BDE 4.18.0 Release

Schedule

• The BDE team announces that the BDE 4.18.0 production release was completed on Monday, Dec 16th, 2024.

BDE 4.18.0 Release Highlights

New Transparent Hasher Tailored for Strings: bdlb::TransparentStringHash

This release introduces the component bdlb::TransparentStringHash that provides a transparent hash function that is specifically tailored to be used with bsl::string keys.

A transparent hash function combined with using a transparent comparator allows to avoid unnecessary conversions when looking up a key in unordered containers. The most common use case is to avoid constructing a bsl::string when looking up a string key represented by a const char * or bsl::string_view. Without transparent hash and comparator, looking up a const char * in a bsl::unordered_set<bsl::string> would incur a conversion to bsl::string and possibly an unnecessary allocation. Furthermore, attempting to look up a bsl::string_view fails to compile since there is no implicit conversion from bsl::string_view to bsl::string:

bsl::unordered_set<bsl::string> set{"hello", "goodbye"};

assert(set.contains("hello"));                    // Passes, but creates a temporary ``bsl::string``!
assert(set.contains(bsl::string_view("hello")));  // Does not compile - no implicit conversion!

Although using bdlb::TransparentHash and bdlb::TransparentEqualTo allows to look up a bsl::string_view and removes the unnecessary conversion, it comes with a surprising result where strings seemingly having the same value are no longer found in the set:

bsl::unordered_set<bsl::string,
    bdlb::TransparentHash,
    bdlb::TransparentEqualTo> set{"hello", "goodbye"};

assert(set.contains("hello"));                    // No allocation, assertion FAILS!
assert(set.contains(bsl::string_view("hello")));  // No allocation, assertion PASSES.

This behavior is due to the default hasher, to which bdlb::TransparentHash delegates, not treating const char * as a string, but rather as a raw pointer, hashing the pointer value itself. With bdlb::TransparentStringHash, however, const char * is treated as a C-style string and is hashed accordingly, producing the expected result:

bsl::unordered_set<bsl::string,
    bdlb::TransparentStringHash,
    bdlb::TransparentEqualTo> set{"hello", "goodbye"};

assert(set.contains("hello"));                    // No allocation, assertion PASSES.
assert(set.contains(bsl::string_view("hello")));  // No allocation, assertion PASSES.

bslim::Printer Now Supports Tuples

This release adds support to bslim::Printer for printing objects of type bsl::tuple, provided that each element type can be printed using bslim::Printer. For example:

bsl::tuple<int, const char*, double> t(1, "two", 3.14);
bslim::Printer                       p(&bsl::cout, 0, -1);

p.printAttribute("tuple", t);

will output:

tuple = [ 1 "two" 3.14 ]

Note that bsl::tuple is an alias to std::tuple.

Fixed DRQSs:

Summary
bdlt::Datetime::add*IfValid – internal over/underflow undermines wide contracts
Add support for Linux ARM64 - fix tests
Support bsl::tuple in bslim::Printer
bdlde_base64-en-de-coderoptions documentation
please correct/reduce bdlcc_deque case 15 intermittent issue
Backport some C++20 type traits
Add a release method to TempDirectoryGuard
Add bdlcc::TimeQueue::removeIf w/ predicate
Transparent Comparators For String
Restore comments regarding key construction in map
Fix ryu build for cmake_build.py
Fix build error in bslmf_unwrapreference