adding examples, improving any_arg, improving error messages

example/is_constexpr/function_object.cpp

@@ -0,0 +1,73 @@
+/*
+Copyright Barrett Adair 2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http ://boost.org/LICENSE_1_0.txt)
+*/
+
+#include <type_traits>
+#include <callable_traits/callable_traits.hpp>
+
+namespace ct = callable_traits;
+
+//this is a constexpr function object (non-templated)
+struct zero {
+
+    constexpr auto operator()() {
+        return 0;
+    }
+};
+
+static_assert(ct::is_constexpr<zero>(), "");
+static_assert(ct::is_constexpr(zero{}), "");
+
+
+
+//this is a constexpr function object (templated)
+struct subtract {
+
+    // For callable_traits::is_constexpr, generic function objects
+    // only need to be SFINAE-friendly if the body of the operator()
+    // function accesses member names besides "type" and "value".
+    // Unary/binary operators and constructor calls are okay to use.
+    template<typename T1, typename T2>
+    constexpr auto operator()(T1, T2) {
+        return T1{} - T2{};
+    }
+};
+
+static_assert(ct::is_constexpr<subtract>(), "");
+static_assert(ct::is_constexpr(subtract{}), "");
+
+
+
+//this is NOT a constexpr function object
+struct add {
+    template<typename T1, typename T2>
+    auto operator()(T1, T2) {
+        return T1{} + T2{};
+    }
+};
+
+static_assert(!ct::is_constexpr<add>(), "");
+static_assert(!ct::is_constexpr(add{}), "");
+
+
+
+
+/*
+// The case below fails to compile, because lambdas are not trivially
+// default constructible. (Note: This restriction also applies to the
+// argument types). Error message: "Cannot perform constexpr checks
+// with this type, because it is not trivially default constructible."
+
+auto multiply = [](auto t1, auto t2) -> decltype(t1.value * t2.value) {
+    return t1.value * t2.value;
+};
+
+static_assert(!ct::is_constexpr<decltype(multiply)>(), "");
+static_assert(!ct::is_constexpr(multiply), "");
+*/
+
+
+
+int main() {}