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cmake/external/glib/cppgir/expected-lite/example/02-required.cpp

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+// Use a non-ignorable value with expected.
+
+#include "nonstd/expected.hpp"
+#include <iostream>
+
+using namespace nonstd;
+
+template< typename T >
+class required
+{
+public:
+    required( T const & value)
+    : content( value ) {}
+
+    required( required && other )
+    : content( other.content )
+    , ignored( other.ignored )
+    {
+        other.ignored = false;
+    }
+
+    required( required const & other ) = delete;
+
+    ~required() noexcept( false )
+    {
+        if ( ignored )
+            throw std::runtime_error("required: content unobserved");
+    };
+
+    T const & operator *() const { ignored = false; return content; }
+
+private:
+    T content;
+    mutable bool ignored = true;
+};
+
+template< typename T >
+auto make_required( T value ) -> required<T>
+{
+    return required<T>( std::move(value) );
+}
+
+using unused_type = char;
+
+auto produce( int value ) -> expected< required<int>, unused_type >
+{
+    return make_required( std::move(value) );
+}
+
+int main( int argc, char * argv[] )
+{
+    try
+    {
+        auto er42 = produce( 42 );
+        auto er13 = produce( 13 );
+
+        std::cout << "value: " << **er42 << "\n";
+    }
+    catch ( std::exception const & e )
+    {
+        std::cout << "Error: " << e.what();
+    }
+}
+
+// cl -EHsc -wd4814 -Zc:implicitNoexcept- -I../include 02-required.cpp && 02-required.exe
+// g++ -std=c++14 -Wall -I../include -o 02-required.exe 02-required.cpp && 02-required.exe
+// value: 42
+// Error: required: content unobserved