4398. enable_nonlocking_formatter_optimization should be disabled for container adaptors

Section: 23.6.2 [queue.syn], 23.6.5 [stack.syn] Status: New Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-10-10

Priority: Not Prioritized

View all issues with New status.

Discussion:

As the standard currently defines formatters for queue, prioriy_queue, and stack enable_nonlocking_formatter_optimization is specialized to true for these adaptors per 28.5.6.4 [format.formatter.spec] p3:

Unless specified otherwise, for each type T for which a formatter specialization is provided by the library, each of the headers provides the following specialization:

template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

However, formatting an adaptor requires formatting of the underlying range in terms of ranges::ref_view, and we disable the nonlocking_optimizations for all ranges, including ranges::ref_view.

This problem does not occur for the flat_set, flat_map adaptors, which are also ranges, but unlike stack etc. they do not have a specialized formatter. They use the formatter specialization for ranges and we already disable the optimization for that formatter.

The proposed wording has recently been implemented in gcc's libstdc++.

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.6.2 [queue.syn], header <queue> synopsis, as indicated:

   […]
   // 23.6.13 [container.adaptors.format], formatter specialization for queue
   template<class charT, class T, formattable<charT> Container>
     struct formatter<queue<T, Container>, charT>;
     
   template<class T, class Container>
     constexpr bool enable_nonlocking_formatter_optimization<queue<T, Container>> = false;
   
   // 23.6.4 [priority.queue], class template priority_queue
   template<class T, class Container = vector<T>,
            class Compare = less<typename Container::value_type>>
     class priority_queue;
   […]
   // 23.6.13 [container.adaptors.format], formatter specialization for priority_queue
   template<class charT, class T, formattable<charT> Container, class Compare>
     struct formatter<priority_queue<T, Container, Compare>, charT>;
     
   template<class T, class Container, class Compare>
     constexpr bool enable_nonlocking_formatter_optimization<priority_queue<T, Container, Compare>> = false;
   […]
   

2. Modify 23.6.5 [stack.syn], header <stack> synopsis, as indicated:

   […]
   
   // 23.6.13 [container.adaptors.format], formatter specialization for stack
   template<class charT, class T, formattable<charT> Container>
     struct formatter<stack<T, Container>, charT>;
   
   template<class T, class Container>
     constexpr bool enable_nonlocking_formatter_optimization<stack<T, Container>> = false;
   
   […]