Token – Shared Pointer Preparation

The first attempt to change token pointers to shared pointers failed (after the change, the code had many memory errors and crashes).  The problems may have been caused by the various containers holding token values being passed as pointers (RPN items in the RPN output list, and items on the translator hold and done stacks).  These containers have now been updated so that their contents are destructed properly and automatically.

The change to shared pointers is being crept up on.  The next step was to replace all references to Token* (pointers to tokens) with the alias TokenPtr:

using TokenPtr = Token *;

This will allow for an easy change to shared pointers.  The changes made during the previous failed attempt were extensive, and it was hard to determine exactly which changes caused the problems.  So the alias change will be separate from the shared pointer change.  The alias change was made first and thoroughly tested (using the memory test script).

Another preparatory change made included passing a constant reference to a token pointer to functions that don't modify the token passed.  This doesn't have much effect for simple pointers, but for shared pointers, it will prevent a copy from being made of the pointer (which incremented its used count, only to be decremented when the argument goes out of scope at the end of the function).

The use of the NULL definition for checking if a token pointer is unset or to assign it to an unset value was changed to the C++11 nullptr, an actual null pointer.  For tokens pointers initialized to a null pointer, the {} C++11 initializer syntax was used.

[branch cpp11 commit 233a8f4017]

Translator – Done Stack As Standard Stack

There were no surprises with changing the done stack to a standard stack, which went like that of the hold stack.  The Done Stack class was replaced with a single alias to std::stack and the functions were changed accordingly.

With the change of the push calls to emplace calls, a constructor was required in the Done Item class.  The push contained arguments for the RPN item pointer, first and last token pointers, where the first and last pointers were optional (defaulting to a null pointer).  The use patterns for the push function were:

• RPN item pointer only
• RPN item pointer and last token pointer
• RPN item pointer, first and last token pointers

A similar constructor with the three arguments and the same optional arguments could have been added, but this would continue to require a null pointer for the middle use pattern.  The first and last tokens pointers could have been switched, but this could lead to confusion.  Instead, three specific constructors were added for the the above use patterns with no optional arguments.

[branch cpp11 commit ca85a259fa]

Translator – Done Stack Realignment

The done stack of the translator is used for temporarily holding RPN items appended to the RPN output list to be consumed as operands by operators, functions and commands.  This will be the next stack changed to std::stack.  But first some realignment was required to move functionality from the Done Stack class to the Done Item structure.  Like the Hold Stack, the Done Stack also has QStack as its base class with functionality added.  Unlike the Hold Stack class, this added functionality did more than just add stack convenience functions.

Each item on the done stack contains a shared pointer to the RPN item plus token pointers to the first and last token of the expression of the RPN item, which may contain open and closing parentheses tokens that will eventually be dropped (in most cases).  These are for reporting errors to an entire expression including open and closing parentheses.  The parentheses token are deleted when they are no longer needed.  A closing parentheses is not deleted for extra parentheses that need recreated.

The non-stack functionality added was for handling these first and last tokens.   The added pop function only returned the RPN item of the done item popped from the stack, but also deleted the first and last tokens if they contained parentheses (checking if unused for the last token).  Similarly, the drop function also deleted the first and last tokens if parentheses.  These called functions in Done Item structure to do the check and delete of the tokens.  The final non-stack function replaced the first and last tokens of the done item on top of the stack.

This non-stack functionality was moved to the Done Item structure.  The replace top first last function of the done stack called the replace first and replace last functions of the done item (the only caller to these functions).  Each replace function deleted the token being replaced if a parentheses token.  These functions were combined into a single replace first last function, which is now called directly for the top item on the stack and the done stack function was removed.

