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libs/lexical_cast/example/variant_to_long_double.cpp

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+// Copyright 2013 Antony Polukhin
+
+// Distributed under the Boost Software License, Version 1.0.
+// (See the accompanying file LICENSE_1_0.txt
+// or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)
+
+
+//[lexical_cast_variant_to_long_double
+/*`
+    In this example we'll make a `to_long_double` method that converts value of the Boost.Variant to `long double`.
+*/
+
+#include <boost/lexical_cast.hpp>
+#include <boost/variant.hpp>
+#include <cassert>
+
+struct to_long_double_functor: boost::static_visitor<long double> {
+    template <class T>
+    long double operator()(const T& v) const {
+        // Lexical cast has many optimizations including optimizations for situations that usually 
+        // occur in generic programming, like std::string to std::string or arithmetic type to arithmetic type conversion. 
+        return boost::lexical_cast<long double>(v);
+    }
+};
+
+// Throws `boost::bad_lexical_cast` if value of the variant is not convertible to `long double`
+template <class Variant>
+long double to_long_double(const Variant& v) {
+    return boost::apply_visitor(to_long_double_functor(), v);
+}
+
+int main() {
+    boost::variant<char, int, std::string> v1('0'), v2("10.0001"), v3(1);
+
+    long double sum = to_long_double(v1) + to_long_double(v2) + to_long_double(v3);
+    assert(sum > 11 && sum < 11.1);
+}
+
+//] [/lexical_cast_variant_to_long_double]