STLplus C++ Library Collection - subsystems/library_manager.hpp - A General-Purpose Library System
Author: Andy Rushton

# STLplus C++ Library Collection

Website Collection Library

# subsystems/library_manager.hpp
A General-Purpose Library System

- library_manager.hpp
- Introduction
- Why Use It?
- Library Manager Concepts
- Creating a Library Manager
- Registering a New Type
- Dummy Types
- Library Mappings
- Library Management
- The Work Library
- Library Unit Management
- Dependency Checking

## Introduction

This component is a general purpose library manager which allows one or more libraries to be managed across a file system. The purpose is to collect information in libraries such that it can be accessed easily, but without cluttering up the file system with arbitrarily-named files.

The library manager originated from the VHDL compiler project, since VHDL is a library-based language. However, this STL+ version is generalised for use in any product which needs to manage a lot of data.

## Why Use It?

The first question your average hacker will ask at this point is "what use is it". After all, if the library manager simply stores information as files in subdirectories, then you can do that without the library manager, can't you?

The quick answer is that hackers never see the point of progress - similar questions were asked of high-level languages by assembler hackers.

The long answer is that the library manager does more than just allow you to create files in directories. It provides an interface which does much more than that. First, it abstracts away from the file system (for example, I could create a version that used a database rather than individual files. The interface would be the same). It provides an equivalence or mapping between a data file and a data structure. It requires that you design your information as a persistent data structure, but having done that, the library manager will manage the data persistence for you. It provides ways of seeing the contents of a library and obtaining summary data in the contents without opening any of the data files and it provides a mechanism where a data file is only opened and read on demand (a kind of just-in-time loading). Above all it allows data to be handled in a consistent, coherent and user-friendly manner throughout a large software project, rather than spraying data files all over the user's workspace.

## Library Manager Concepts

The library manager is presented as a C++ class with a large (and growing) number of access functions.

A library manager is made unique to a product by an owner string. This will typically be the name of the product in lowercase (e.g. "moods" for the Moods product). A library manager can only open and manipulate libraries created by the owner product. This allows many products to co-exist on the same computer without any possibility of confusion as to which libraries are owned by which product.

A library manager contains any number of libraries. Each library has a name (a string) and a file-system location for the directory where the library's data is stored. The location (or path) is only used to create and open libraries. Once a library is open and therefore part of the library manager, it is referred to by its name. Of course, all libraries managed by the library manager at any time must have unique names.

The library name is stored in the library itself, so once a library has a name it must keep it. This is because, as will be seen later, the library manager also manages cross-library dependencies and these use the library name, not its path to represent a dependency. Allowing the library to have a different name each time would break the dependency management.

A library contains a set of units. Each unit is referred to by a name and a type (both std::strings). A unit name must be unique within its type, but it is perfectly legal to have units of different types having the same name. The combination of name and type together are referred to as the unit-name.

The type of a unit is used to differentiate between different data types. Every unit of the same type has the same data structure (presumable containing different data though). However, each type can have a different data structure. As an example, consider a library-based compiler: the compilation generates a parse tree data structure which is stored in the library manager as type "tree" (as in "parse tree"). The intermediate code generated by the compiler is also stored in the library manager, but this time using an intermediate code data structure which is stored in the library manager as type "object". Types are typically abbreviations although they don't have to be. They currently become the extension of the data file (so type "tree" is stored as a file with a ".tree" extension), but remember that the library manager is an abstraction and could be replaced by a database. Therefore the type should be thought of as a unique signature that identifies a data type and not as a file extension.

Each unit contains two types of information. There is "header" information which is common to all types and contains information such as the unit's name, its type, its source file (if relevant) and its dependencies (if relevant). This header information is always available and is stored separately by the library manager. Then there is "body" information which is type specific - this is the data structure associated with that type and is stored in that data structure's file format. Body data is not loaded until it is requested - a policy designed to minimise the initialisation time of the library manager and which also minimises the memory requirements.

## Creating a Library Manager

The constructor to the library manager takes one required and two optional arguments:

    library_manager::library_manager(const std::string& owner, bool library_case = false, bool unit_case = false);

The owner string is a unique string that identifies the product - in lowercase. The library manager will only recognise libraries created by the owner product. Thus, for example, all tools in the Moods product would use the owner string "moods" so that all tools recognise the set of Moods libraries. However, a second product Tempers would use the owner string "tempers" and this would not be able to access the Moods libraries, nor vice-versa.