The added pop and drop functions both deleted the first and last tokens if parentheses.  These functions also decreased the size of the stack (by a stack pop or resize down one element).  This would cause the destructor of the done item to be called.  No destructor was declared, but since the RPN item was changed to a shared pointer, the compiler generated a default destructor to call the destructor of the shared pointer.  The last and first token pointers were plain old data, so they were not affected.  The delete parentheses calls were removed from the pop and drop functions, and added to a new done item destructor (the RPN item destructor is still handled automatically).  However, this caused a curious problem in pop function (a multiple deleted closing parentheses token), which now contained the single statement:

return QStack::pop().rpnItem;

The pop() returns a copy of the done item on top of the stack.  The item on top of the stack is then removed, which calls the new done item destructor for the item (deleting a closing parentheses in the last token pointer).  The copy is used to get the RPN item, and then the copy goes out of scope, which calls the done item destructor.  Since the copy has the same last parentheses token as the first, it gets erroneously deleted twice.  This was happening previously, but since the [default] destructor was not affecting the first and last tokens, no double delete occurred.  This was corrected by replacing the above statement with:

RpnItemPtr rpnItem = top().rpnItem;
drop();
return rpnItem;

Now a temporary of only the RPN item is made.  The drop() call resizes the stack down one item, causing the new destructor to be called, deleting the last token holding an unused closing parentheses.  The RPN item is then returned.  This also matches the operations that will be required for std::stack.  These specialized delete parentheses token functions won't be needed once shared pointers are used for token pointers.

[branch cpp11 commit c23603ea2e]

Translator – Hold Stack As Standard Stack

Continuing with the transition to STL classes and for the preparation of changing token pointers to a shared smart pointer, the hold stack, which holds token pointers, was changed from a QStack to a std::stack.  This stack contains Hold Item structures, each with a pointer to a token and a pointer to the token of the first operand if the token is an operator.  The Hold Stack class was created to add two functions to the QStack used as its base class.

The added push function contained arguments for the pointer to a token and an optional pointer first operand token.  The stack was resized up by one element and the new top element was set to the token pointers provided.  This was an attempt to optimize the push operation.  The default constructor would be called to the stack item added by the resize, but since the Hold Item structure only contained plain old data members (pointers), there was no constructor.

The added drop function resized the stack down one element.  The destructor for the element dropped would be called, but again, since the element was a simple structure, there was no destructor.

The functions of std::stack work differently.  With C++11, the std::stack gains the emplace function, which as previously described, does an in-place construction of the item being pushed.  This is exactly what the added push function was implemented to achieve, so all push calls were changed to emplace although this required that the Hold Item have a constructor with arguments for the token pointer and optional first token pointer (like the added push function).

The pop function of the std::stack class does not return a reference like with QStack and it only pops the top item off of the stack (there is no return value).  Therefore, all pop calls had to be changed to top calls (to get the value) followed by a pop call.  However, the pop function is identical to the added drop function, so these were changed to pop calls.  Finally, the isEmpty() calls were changed to the standard equivalent empty() and the Hold Stack class was removed.

An attempt was made to modify the Hold Stack class to combine the top and pop functions, but the resulting code was slightly larger and slower than using std::stack directly (determined by using a small test program with many iterations).  This is probably why these functions were implemented the way they were.  Therefore, these functions will be used as is.

The std::stack class is an adapter for the underlying class, which by default is the std::deque ("deck") class that works like both a vector (array) and double linked list (sounds expensive).  The std::stack class can work with any class that supports empty, size, back, push back and pop back.  The class is specified as the second template parameter after the stack element type.  Using a test program, a std::vector and std::list were also tried, but the default std::deque class was the fastest (though not the smallest).  The std::forward_list class, a simple single linked list, was also tried as a stack, though it can't be used with the std::stack class.  And while std::forward_list is the simplest, it was also the slowest (by over four times).  Therefore, the default std::stack will be used.

[branch cpp11 commit e4d568e43f]

RPN List – Pass By Value

The decode and translate now return RPN lists by value, either from a local variable or member variable as described in recent posts.  For the functions that had an RPN list pointer as an argument, the argument was changed to a constant reference (constant because the RPN list is not modified).  The callers that receive RPN lists no longer are responsible for deleting the instance, which will occur automatically when the variable goes out of scope, so all the delete calls were removed.

One issue that was discovered during compiling of functions taking a constant reference to an RPN list was an error when using the code size access function.  This function only returned the code size member and did not modify the object, so it should haven declared as a constant function.

The translate input routine in the Tester class previously returned a null pointer to indicate an error.  Since a pointer is no longer returned, the RPN list is cleared and an empty list is returned to indicate an error.  An empty list could also mean an empty BASIC line (which is allowed), but empty lines are already being ignored and are not translated duing testing.

