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++.md             |  4 +++
 src/development/c++/Makefile       |  5 ++-
 src/development/c++/concepts-11.cc |  3 +-
 src/development/c++/tmpl-pair.cc   | 73 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 82 insertions(+), 3 deletions(-)
 create mode 100644 src/development/c++/tmpl-pair.cc

(limited to 'src')

diff --git a/src/development/c++.md b/src/development/c++.md
index e6fcfef..9dc8fb0 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -69,6 +69,10 @@ struct is_valid<T, std::void_t<
 ```
 > `std::declval<T>()` creates an instance of type T in an unevaluated context.
 
+## Template selection with partially / fully specializations.
+```cpp
+{{#include c++/tmpl-pair.cc:3:}}
+```
 
 [gist-strict-asliasing]: https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
 [parameter-pack]: https://en.cppreference.com/w/cpp/language/parameter_pack
diff --git a/src/development/c++/Makefile b/src/development/c++/Makefile
index 610b5a2..1d5172b 100644
--- a/src/development/c++/Makefile
+++ b/src/development/c++/Makefile
@@ -1,4 +1,4 @@
-SRC = concepts-11.cc meta.cc meta2.cc meta4.cc
+SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc
 BIN = $(SRC:.cc=)
 
 all: $(BIN)
@@ -6,5 +6,8 @@ all: $(BIN)
 %: %.cc
 	$(CXX) -o $* $^ -std=c++17 -g -fsanitize=address -fsanitize=undefined -fsanitize=leak
 
+fmt:
+	clang-format -i $(SRC)
+
 clean:
 	$(RM) $(BIN)
diff --git a/src/development/c++/concepts-11.cc b/src/development/c++/concepts-11.cc
index 888ff4d..cec8716 100644
--- a/src/development/c++/concepts-11.cc
+++ b/src/development/c++/concepts-11.cc
@@ -32,8 +32,7 @@ struct is_entry {
     template<typename TT>
     using val = decltype(std::declval<TT>().val());
 
-    static constexpr bool value = is_valid_v<T, init> &&
-                                  is_valid_with_ret_v<T, int, tag> &&
+    static constexpr bool value = is_valid_v<T, init> && is_valid_with_ret_v<T, int, tag> &&
                                   is_valid_with_ret_v<T, typename T::Type, val>;
 };
 
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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