The case sensitivity switches determine whether library names and unit names are case-sensitive.

Note: at present there is a problem if you set `unit_case` to true. On Windows, filenames are case-insensitive, but the library manager uses the unit name to create its filenames. Thus setting the library manager to use case-sensitive unit names means that more than one unit name maps onto the same filename. For the moment, until I come up with a solution for this, you are recommended to use the library manager with unit_case set to false and if necessary find a way of mapping case-sensitive unit names onto case-insensitive strings. There is no such problem with setting `library_case` to true - library names can be case-sensitive.

## Registering a New Type

This section will explain how to add a new type of object to the library manager. It is a simple process but one which needs to be explained. As an example, I will demonstrate the creation of a trivial data type which contains a single std::string.

The first stage is to create a class which represents your data in the library manager. It should be derived from stlplus::lm_unit (the base class for all library units) and should contain the data structure which you wish to have managed. This class should have a constructor, destructor, a read and a write routine and a couple of other functions which will be explained later. For the example, the class will be called string_unit:

    class string_unit : public stlplus::lm_unit
    {
    private:
      std::string m_data;
    public:
      ...
    };

The constructor should take at least two arguments - the unit-name and a library pointer. These are then passed to the superclass lm_unit constructor to initialise the superclass data structures. The destructor should destroy only your own data structures if necessary.

    string_unit::string_unit(const stlplus::lm_unit_name& name, stlplus::lm_library* library) :
      stlplus::lm_unit(name,library) {}

    string_unit::~string_unit(void) {}

Note that the constructor does nothing after initialising the baseclass because the default construction behaviour of a string is exactly what we want anyway.

The destructor likewise does nothing because the default string destructor does what we want (deallocate the string).

The next stage is to write the read and write methods. If you think of the data structure as a persistent type, these methods are the converters that go from the file format to the data structure and vice versa. Here are the read and write methods for the example:

    virtual bool string_unit::read(std::istream& input, void* type_data)
    {
      std::getline(input,m_data);
      return !input.error();
    }

    virtual bool string_unit::write(std::ostream& output, void* type_data)
    {
      output << m_data << std::endl;
      return !output.error();
    }

As you can see, for this trivial data structure, the read and write methods are also trivial. Note that both work on IOStream devices. This is part of the abstraction. If the library manager had been written based on FILE* so that you could use fprintf, this would lock it into a file-based store and prevent the possibility of reworking the library manager to work on a database. However, IOStream is an abstract I/O system and so keeps that option only. Basically, the device is already opened by the library manager when the read or write is called, so all you have to do is the reading and writing of the file format. Each function should return false if it fails, true if it succeeds.

The read and write methods also have a void* type data field passed to them by the library manager. This type data field is common to all units of the same type but can be different for different types. This is used to pass any extra data into the read/write routines or into the constructor. For example, this could be a pointer to an stlplus::message_handler which could then be used to report errors when reading the file, if appropriate. If an error occurs, the read/write methods should return false whether or not it reports any errors. It should return true on success. This allows the library manager to keep track of which body files have been loaded or saved successfully.

You can optionally provide a purge method. Purging is used by the library manager to recover memory by clearing the in-memory representation (the body data) of each unit. It is optional because small data structures are not worth purging. However, although this is a small data structure (of course a string could be megabytes), I will show how to write one. Here's the string_unit purge routine:

    virtual bool string_unit::purge(void)
    {
      m_data.clear();
      return true;
    }

You are required to provide a clone method. The reason is that units are stored in the library manager using a smart pointer - specifically the stlplus::smart_ptr_clone variant. The requirement for a clone method is simply a work round for a limitation of C++ that makes it impossible to copy a subclass object when all you have is a superclass pointer. The clone method works round this by copying an object and returning it as a superclass pointer. Thus, for the string_unit class it look like this:

    virtual lm_unit* string_unit::clone(void) const
    {
      return new string_unit(*this);
    }

Note that the body of the function creates a new subclass (the subclass type being string_unit), uses that new object's copy constructor to copy the current object (represented by *this) and returns the result as a superclass pointer (in this case an stlplus::lm_unit*). You don't have to understand all this though, just copy this example and modify to match the name of your type.