The null pointer initializer for the output list member was removed from the constructor of the Translator class since it is no longer a pointer - it is now initialized using the default constructor for the RPN List class.  All uses accessing the pointer member were changed to regular member (dot) operators.

[branch cpp11 commit 5b9245cf0a]

RPN List – Default Move Not Sufficient

The RPN list class contains other members in addition to the list including the code size, error column, error length and error message members.  Since the list member is a standard list with a move operation, it is cleared by the default generated move constructor when calling the standard move in the return statement of the translate routine.  All but the error message are integers with no default move operation so these members are not affected.  The error message is currently a QString, which also does not have a move operation (the Qt5 classes do) and was also not affected.

When a new RPN list instance was allocated at the beginning of the translate routine, these other  members were initialized according to the constructor (though only the error members were initialized).  Now, at the return statement, the list of the RPN list member is cleared from the default move, but with no move operations for the other members, these members retained their values.

This was detected while running the tests.  Once a line with an error was reported, that same error was reported for every line afterward (at least until there was a different error).  This occurred because the error column member (used to test if the RPN list has an error) was not initialized for the next translation.

One solution would be to reset these other members at the at the beginning where the allocation was.  The better solution of course is to add move constructor and assignment functions.  The move constructor initializes each of the members from the other RPN list instance.  In the function body, each of the members of the other RPN list instance are set to an initialized state.  The list and error message members are cleared and the integer variables are set to appropriate default values.  The move assignment swaps all the members before returning a reference to the instance.

RPN List – Return Non-Local Variable

Both the decode and translate routines generate an RPN list and instead of a pointer to the list will return the list by value, preferably by a move operation.  The decode routine (of the Program Model class) contains a local RPN list variable, and so the value of the variable is returned by a move operation since the variable is going out of scope.  However, the translate routine builds the RPN list in a member value of the Translator class (so all translator functions have access to the list).  If this variable is returned by value, the value is copied because the variable is not going out of scope.

With the pointer implementation when a pointer was returned, since the translate routine no longer needed the list, it made a temporary copy of the member list pointer, set the member pointer to null and returned the temporary copy of the pointer (this transferred the list to the caller who was then responsible for deleting the list):

RpnList *output = m_output;
m_output = NULL;
return output;

The Translator class RPN list member will be the actual list, not a pointer to a list.  When returning from the translate routine, the RPN list is no longer needed, so it can be transferred to the caller with a move operation.  This move does not occur by just returning the variable because it is not going out of scope.  The move operation can be forced by using the standard move (which resets the member variable to an empty list and transfers its value to the receiving variable of the caller):

return std::move(m_output);

By default, C++ provides a default constructor, copy constructor, copy assignment, move constructor, move assignment and destructor for a class if a program uses them.  If a constructor is declared, a default constructor is not generated.  If a copy operation, move operation or destructor is declared, then no default copy operation, move operation or destructor is generated.

Defining a destructor indicates that something other than the default operation needs to be done for a copy or move (for example, dealing with an allocated resource).  However, these rules are not completely enforced.  For backward compatibility, GCC still generates default copy operations if a destructor is defined without a warning.

The upstream problem mentioned a few posts ago http://interactivebasiccompilerproject.blogspot.com/2014/09/rpn-list-base-class-to-member.html was thought to have occurred due the destructor of the list base class was not being called because the above return move statement was not clearing the list in the member variable.  The problem was finally identified to be caused by the destructor (which wasn't doing anything) declaration was removed.  With the destructor, there were no move operations, the standard move performed a copy.  Removing the destructor allowed the default move operations to be generated.  A default move operation can be generated using the new C++11 syntax:

ClassName(ClassName &&other) = default;

This solved the issue of the member list not being cleared.  However, the default move operations were not sufficient as there was another problem...

C++11 Move Constructors and Assignments

The actual instances of RPN lists will be returned from functions instead of a pointer to an allocated instance that requires the receiver to delete the instance when it is done with it (or a memory leak occurs).  The new C++11 move constructors and assignments feature makes this efficient.

