Replace many `const` variables with `constexpr`

constexpr is like const but (for variables) guarantees evaluation at
compile time (as opposed to runtime).

Likely this change will do nothing on its own (though it could, since
it gives the compiler more information). But it still has benefits.

It allows programmer to know that initialization is not expensive (it
was done at compile time), and reduces risk of a refactoring
regressing the compiletimeness.

Runtime initialization can be nonobvious in larger codebases. E.g.:
  struct S {
    static int foo();
  };
  const int bar = S::foo();  // Called and initialized at *runtime*.
  int S::foo() { return 10; }

With constexpr:
  struct S {
    static constexpr int foo();
  };
  constexpr int bar = S::foo();   // Error: used before definition.
  constexpr int S::foo() { return 10; }

Initializing at runtime is not just startup costs, but also can save
memory since it'll end up in a R/O section of a binary and therefore
doesn't need to be swapped out, but can be shared (in the mmap() sense
of the word).

2 files changed, 8 insertions, 8 deletions

diff --git a/gr-filter/lib/gen_interpolator_taps/gen_interp_differentiator_taps.c b/gr-filter/lib/gen_interpolator_taps/gen_interp_differentiator_taps.c
index aadd80114..a162058fd 100644
--- a/gr-filter/lib/gen_interpolator_taps/gen_interp_differentiator_taps.c
+++ b/gr-filter/lib/gen_interpolator_taps/gen_interp_differentiator_taps.c
@@ -162,11 +162,11 @@ int main(int argc, char** argv)
  */\n\n");
 
 
-    printf("static const int	DNTAPS     = %4d;\n", ntaps);
-    printf("static const int	DNSTEPS    = %4d;\n", nsteps);
-    printf("static const double	DBANDWIDTH = %g;\n\n", global_B);
+    printf("static constexpr int	DNTAPS     = %4d;\n", ntaps);
+    printf("static constexpr int	DNSTEPS    = %4d;\n", nsteps);
+    printf("static constexpr double	DBANDWIDTH = %g;\n\n", global_B);
 
-    printf("static const float Dtaps[DNSTEPS+1][DNTAPS] = {\n");
+    printf("static constexpr float Dtaps[DNSTEPS+1][DNTAPS] = {\n");
     printf("  //    -4            -3            -2            -1             0           "
            "  1             2             3                mu\n");
 
diff --git a/gr-filter/lib/gen_interpolator_taps/gen_interpolator_taps.c b/gr-filter/lib/gen_interpolator_taps/gen_interpolator_taps.c
index 3b22c74c2..4dd13a40a 100644
--- a/gr-filter/lib/gen_interpolator_taps/gen_interpolator_taps.c
+++ b/gr-filter/lib/gen_interpolator_taps/gen_interpolator_taps.c
@@ -177,11 +177,11 @@ int main(int argc, char** argv)
  */\n\n");
 
 
-    printf("static const int	NTAPS     = %4d;\n", ntaps);
-    printf("static const int	NSTEPS    = %4d;\n", nsteps);
-    printf("static const double	BANDWIDTH = %g;\n\n", global_B);
+    printf("static constexpr int	NTAPS     = %4d;\n", ntaps);
+    printf("static constexpr int	NSTEPS    = %4d;\n", nsteps);
+    printf("static constexpr double	BANDWIDTH = %g;\n\n", global_B);
 
-    printf("static const float taps[NSTEPS+1][NTAPS] = {\n");
+    printf("static constexpr float taps[NSTEPS+1][NTAPS] = {\n");
     printf("  //    -4            -3            -2            -1             0           "
            "  1             2             3                mu\n");