Cache GetSystemMetrics values with variable template.

The variable template could be generalized like this:

  template <auto F, auto... A> const auto cache = F(A...);

and instantiated like:

  cache<GetSystemMetrics, SM_CXVSCROLL>

It would still be limited to constant function arguments, which
usually isn't a problem for GetSystemMetrics, but might be for
other functions.

1 files changed, 9 insertions, 5 deletions

diff --git a/c/common.h b/c/common.h
index 2c0be13..c6c7dbd 100644
--- a/c/common.h
+++ b/c/common.h
@@ -54,7 +54,11 @@ std::optional<T> maybe_make(U... xs)
 	}
 }
 
-/* Call Windows API function, throwing error on NULL. */
+/* Variable template for caching values from GetSystemMetrics. */
+template <int I>
+const auto Metric = GetSystemMetrics(I);
+
+/* Check result of Windows API call, throwing error on NULL. */
 template <typename T>
 inline T require(const T x)
 {
@@ -62,7 +66,7 @@ inline T require(const T x)
 	return x;
 }
 
-/* Call Windows API function, showing a warning on NULL. */
+/* Check result of Windows API call, showing a warning on NULL. */
 template <typename T>
 inline T prefer(const T x)
 {
@@ -75,7 +79,7 @@ inline T prefer(const T x)
 }
 
 /* Conditionally choose between two values. This template is necessary
- * because the ternary conditional operator chooses only between
+ * because the ternary conditional operator can choose only between
  * values of the same type. */
 template <bool B, typename X, typename Y>
 constexpr std::enable_if_t<B == true, X> choose(X x, Y) { return x; }
@@ -83,10 +87,10 @@ template <bool B, typename X, typename Y>
 constexpr std::enable_if_t<B == false, Y> choose(X, Y y) { return y; }
 
 /* Conditionally choose between ANSI and wide string literal. */
-#define AWTEXT(t, s) choose<std::is_same_v<t, char>>(s, L"" s)
+#define AWTEXT(t, s) choose<std::is_same_v<t, wchar_t>>(L"" s, s)
 
 /* Conditionally choose between ANSI and wide Windows API function. */
-#define AWFUN(t, f) choose<std::is_same_v<t, char>>(f##A, f##W)
+#define AWFUN(t, f) choose<std::is_same_v<t, wchar_t>>(f##W, f##A)
 
 /* Return integer scaled for current DPI. */
 inline int Dpi(const int i)