From dc3a96a29bb407bae9804d33f39d2b0dfc426016 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ion=20Gazta=C3=B1aga?= <igaztanaga@gmail.com>
Date: Wed, 21 Jan 2026 23:40:18 +0100
Subject: [PATCH] Add C++20 "starts_with" overloads

---
 doc/container.qbk                  |  2 +-
 include/boost/container/string.hpp | 32 ++++++++++++++++++++++++++++++
 test/string_test.cpp               | 29 ++++++++-------------------
 3 files changed, 41 insertions(+), 22 deletions(-)

diff --git a/doc/container.qbk b/doc/container.qbk
index 51ac0c1..f1664ee 100644
--- a/doc/container.qbk
+++ b/doc/container.qbk
@@ -1461,7 +1461,7 @@ use [*Boost.Container]? There are several reasons for that:
 * Implemented overaligned allocation support for `adaptive_pool`and `node_allocator`
 * Updated `basic_string` to the latest standard API:
    * Added missing `string_view` members and updated `operator[]` to be able to return the terminating null.
-   * Added C++23 `contains` overloads
+   * Added C++20 `starts_with` and C++23 `contains` overloads.
 * Fixed bugs/issues:
    * [@https://github.com/boostorg/container/issues/323 GitHub #323: ['"flat_tree::try_emplace UB"]].
    * [@https://github.com/boostorg/container/issues/328 GitHub #328: ['"boost::container::deque stores a redundant copy of the allocator, increasing size"]].
diff --git a/include/boost/container/string.hpp b/include/boost/container/string.hpp
index a8d0e89..fc88a54 100644
--- a/include/boost/container/string.hpp
+++ b/include/boost/container/string.hpp
@@ -2981,6 +2981,38 @@ class basic_string
       bool contains(const CharT* s) const BOOST_NOEXCEPT
    {  return this->find(s) != npos; }
 
+   //! <b>Effects</b>: Checks if the string begins with the given prefix
+   //!
+   //! <b>Throws</b>: Nothing
+   //!
+   //! <b>Returns</b>: true if the string begins with the provided prefix, false otherwise.
+   template<template <class, class> class BasicStringView>
+   BOOST_CONTAINER_NODISCARD inline
+      bool starts_with(BasicStringView<CharT, Traits> sv) const BOOST_NOEXCEPT
+   {  return this->size() >= sv.size() && Traits::compare(this->data(), sv.data(), sv.size()) == 0;  }
+
+   //! <b>Effects</b>: Checks if the string begins with the given prefix
+   //!
+   //! <b>Throws</b>: Nothing
+   //!
+   //! <b>Returns</b>: true if the string begins with the provided prefix, false otherwise.
+   BOOST_CONTAINER_NODISCARD inline
+      bool starts_with(CharT c) const BOOST_NOEXCEPT
+   {  return !empty() && Traits::eq(front(), c);  }
+
+   //! <b>Effects</b>: Checks if the string begins with the given prefix
+   //!
+   //! <b>Throws</b>: Nothing
+   //!
+   //! <b>Returns</b>: true if the string begins with the provided prefix, false otherwise.
+   BOOST_CONTAINER_NODISCARD inline
+      bool starts_with(const CharT* s) const BOOST_NOEXCEPT
+   {
+      const size_type s_sz = Traits::length(s);
+      const size_type t_sz = this->size();
+      return t_sz >= s_sz && Traits::compare(this->data(), s, s_sz) == 0;
+   }
+
    #ifndef BOOST_CONTAINER_DOXYGEN_INVOKED
    private:
    void priv_move_assign(BOOST_RV_REF(basic_string) x, dtl::bool_<true> /*steal_resources*/)
diff --git a/test/string_test.cpp b/test/string_test.cpp
index ec30592..167ed7d 100644
--- a/test/string_test.cpp
+++ b/test/string_test.cpp
@@ -1481,13 +1481,7 @@ void test_compare()
 // Helper functions for starts_with/ends_with/contains since boost::container::string
 // might not have these methods in older versions
 namespace test_helpers {
-   bool starts_with(const string& s, const char* prefix) {
-      string::size_type len = std::strlen(prefix);
-      return s.size() >= len && s.compare(0, len, prefix) == 0;
-   }
-   bool starts_with(const string& s, char c) {
-      return !s.empty() && s[0] == c;
-   }
+
    bool ends_with(const string& s, const char* suffix) {
       string::size_type len = std::strlen(suffix);
       return s.size() >= len && s.compare(s.size() - len, len, suffix) == 0;
@@ -1495,27 +1489,20 @@ namespace test_helpers {
    bool ends_with(const string& s, char c) {
       return !s.empty() && s[s.size() - 1] == c;
    }
-   bool contains(const string& s, const char* substr) {
-      return s.find(substr) != string::npos;
-   }
-   bool contains(const string& s, char c) {
-      return s.find(c) != string::npos;
-   }
 }
 
 void test_starts_with()
 {
    string s("Hello, World!");
 
-   using test_helpers::starts_with;
-   BOOST_TEST(starts_with(s, "Hello"));
-   BOOST_TEST(starts_with(s, "H"));
-   BOOST_TEST(starts_with(s, ""));
-   BOOST_TEST(starts_with(s, 'H'));
+   BOOST_TEST(s.starts_with("Hello"));
+   BOOST_TEST(s.starts_with("H"));
+   BOOST_TEST(s.starts_with(""));
+   BOOST_TEST(s.starts_with('H'));
 
-   BOOST_TEST(!starts_with(s, "World"));
-   BOOST_TEST(!starts_with(s, "hello")); // case-sensitive
-   BOOST_TEST(!starts_with(s, 'W'));
+   BOOST_TEST(!s.starts_with("World"));
+   BOOST_TEST(!s.starts_with("hello")); // case-sensitive
+   BOOST_TEST(!s.starts_with('W'));
 }
 
 void test_ends_with()