From 0394ea1feda98e5e28be57b9b4efbdb9e4dbc400 Mon Sep 17 00:00:00 2001
From: Daniel Lemire <daniel@lemire.me>
Date: Mon, 27 Mar 2023 13:36:46 -0400
Subject: [PATCH 1/2] Compile-time evaluation

---
 README.md              | 21 +++++++++++++++++++++
 tests/example_test.cpp | 21 +++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/README.md b/README.md
index 3e53871..2cc5626 100644
--- a/README.md
+++ b/README.md
@@ -72,6 +72,27 @@ We support Visual Studio, macOS, Linux, freeBSD. We support big and little endia
 
 We assume that the rounding mode is set to nearest (`std::fegetround() == FE_TONEAREST`).
 
+## C++20: compile-time evaluation (constexpr)
+
+In C++20, you may use `fast_float::from_chars` to parse strings
+at compile-time, as in the following example:
+
+```C++
+// consteval forces compile-time evaluation of the function in C++20.
+consteval double parse(std::string_view input) {
+  double result;
+  auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
+  if(answer.ec != std::errc()) { return -1.0; }
+  return result;
+}
+
+// This function should compile to a function which
+// merely returns 3.1415.
+double constexptest() {
+  return parse("3.1415 input");
+}
+```
+
 ## Using commas as decimal separator
 
 
diff --git a/tests/example_test.cpp b/tests/example_test.cpp
index f2a0bce..cb9be38 100644
--- a/tests/example_test.cpp
+++ b/tests/example_test.cpp
@@ -43,6 +43,22 @@ void many_loop() {
   }
 }
 
+#if FASTFLOAT_IS_CONSTEXPR
+// consteval forces compile-time evaluation of the function in C++20.
+consteval double parse(std::string_view input) {
+  double result;
+  auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
+  if(answer.ec != std::errc()) { return -1.0; }
+  return result;
+}
+
+// This function should compile to a function which
+// merely returns 3.1415.
+double constexptest() {
+  return parse("3.1415 input");
+}
+#endif
+
 int main() {
     const std::string input =  "3.1416 xyz ";
     double result;
@@ -55,5 +71,10 @@ int main() {
         return EXIT_FAILURE;
     }
     many_loop();
+#if FASTFLOAT_IS_CONSTEXPR
+    if constexpr(constexptest() != 3.1415) {
+      return EXIT_FAILURE;
+    }
+#endif
     return EXIT_SUCCESS;
 }

From d7ba016c736a2812c9c742e3119b55f83ec00131 Mon Sep 17 00:00:00 2001
From: Daniel Lemire <daniel@lemire.me>
Date: Mon, 27 Mar 2023 13:46:51 -0400
Subject: [PATCH 2/2] Fix.

---
 README.md              | 2 +-
 tests/example_test.cpp | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 2cc5626..63c2c8c 100644
--- a/README.md
+++ b/README.md
@@ -88,7 +88,7 @@ consteval double parse(std::string_view input) {
 
 // This function should compile to a function which
 // merely returns 3.1415.
-double constexptest() {
+constexpr double constexptest() {
   return parse("3.1415 input");
 }
 ```
diff --git a/tests/example_test.cpp b/tests/example_test.cpp
index cb9be38..91437de 100644
--- a/tests/example_test.cpp
+++ b/tests/example_test.cpp
@@ -54,7 +54,7 @@ consteval double parse(std::string_view input) {
 
 // This function should compile to a function which
 // merely returns 3.1415.
-double constexptest() {
+constexpr double constexptest() {
   return parse("3.1415 input");
 }
 #endif