R4 standardizes what implementations already do.

The standardized version of hardening becomes the first use of contracts (specified in P2900). This aligns perfectly with the direction of the language.