When a function is called that has a return value, temporary memory (usually on the stack) needs to set aside hold the return value because the variable holding the return value in called function goes out of scope and will be destroyed (destructor called) if not a plain old data (POD) type.  Prior to C++11, the return value was copied from the temporary into a receiving variable using a copy constructor if not a POD type.  If the variable was a large container like a list or vector, this was not efficient.  This is why RPN lists were returned by a pointer.

However, with a C++11 move constructor, the return value is moved directly into the variable being constructed or assigned, each having a slightly different mechanism.  For a container class like a list or vector, the only thing that is moved is the pointer to the data, not the actual data itself.  Consider this example of a function that returns a list and code that calls the function:

std::list<SomeType> someFunction()
{
    std::list<SomeType> localList;
    ... list created here ...
    return localList;
}
...
std::list<SomeType> list {someFunction()};     ← caller

The new C++11 initializer syntax is used to show that the move constructor is called (but the old assignment syntax does the same thing).  Without a move constructor, the local list variable destructor would be called after the return value was copied to a temporary.  With the move constructor, the return value is moved directly into the receiving variable and the local variable is reset, and when the destructor for the local is called, it has nothing to do.  With a container class, minimal values are copied (essentially a pointer with a little other possible housekeeping like a size).  A move constructor has this basic layout:

ClassName(ClassName &&other) :
    member1{other.member1},
    member2{other.member2},
    ...
{
    other.member1{};
    other.member2{};
    ...
}

First the members of the receiving variable are initialized with another instance (the return variable).  In the body of the move constructor, the other members are initialized to defaults.  If one of the members was a pointer to allocated data, the pointer is transferred, and the pointer in the return variable that is about to go out of scope is set to null, so when its destructor is called, it has nothing to delete.

A move assignment is similar, but is used when the receiving variable (that already exists) is being assigned to a return value, where the caller looks like this:

std::list<SomeType> list;
...
list = someFunction();

In this case, the value in the receiving variable must be destroyed first and then the return value moved in.  Without a move assignment, this is handled by the copy constructor, which destroys the old value, and copies the new value.  The destructor for the local variable is called.  A move assignment has this basic layout:

ClassName &operator=(ClassName &&other)
{
    std::swap(member1, other.member1);
    std::swap(member2, other.member2);
    ...
    return *this;
}

For each member, the values of the members are swapped.  The standard swap function does this efficiently with move operations where a member is moved into a temporary variable, the other member is moved into the current member, and the temporary moved into the other member.  No destructors are called for any of the moves.  Here, the other instance is the local variable of the function that is going out of scope.  What this does is moves the value of the receiving variable into the local variable, which then gets destroyed.  As with any assignment, the reference of the object is returned.

The above descriptions may not be exactly what occurs internally, but it is my interpretation and explains the basic concept behind the move mechanisms.  One final mention is that if the object does not have a move constructor or assignment, then the old copy through a temporary mechanism (copy constructor) is used.  There are reasons why an object may not have move operations, which will be covered in the next post.

One last note.  The implicit sharing feature of the container classes of Qt is another mechanism to make returning instances more efficient.  This feature acts like a shared smart pointer.  The return value is copied to the temporary, and the use count is incremented.  When the local variable is destroyed, the use count is decremented and nothing needs to be destroyed.  Shared smart pointers is another mechanism (as already used for RPN item pointers).

RPN List – To Standard List

The RPN List class list member was changed from QList to std::list.  The largest part of this change is due to standard lists not being indexed by a simple integer like a QList (which is implemented as an array internally), but with iterators as the standard list is a double linked list internally.

The significant parts of this change include iterating over the items in the list (changing regular indexed for loops to C++11 range-for loops) where the get item at index function was removed and access to the list's constant begin and end iterators was added; updating the RPN List class access functions for adding to or inserting into the list; and updating the INPUT command translate function to handle an iterator for inserting input parse codes into the list instead of an index.  Click Continue... for details of these changes.

[branch cpp11 commit 9bca14b4b5]

RPN List – Base Class To Member

The next step is to replace the use of pointers to RPN lists.  Currently, an RPN list is allocated in the translator and decoder, a pointer to which is returned.  It is then the caller's responsibility to delete the RPN list.  This is the so called naked new and delete that can be problematic (cause memory leaks if not careful).  This could be another use for a shared pointer.  However, for this case, there is a another better C++11 solution.  This change involves several distinct parts.