Warning: this clone function calls the copy constructor for string_unit, but I haven't written one. This means that the default copy constructor is used which simply copies each field using its copy constructor. The baseclass stlplus::lm_unit is designed to be copied this way, so no problem there. The string_unit data field is of type std::string, which copies correctly, so no problem there, so overall this is correct. However, beware if you have a pointer to your data type. Pointers do not copy safely because you end up with two pointers to the same object. When the two objects are destroyed, both pointers get deleted and the memory gets deallocated twice. Result: corrupted memory. If you have a pointer to your data structure, you must write a copy constructor that genuinely copies the structure pointed to. Or use the smart_ptr class.

That's it for the library manager interface of the data structure, though you will probably want access functions to the data field (note that the string is private. So, to complete the class, here's the access functions:

    const std::string& string_unit::data(void) const
    {
      return m_data;
    }

    void string_unit::set_data(const std::string& data)
    {
      m_data = data;
      mark();
    }

The interesting point here is the call to mark() in the set_data function. This concerns the unit's persistence. If you change the data structure for any reason, you need to call mark() to tell the library manager that the data has been changed. This will be used later by the library manager to determine whether to save the unit. If you don't call mark() after a change, the data will never be saved to disk. You don't have to encapsulate the call to mark in the access function like this, you could call it from your own code. The mark function is a member of the superclass stlplus::lm_unit and so is inherited by the string_unit class.

Now that the data structure for the string_unit class is ready, we can tell the library manager about it. This is known as registering a type. There are three things needed to register a type: a signature (the extension if you like), a human readable name for the type (for printouts) and a create callback for the data type.

The callback function is another work-around for C++ since it is not possible to create an object of a type just from its name. Thus we have to provide a function to create units. The callback is trivial and here's the callback for the string_unit class to prove it:

    stlplus::lm_unit* create_string_unit(const stlplus::lm_unit_name& name,
                                         stlplus::lm_library* library,
                                         void* type_data)
    {
      return new string_unit(name,library);
    }

Note: the callback is not a member of the class, it is a stand-alone C++ function.

The callback takes three arguments, the unit-name, a pointer to a library and a void* hook. The first two values are both generated and passed to the function by the library manager. The function simply creates a dynamic object of the string_unit class and constructs it using these two parameters (now you know why the string_unit constructor had to take these two parameters). That's it. The callback also receives the void* type_data field. In this case it has been ignored, but it could have also been passed into the constructor if extra data was required there.

Now at last the new type can be registered with the library manager. This is done with the add_type member function:

    bool library_manager::add_type(const std::string& type,
                                   const std::string& description,
                                   stlplus::lm_create_callback fn = 0,
                                   void* type_data = 0);

The following code extract shows how this is used:

    stlplus::library_manager libraries;
    libraries.add_type("sun", "String", create_string_unit);

The first line creates a library manager which will initially be empty (no types and no libraries). The second statement registers the string_unit class. In this case, I've chosen the signature "sun" to represent a string_unit in the library manager. I've given it the description "String" for human readable printouts, and I've registered the create callback written above so that object of this type can be created by the library manager. Note that a function is passed as a parameter by just using the function's name as if it were a variable. This is the standard C++ way of representing a pointer to a function. Finally, I've left the type_data field undefined which causes it to default to null, because in this example it is not used.

## Dummy Types

Sometimes you will want to get access to the header data for a type without being interested in the body data. This might be, for example, to list the contents of a library or to perform dependency checking on the contents of a library. These operations can be achieved solely from header data and therefore there is no need to load the units' bodies.

To make this usage of the library manager simpler, it is possible to register a dummy type with the library manager. Remember how the string unit was registered with the library manager:

    library_manager libraries;
    libraries.add_type("sun", "String", create_string_unit);

This can be changed into registration of a dummy string unit by omitting the creation callback, using a null pointer instead. Indeed, the default value of the callback is a null pointer, so a dummy string unit can be registered like this:

    library_manager libraries;
    libraries.add_type("sun", "String");

This will allow you to perform any header-specific operations on the string units except for loading the unit itself. The unit is represented by the superclass stlplus::lm_unit rather than the derivative string_unit. It is illegal to try to load an stlplus::lm_unit since it has no on-disk representation, so any attempt to load the unit will fail (the lm_unit::load() function will return false). However, all the header information such as source file, dependencies, modified time etc. will be available.

## Library Mappings

Library mappings are the directory paths to the set of libraries that you wish to be loaded into the library manager. A library mapping is just a path - the library's name is stored in the library itself.

