127 lines
4.9 KiB
C++
127 lines
4.9 KiB
C++
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// Copyright 2012 Google Inc.
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above copyright
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// notice, this list of conditions and the following disclaimer in the
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// documentation and/or other materials provided with the distribution.
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// * Neither the name of Google Inc. nor the names of its contributors
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// may be used to endorse or promote products derived from this software
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// without specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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/// \file examples/raii.cpp
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/// Demonstrates how RAII helps in keeping the Lua state consistent.
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///
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/// One of the major complains that is raised against the Lua C API is that it
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/// is very hard to ensure it remains consistent during the execution of the
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/// program. In the case of native C code, there exist many tools that help the
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/// developer catch memory leaks, access to uninitialized variables, etc.
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/// However, when using the Lua C API, none of these tools can validate that,
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/// for example, the Lua stack remains balanced across calls.
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///
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/// Enter RAII. The RAII pattern, intensively applied by Lutok, helps the
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/// developer in maintaining the Lua state consistent at all times in a
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/// transparent manner. This example program attempts to illustrate this.
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#include <cassert>
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#include <cstdlib>
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#include <iostream>
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#include <string>
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#include <lutok/operations.hpp>
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#include <lutok/stack_cleaner.hpp>
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#include <lutok/state.ipp>
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/// Prints the string-typed field of a table.
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///
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/// If the field contains a string, this function prints its value. If the
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/// field contains any other type, this prints an error message.
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///
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/// \pre The top of the Lua stack in 'state' references a table.
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///
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/// \param state The Lua state.
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/// \param field The name of the string-typed field.
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static void
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print_table_field(lutok::state& state, const std::string& field)
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{
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assert(state.is_table(-1));
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// Bring in some RAII magic: the stack_cleaner object captures the current
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// height of the Lua stack at this point. Whenever the object goes out of
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// scope, it will pop as many entries from the stack as necessary to restore
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// the stack to its previous level.
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//
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// This ensures that, no matter how we exit the function, we do not leak
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// objects in the stack.
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lutok::stack_cleaner cleaner(state);
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// Stack contents: -1: table.
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state.push_string(field);
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// Stack contents: -2: table, -1: field name.
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state.get_table(-2);
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// Stack contents: -2: table, -1: field value.
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if (!state.is_string(-1)) {
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std::cout << "The field " << field << " does not contain a string\n";
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// Stack contents: -2: table, -1: field value.
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//
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// This is different than when we started! We should pop our extra
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// value from the stack at this point. However, it is extremely common
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// for software to have bugs (in this case, leaks) in error paths,
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// mostly because such code paths are rarely exercised.
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//
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// By using the stack_cleaner object, we can be confident that the Lua
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// stack will be cleared for us at this point, no matter what happened
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// earlier on the stack nor how we exit the function.
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return;
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}
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std::cout << "String in field " << field << ": " << state.to_string(-1)
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<< '\n';
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// A well-behaved program explicitly pops anything extra from the stack to
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// return it to its original state. Mostly for clarity.
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state.pop(1);
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// Stack contents: -1: table. Same as when we started.
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}
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/// Program's entry point.
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///
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/// \return A system exit code.
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int
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main(void)
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{
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lutok::state state;
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state.open_base();
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lutok::do_string(state, "example = {foo='hello', bar=123, baz='bye'}",
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0, 0, 0);
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state.get_global("example");
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print_table_field(state, "foo");
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print_table_field(state, "bar");
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print_table_field(state, "baz");
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state.pop(1);
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return EXIT_SUCCESS;
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}
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