During these changes, a problem was found upstream with the RPN List class that was misinterpreted as being caused by using a list class as a base class.  The list classes (QList or std::list) were not designed to be used as base classes.  The problem was perceived to be due to these classes not having virtual destructors, but this was not the case.  A bigger issue is that having the list class as a public base class opens up all of the public list class members for access to users of the RPN List class, which is not good object-oriented design.

Therefore, the first part of the next set of changes was to change from using the list class as a base class to having a list member variable.  Access functions to the list member were added, which includes getting the list size, checking if the list is empty, getting the token of the last item, getting an item at an index, and clearing the list.  There are already access functions for appending items to the list, comparing lists (equality operators), inserting into the list, and converting the list to text (for test output).

The Translator class also contained access functions to the output list (for the various external translate functions) and these were insured to call the RPN List class access functions instead of the list directly.  Only the output last token function needed to be modified since it was accessing the list directly.

Finally, the RPN List class destructor was calling the clear function of the list, but this is no longer necessary as this will happen automatically when each member (including the new list member) is destroyed.  This was necessary previously because of the problem mentioned above that the base list class destructor was not defined as virtual and therefore was not being called (the RPN List class destructor overrode the list class destructor).

[branch cpp11 commit 32ab971b45]

Token Text – Standard String

Continuing with the change to the STL, the text function of the Token class was also modified to build the string into a standard string stream.  Like with the text function of the RPN List class, the with indexes option argument was not being utilized, so it was removed.

For the Token class, the return value of the text function was changed to a standard string.  Callers were modified accordingly for this change.  Only one caller remains that expects a QString value, the delete operator function.  This function will not been needed once smart (shared) pointers are used for token pointers.

The way the private text operand helper function was used in the text function was changed making a separate function unnecessary.  This helper function surrounded the token text with vertical bars and only two call located were left once the with indexes feature was removed.  Instead of setting the second string, the string was changed to a flag and at the end of the function, if set, the vertical bars and the token string are added to the string stream.

[branch cpp11 commit 47766746a1]

RPN Item – Index Member and Text Function

The next step will be to use smart pointers for RPN list pointers.  In keeping with the change to STL classes, the RPN list will be changed from a QList to a std::list.  Even though QList is like std::vector internally (an allocated array), a double linked list container is more appropriate because there is one instance so far (the INPUT command) where RPN items are inserted into the list.  Inserting into an array requires moving all the elements from the insertion point.

While reviewing the code for these changes, it was noticed that the index member of the RPN item was only being used for test output.  This index is set when an RPN item is appended to the list.  If an item is inserted into the list, the index of every item after the inserted item needs to be incremented.  This index is only output on attached items as a check to make sure the correct item is attached.

Current, attached RPN items always occur before the item that are attached to, which means the indexes could be assigned to an item as it is converted to text.  Therefore the index of each item will be assigned and temporarily held in an unordered map (RPN item pointer to index) instead of assigning an index when the item is appended (and incremented later for inserts).  The index was removed from the RPN Item class.

The text function of the RPN List class was modified to handle the conversion of each item to text using access functions of the RPN Item class so that the indexes can be added (from the temporary map) instead of calling the text function of the RPN item (which would not have the temporary indexes).  Since the text function of the RPN Item class is no longer needed, it was removed.  Also, the with indexes option argument was not being utilized and so this feature was also removed.

The text function was also modified to build the text of the RPN List into a standard string stream (std::stringstream, which works like an output stream but puts the result into a string).  This is easier for building a string than using the standard string which is feature limited.  For now, when the final standard string is obtained from the string steam, it is converted to a QString (since that it how the caller uses it, for now).  Also, the text from the token needs to be converted from a QString to a standard string before adding to the string stream.

[branch cpp11 commit da4fcfbb23]

RPN Item Pointers As Shared Pointers

An attempt was made to replace all the token pointers with a shared pointer class (QSharedPointer).  A lot of changes were required that created many compile issues to correct.  Worst, when running the first time, there were many crashes that were not being easy to resolve.  I believe some of the problems were related to the RPN items (that hold token pointers) stored in the RPN output list.  So, these changes were temporarily stashed.