There are two ways of managing library mappings - builtin and do-it-yourself. The builtin method is switched on by simply providing a library mapping file to the library manager. The following functions relate to the management of library mapping files:

    void library_manager::set_mapping_file(const std::string& mapping_file);
    bool library_manager::load_mappings (const std::string& mapping_file);
    bool library_manager::save_mappings (void) const;

The set_mapping_file method sets the library manager's mapping file field but does not attempt to open let alone read the file. It is used to initialise an empty mapping file. If the file already exists, it will eventually be overwritten.

The load_mappings method sets the library mapping file and attempts to read it. If the file does not exist it will eventually be created. If it does exist, the library mappings in it will be loaded and all the libraries listed will be opened. The library manager has the concept of a current working library - this is also optionally stored in the library mapping file, so this will be set if it is present. It is not required.

Finally, the save_mappings method saves the current set of libraries to the library mapping file. If the library mapping file has not been set, it does nothing.

Of course, the set of libraries can be changed using the library management functions described in the next section.

You are not obliged to use the library mapping file to manage library mappings. You can use your own method. To add an existing library to the set of libraries in the maneger, use the open function:

    stlplus::lm_library* library_manager::open(const std::string& path);

The return value is a handle to the library opened. It is null if the library did not exist.

Saving the set of library paths is a two stage process: get the set of library names, then get the path for each library:

    std::vector<std::string> library_manager::names(void) const;
    std::string library_manager::path(const std::string& library) const;

## Library Management

This section deals with the set of methods which manipulate whole libraries, creating or deleting them adding them to or removing them from the library manager.

    static bool library_manager::is_library(const std::string& path);
    stlplus::lm_library* library_manager::create(const std::string& name, const std::string& path, bool writable = true);
    stlplus::lm_library* library_manager::open(const std::string& path);
    bool library_manager::close(const std::string& name);
    bool library_manager::erase(const std::string& name);

The static function is_library tests whether a particular directory represents a library. Because it is static, it is not necessary to create a library manager object to use it - you can use it in the form:

    if (stlplus::library_manager::is_library(path))
      ...

If you try to open a path that does not represent a library, the open will fail anyway, so that is an alternative way of testing the validity of a path.

The create method will create a library in the specified directory and give it the specified name. If the directory is already a library, the behaviour is the same as for the open method. If the directory does not exist it will be created and made into a library. If the directory does already exist, it will simply be made into a library. If the directory cannot be created, the create function fails and returns null. Otherwise the library is added to the library manager and a handle to the new library returned.

The open method tests whether the specified directory is a library. If it is not, the function returns a null handle. If it is, that library is loaded and added to the library manager. A handle to the library is then returned.

The close method closes the named library by saving it and then removing it from the library manager. It does not remove the directory. Thus it can be re-opened later. By contrast, the erase function closes the library, removes it from the library manager and deletes the directory.

Note: once a library has been opened, either by open or create, it is referred to either by its name or by its handle (the lm_library* type). It is possible to convert from a library name to a handle using the find function:

    stlplus::lm_library* library_manager::find(const std::string& name);
    const stlplus::lm_library* library_manager::find(const std::string& name) const;

It is also possible to get the name from the handle by using the lm_library's name method:

    const std::string& lm_library::name(void) const;

This is used by dereferencing the library handle in the normal C++ way:

    stlplus::lm_library* library = libraries.open("libdir")
    if (library)
      std::cerr << "opened library " << library->name() << std::endl;

The set of names or handles for all the libraries in a library manager can be obtained by a single function call:

    std::vector<std::string> library_manager::names(void) const;
    std::vector<const lm_library*> library_manager::handles(void) const;

A library can be made read-only or writable. This can be done through the library manager or through the operating system. Each library has a flag stored internally which is the write status of the library. This tag can be set through the library manager interface. However, if the directory containing the library does not have write access (set from the command-line or Windows), this will also cause it to be treated as a read-only library. The difference is that this cannot be set or cleared through the library manager interface. It is possible to find out why a library is read-only though. The functions for supporting this functionality are:

    bool set_writable(const std::string& library);
    bool set_read_only(const std::string& library);
    bool writable(const std::string& library) const;
    bool read_only(const std::string& library) const;
    bool os_writable(const std::string& library) const;
    bool os_read_only(const std::string& library) const;
    bool lm_writable(const std::string& library) const;
    bool lm_read_only(const std::string& library) const;

The set_writable and set_read_only functions change the internal flag bit. However, if the library is read-only because of access permissions, it will remain read-only regardless of this internal flag. The writable and read_only functions allow the read-only status of the library to be tested. For diagnostic purposes, the os_read_only function tells you whether the library is read only due to the operating system (cannot be changed) and lm_read_only tells you whether it is read-only because of the internal flag (can be changed). Bear in mind that if the OS has locked the library, the internal flag will have no effect and cannot itself be changed.

