[lmi-commits] [lmi] master 28d7b307 1/4: Assert a necessary precondition

[Greg Chicares]

branch: master
commit 28d7b307b5466a9105503e009d32e763f8878ce1
Author: Gregory W. Chicares <gchicares@sbcglobal.net>
Commit: Gregory W. Chicares <gchicares@sbcglobal.net>

    Assert a necessary precondition
    
    This may seem plausible:
        double e1 = nonstd::power
            (static_cast<double>(double_traits::radix)
            ,static_cast<double>(double_traits::digits)
            );
    but it doesn't compile:
        invalid 'double' operand to binary '&'
    Added a static assertion that type 'Integral' is integral.
    
    Noted that the code really assumes that type 'Integral' is not merely
    integral, but also unsigned.
---
 stl_extensions.hpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/stl_extensions.hpp b/stl_extensions.hpp
index f46d66d5..813820f7 100644
--- a/stl_extensions.hpp
+++ b/stl_extensions.hpp
@@ -55,6 +55,7 @@
 
 #include <functional>                   // multiplies, plus
 #include <stdexcept>                    // logic_error
+#include <type_traits>                  // is_integral_v
 
 namespace nonstd
 {
@@ -78,11 +79,15 @@ template <typename T> inline T 
identity_element(std::multiplies<T>)
 /// necessarily commutative.
 ///
 /// GWC modification: throw on negative exponent--otherwise, the loop
-/// appears not to terminate.
+/// may never terminate, because the bitwise operators don't work as
+/// intended with negative values. Alternative not used: assert that
+/// type Integer is unsigned, as the author evidently assumed--but
+/// imposing that requirement now breaks too much existing lmi code.
 
 template <typename T, typename Integer, typename MonoidOperation>
 T power(T x, Integer n, MonoidOperation opr)
 {
+    static_assert(std::is_integral_v<Integer>);
     if(n < 0)
         {
         throw std::logic_error("power() called with negative exponent.");