Instead of starting with the token pointers, the simpler RPN item pointers were changed to use QSharedPointer.  This was successful, however, I decided to try using the STL std::shared_ptr class (C++11 only) instead .  After some testing, it appears that STL classes are faster than the equivalent Qt classes.

Originally, STL classes were avoided in favor of Qt classes, since this is a Qt based GUI application.  With the recent desire to use C++11 and considering that the STL classes were enhanced and optimized to use C++11, STL classes will now be used as much as possible except when dealing specifically with GUI elements requiring Qt classes or when Qt classes have features not available in the STL classes (for example, the STL std::string class does not have a case insensitive comparison, but the QString class does).

To start the changes for the RPN item pointers, all instances of RpnItem* were changed to RpnItemPtr, which was defined as an alias (this using syntax is new to C++11 and is just an easier form of the typedef statement):

using RpnItemPtr = QSharePointer<RpnItem>;

Many functions that had an RPN item pointer as an argument were changed to a reference to a RpnItemPtr so that the use count of the isn't unnecessarily incremented when a copy is made for the function call argument (and then decremented when the function returns).  If the item is copied inside the function, the use count will get incremented.  RPN items no longer need to be deleted as they now will be done automatically when they they go out of scope or their container is deleted or goes out of scope.

The RPN item constructor was changed to use the new initializer syntax instead of setting the member inside the constructor body.  This is more efficient because otherwise, the default constructor is first called for each non-plain member and then a value is copied into the member.  The RPN list class also no longer needed a clear function to delete the RPN items (the items get deleted automatically).

The attached array (with a count) in the RPN item class was naively implemented as a plain C array, which required naked new and delete operations and was changed to a QList.  This change eliminates the need for a count variable (QList maintains its size) and also eliminates the need to delete the attached RPN items (automatic when the RPN item is deleted automatically).

When the RPN item pointer was changed to the STL std::shared_ptr class, the changes with QSharedPointer were put on a branch and the branch abandoned.  The change in shared pointer class only required minor changes, which included changing the alias above and adding an alias for RPN item pointer vector:

using RpnItemPtr = std::shared_ptr<RpnItem>;
using RpnItemPtrVector = std::vector<RpnItemPtr>;

The latter alias is used for the attached member (just changed to a QList) was changed to a std::vector (which is more similar to QList than to std::list as QList is an allocated array, std::list a double-linked list).  Also, the size of STL classes is accessed using the size member function compared to the count member functions of the Qt classes.

[branch cpp11 commit a2069aae24]
[branch rpnitem-qt-sharedptr commit 7840954d5e]

C++ Smart Pointers

C++ smart pointers are not a feature of the C++ language put a utilization of the language.  Consider the function below, which is translation of the C way of handling allocated resources:

bool function()
{
    Class *a {new Class};
    // ... do some work ...
    if (error)
    {
        delete a1;
        return false;
    }
    Class *b {new Class};
    // ... do some more work ...
    if (error)
    {
        delete b;
        delete a;
        return false;
    }
    // ... do final work ...
    delete b;
    delete a;
    return true;
}

Care must be taken to release the resources that were allocated to prevent memory leaks.  This puts the burden on the programmer to make sure resources are released.  Consider that if the class instances were local variables, their destructors would be called automatically when they go out of scope.  This is the basic idea of a smart pointer, consider this simple smart pointer class:

class SmartPtr
{
    Class *ptr;    // hold pointer to allocated resources
public:
    SmartPtr(Class *p) : ptr {p} {}
    ~SmartPtr() { delete ptr; }
    const Class *pointer() const { return ptr; }
};

The class holds the pointer to the allocated resource.  When the smart pointer instance goes out of scope, its destructor will release the resource it is holding.  The smart pointer provides access to the pointer it is holding.  This class can be made more elaborate and convenient (for example, allowing the pointer to be replaced, allowing the pointer to be cleared, providing the indirection and arrow operators so that it behaves like a pointer, etc.), but this definition is sufficient to show the basic concept of smart pointers.  The original function can be rewritten as:

bool function()
{
    SmartPtr a {new Class};
    // ... do some work ...
    if (error)
    {
        return false;
    }
    SmartPtr b {new Class};
    // ... do some more work ...
    if (error)
    {
        return false;
    }
    // ... do final work ...
    return true;
}

Note that it is no longer necessary to release the resources.  The new calls in the first function are known as naked, each of which requires a corresponding naked delete.  In the second function, the smart pointers hold the allocated resources (the new is enclosed).  When the smart pointer goes out of scope, it destructor is called releasing the resource.

