From a6d3ae16736d7746ad6827f10c299ad84aa78a5b Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Wed, 12 Apr 2023 22:41:33 +0200
Subject: c++: add example for picking template with partial specialzations

---
 src/development/c++/tmpl-pair.cc | 73 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 src/development/c++/tmpl-pair.cc

(limited to 'src/development/c++/tmpl-pair.cc')

diff --git a/src/development/c++/tmpl-pair.cc b/src/development/c++/tmpl-pair.cc
new file mode 100644
index 0000000..a8acaed
--- /dev/null
+++ b/src/development/c++/tmpl-pair.cc
@@ -0,0 +1,73 @@
+// Copyright (C) 2023 johannst
+
+enum Kind {
+    kPrimary,
+    kTT,
+    kIntBool,
+    kIntInt,
+};
+
+// (1) Primary template.
+template<typename T, typename U = bool>
+struct pair {
+    static constexpr Kind kind = kPrimary;
+};
+
+// (2) Partial template specialization.
+template<typename T>
+struct pair<T, T> {
+    static constexpr Kind kind = kTT;
+};
+
+// (3) Template specialization.
+template<>
+struct pair<int, bool> {
+    static constexpr Kind kind = kIntBool;
+};
+
+// (4) Template specialization.
+template<>
+struct pair<int, int> {
+    static constexpr Kind kind = kIntInt;
+};
+
+int main() {
+    static_assert(pair<int>::kind == kIntBool, "");
+    // * Compare template arg list with primary template, we only supplied one
+    //   arg, the second one will be defaulted as
+    //   pair<int, bool>
+    // * Compare template arg list against available specializations, this will
+    //   try to match the pattern <int, bool> against the patterns defined in the
+    //   partial specializations.
+    // * (2) <int, bool> pattern does not match
+    // * (3) <int, bool> pattern does match
+    // * (4) <int, bool> pattern does not match
+    // * Pick the most specialized version -> (3)
+
+    static_assert(pair<char, char>::kind == kTT, "");
+    // * Compare template arg list against available specializations, this will
+    //   try to match the pattern <char, char> against the patterns defined in the
+    //   partial specializations.
+    // * (2) <char, char> pattern does match
+    // * (3) <char, char> pattern does not match
+    // * (4) <char, char> pattern does not match
+    // * Pick the most specialized version -> (2)
+
+    static_assert(pair<int, int>::kind == kIntInt, "");
+    // * Compare template arg list against available specializations, this will
+    //   try to match the pattern <int, int> against the patterns defined in the
+    //   partial specializations.
+    // * (2) <int, int> pattern does match
+    // * (3) <int, int> pattern does match
+    // * (4) <int, int> pattern does not match
+    // * Pick the most specialized version -> (3)
+
+    static_assert(pair<char, short>::kind == kPrimary, "");
+    // * Compare template arg list against available specializations, this will
+    //   try to match the pattern <char, short> against the patterns defined in the
+    //   partial specializations.
+    // * (2) <char, short> pattern does not match
+    // * (3) <char, short> pattern does not match
+    // * (4) <char, short> pattern does not match
+    // * No specialization matches, take the primary template.
+}
-- 
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