Merge pull request #188 from fastfloat/dlemire/compile_time

Compile-time evaluation

Author: lemire

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.
+constexpr double constexptest() {
+  return parse("3.1415 input");
+}
+```
+
 ## Using commas as decimal separator
 
 

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.
+constexpr 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;
 }