minix/external/bsd/lutok/dist/examples/raii.cpp

126 lines
4.9 KiB
C++
Raw Normal View History

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