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This patch was inspired by work by Michiel De Wilde. But he used Boost variants which (i) requires Boost on the user side, (ii) is slow, and (iii) has useless overhead (the parser knows the type of the semantic value there is no reason to duplicate this information as Boost.Variants do). This implementation reserves a buffer large enough to store the largest objects. yy::variant implements this buffer. It was implemented with Quentin Hocquet. * src/output.c (type_names_output): New. (output_skeleton): Invoke it. * data/c++.m4 (b4_variant_if): New. (b4_symbol_value): If needed, provide a definition for variants. * data/lalr1.cc (b4_symbol_value, b4_symbol_action_) (b4_symbol_variant, _b4_char_sizeof_counter, _b4_char_sizeof_dummy) (b4_char_sizeof, yy::variant): New. (parser::parse): If variants are requested, define parser::union_type, parser::variant, change the definition of semantic_type, construct $$ before running the user action instead of performing a default $$ = $1. * examples/variant.yy: New. Based on an example by Michiel De Wilde.
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