If a library is read-only it cannot have new units created in it, nor can existing units be modified (actually, I can't stop you modifying the unit in memory, but the library manager will refuse to save the changes to a read-only library).

The final set of functions manipulate the set of all units in a library or in all libraries to perform system-wide operations:

    bool library_manager::save(const std::string& library);
    bool library_manager::save(void);

    bool library_manager::load(const std::string& library);
    bool library_manager::load(void);

    bool library_manager::purge(const std::string& library);
    bool library_manager::purge(void);

The save methods save all units which have been marked for saving. The first form of save only acts on the named library, whilst the second acts on all libraries. Note that only units which have been loaded and marked as changed will be saved.

The load methods are unlikely to be used in practice. They load the data for every single unit in the named library (first form) or in all libraries (second form). Generally you should load units as they are required (just-in-time loading).

The purge functions save marked units and then delete the in-memory data structures of every unit to recover their memory. Again there are two forms - the first acts on a named library, the second on all libraries.

## The Work Library

The concept of a 'current' library is useful in most applications. The library manager provides some support for this concept by allowing a library name to be stored as the current library. This is all it does - how this concept is used is down to the application. In my compiler project, the library actually named "work" was considered an alias of the current working library. The string "work" is therefore mapped onto the name of the current library and that name is then used to lookup information from the library. I did not build this mapping into the library manager, obviously, because it is too application-specific. Different applications will have different requirements, but most will have a use for the concept of a current working library.

The support functions for the working library are:

    bool library_manager::setwork(const std::string& library);
    bool library_manager::unsetwork(void);
    const lm_library* library_manager::work(void) const;
    lm_library* library_manager::work(void);
    std::string library_manager::work_name(void) const;

The setwork method sets the named library to be the current working library. If the library does not exist, then the function fails and returns false. The unsetwork method clears the current library field (if there is no work library, the string representing the work library is empty). The work method returns a handle to the current work library, null if there isn't one. Finally, the work_name method returns the name of the curring work library, an empty string if there isn't one.

## Library Unit Management

The library manager provides a range of functions for manipulating units within a library. However, manipulation of the unit contents is done by using the unit's member functions. In other words, the relationship of the unit to its library is managed by the library manager, but the unit's internal data is managed by the unit. This section only details the library manager methods.

Units in the library manager are characterised by two fields: the library name and the unit-name (remember that a unit-name is the combination of two strings, a name and a type). You can only manipulate units for a type that is registered with the library manager (see the section above on "Registering a New Type").

Units are stored in the library as smart pointers to the superclass lm_unit. These smart pointers are referred to by the typedef stlplus::lm_unit_ptr, the typedef for which is:

    typedef stlplus::smart_ptr_clone<stlplus::lm_unit> stlplus::lm_unit_ptr;

Remember that in practice, this will be a pointer to a subclass of lm_unit, specifically the subclass registered for the unit's type. You can recover the lm_unit* type from the lm_unit_ptr by using the smart pointer's pointer() function, and then type convert that pointer to the correct subclass. You are responsible for this type conversion because there is no way I could build this into the library manager using C++.

Using the string_unit example from earlier, the derivative unit type can be obtained from an lm_unit_ptr using an old-style C type conversion:

    stlplus::lm_unit_ptr unit = ...
    string_unit* s_unit = (string_unit*)unit.pointer();

C++ now provides a type-safe type conversion which checks that the conversion is legal. This is recommended, but by no means essential. It uses the dynamic_cast template:

    stlplus::lm_unit_ptr unit = ...
    string_unit* s_unit = dynamic_cast<string_unit*>(unit.pointer());

The dynamic_cast template will return null if for any reason you try to typecast onto the wrong derivative type.

You can create a new unit in a library with the create function:

    stlplus::lm_unit_ptr library_manager::create(const std::string& library,
                                                 const stlplus::lm_unit_name& name);

This will overwrite any existing unit of the same unit-name (Note that a unit with the same name but different type will not be affected). It will create an empty unit in the named library and return the smart pointer to that unit. You can then type convert the unit pointer to the right derivative type and start filling in the data structure associated with that type.

You can only create units of a type that has been registered with the library manager. If you try to create an unrecognised unit type, the create function will fail and will return a null pointer to indicate that failure.