Fortunately, very elaborate smart pointer classes have already been implemented.  In the C++11 version of the STL, there is the std::unique_ptr class, which is a more elaborate implementation of the class above.  Qt has the similar QScopedPointer class.

The C++11 STL also has the std::shared_ptr class, which is a smart pointer that can be shared.  These have a use count, which is incremented each time the pointer is copied (for example, put into a different list or stack).  When one of the copies is deleted, the use count is decremented.  When the use count reaches zero, the resource is automatically released.  Qt has the similar QSharePointer class.  These shared smart pointers will be used for the tokens, which get shared (in the RPN output list, done stack, holding stack, pending parentheses token, etc.).

Memory Testing Issues

The next step is to start using C++ smart pointers (described in the next post).  One use for smart pointers will be to replace the complicated code used to track the handling of tokens and the reporting of various errors (for example, memory leaks for tokens not freed).  Smart pointers will make this code unnecessary.  Before starting I thought it was a good idea to make sure the memory test scripts still worked.

There were no memory errors reported when using Qt 4.8.1 (installed with Mint13 or Ubuntu 12.04) or Qt 4.8.2 (installed with the kubuntu backports to get the latest KDE).  However, when building the application with Qt 4.8.6 (the latest Qt 4.8 that was installed from source), memory errors were reported.  The CMake build file was set up to generate the error suppression file from a template using the detected version and directory of Qt.  This file was not tested with newer versions of Qt.

It appeared that with Qt 4.8.6, memory errors were reported differently (which was also the case with Qt 4.8.4, also built from source).  After examining the output for each version of Qt, a common error suppressions file template was created that works with each of these four versions of Qt (other versions were not tested).

As with GDB, valgrind (the memory testing utility) only supports the new debug symbols output from GCC 4.8.1 starting with version 3.8.0 (3.7.0 is installed in Mint13).  Version 3.9.0 is the latest available from source code.  However, version 3.7.0 appears to work fine, including within Qt Creator.  The new errors suppressions file also works with valgrind 3.9.0.  Click Continue... for details on installing 3.9.0 from source, but this shouldn't be necessary on Mint 13 (Unbuntu 12.04).  The valgrind 3.10.0.SVN installed on Mint 17 (Ubuntu 14.04) reports errors differently and does not work with this error suppression file (for now 3.9.0 would have to be installed from source).

[branch cpp11 commit a2069aae24]

Qt Creator (GDB) No Watch Variables With GCC 4.8

The next set of changes are rather complicated and required debugging.  Upon reaching the first breakpoint, no variables were displayed in the Locals and Expressions debugging window in Qt Creator.  After a little research, the problem was determined to be with GCC 4.8.1, which is using a new format (named DWARF-4) to write debugging symbols to the executable.

The problem is that GDB (debugger) does not support this newer format, at least older versions prior to GDB 7.5.  Mint 13 (Ubuntu 12.04) has only version 7.4.  This problem does not affect Windows with the programs installed as recently described (see the Windows tagged posts).  The latest MSYS with MinGW has GDB 7.6.1 and MinGW-w64 has GDB 7.7.  Mint 17 (Ubuntu 14.04) is also fine with GDB 7.7.

There are two ways to solve this problem.  The GCC compiler has an option for generating the older DWARF-3 format debugging symbols.  Instead of permanently adding this option to the CMake build file, the following option can be added to the CMake command line or in Arguments field in the Run CMake wizard within Qt Creator:

-DCMAKE_CXX_FLAGS_DEBUG='-g -gdwarf-3'

The other solution is to build and install GDB from source code.  Click Continue... for details on the procedure for this.