You must remember to call the mark function if you change the data structure so that the library manager knows to save the unit (in fact a created unit is pre-marked). The mark function is a member of the baseclass lm_unit so can be called from either the smart pointer or the typecast pointer:

    unit->mark();

or

    s_unit->mark();

In fact, if you refer back to the definition of string_unit, merely setting the string value implicitly called mark, so the unit data can be changed and the unit marked in one step:

    stlplus::lm_unit_ptr new_unit = libraries.create(libraries.work_name(),stlplus::lm_unit_name("hello", "sun"));
    string_unit* s_unit = dynamic_cast<string_unit*>(new_unit.pointer());
    if (!s_unit)
      // report error
    else
      s_unit->set_string("Hello World!");
    libraries.save();

This example shows a number of features of using the library manager. The first line creates a string_unit unit called "hello". It uses the string_unit's signature "sun" as the type. By the way, it is bad practice to do this with a string literal, ideally you should declare a global constant string containing this signature and then use the global constant throughout. It is then possible to change the signature of string_unit by changing one line of code.

The create routine returns an stlplus::lm_unit_ptr which is then typecast onto string_unit* using the C++ method outlined above. This can return null under two conditions: the wrong type was cast to (a programming error - once you've got it right it will always be right) or if the create failed.

The final stage is to set the string field of the string unit, which marks it for saving. Finally, I've included an explicit save call to ensure it is saved immediately. This is not necessary - indeed save is called by the destructor, so there's no need for an explicit save at all, it depends on the application.

To access an existing unit, use the find functions:

    const stlplus::lm_unit_ptr library_manager::find(const std::string& library, const stlplus::lm_unit_name& name) const;
    stlplus::lm_unit_ptr library_manager::find(const std::string& library, const stlplus::lm_unit_name& name);

There are two variants in keeping with the normal convention that a const library manager returns a const unit pointer and a non-const library manager returns a non-const unit pointer. The find functions take the same arguments as the create function and return the same pointer type. If the unit does not exist, this pointer will be null. Alternatively, you can check whether the unit exists before trying to find it with the exists function:

    bool library_manager::exists(const std::string& library, const stlplus::lm_unit_name& name) const;

Finally, of course, it is possible to delete a unit from the library by using the erase function:

    bool library_manager::erase(const std::string& library, const stlplus::lm_unit_name& name);

Remember that a unit initially only contains header information (as explained in the section "Registering a New Type"). This information can be retrieved from the unit by dereferencing the lm_unit_ptr. Body information (i.e. the type-specific data structure) is not loaded until you request it. This can be loaded through the library manager. Body data can be removed from memory by calling purge and saved to file through calling save. To perform these operations, the following functions are provided:

    bool library_manager::loaded(const std::string& library, const lm_unit_name& name) const;
    bool library_manager::load(const std::string& library, const lm_unit_name& unit);
    bool library_manager::purge(const std::string& library, const lm_unit_name& unit);
    bool library_manager::save(const std::string& library, const lm_unit_name& unit);
    bool library_manager::mark(const std::string& library, const lm_unit_name& name);
    bool library_manager::unmark(const std::string& library, const lm_unit_name& name);
    bool library_manager::marked(const std::string& library, const lm_unit_name& name) const;

The loaded method tests whether a unit is already loaded. The load method loads it. In fact, if a unit is already loaded, the load function does nothing, so it is safe to call it more than once on a unit and the loaded method is really redundant. The purge method saves the unit if necessary and destroys the body data in memory. The save method saves the body data (if marked for saving) but keeps it in memory. The mark method marks a unit for saving, unmark removes that mark and marked tests to see whether a unit has been marked for saving and not yet saved (since the mark is cleared when the unit is saved).

The library manager has a number of methods for accessing the set of all units in a library or all units of a specified type in the library. These units can either be accessed by unit name or by unit pointer:

    std::vector<stlplus::lm_unit_name> names(const std::string& library) const;
    std::vector<std::string> names(const std::string& library, const std::string& type) const;
    std::vector<stlplus::lm_unit_ptr> handles(const std::string& library) const;
    std::vector<stlplus::lm_unit_ptr> handles(const std::string& library, const std::string& type) const;

Note that if you get the set of all names in a library, you get a vector of unit-name since there is more than one type of unit possible in the list. Hoever, if you get tyhe names for a specific type in the library, the vector is of strings representing just the name part, since the type is already known.

## Dependency Checking

to be continued...