gencpp/docs/Parser_Algo.md

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Parser's Algorithim

gencpp uses a hand-written recursive descent parser. Both the lexer and parser currently handle a full C/C++ file in a single pass.

Notable implementation background

Lexer

The lex procedure does the lexical pass of content provided as a Str type.
The tokens are stored (for now) in Lexer_Tokens.

Fields:

Array<Token> Arr;
s32          Idx;

What token types are supported can be found in ETokType.csv you can also find the token types in ETokType.h , which is the generated enum from the csv file.

Tokens are defined with the struct gen::parser::Token:

Fields:

char const* Text;
sptr        Length;
TokType     Type;
s32         Line;
s32         Column;
u32         Flags;

Flags is a bitfield made up of TokFlags (Token Flags):

• TF_Operator : Any operator token used in expressions
• TF_Assign
  • Using statment assignment
  • Parameter argument default value assignment
  • Variable declaration initialization assignment
• TF_Preprocess : Related to a preprocessing directive
• TF_Preprocess_Cond : A preprocess conditional
• TF_Attribute : An attribute token
• TF_AccessSpecifier : An accesor operation token
• TF_Specifier : One of the specifier tokens
• TF_EndDefinition : Can be interpreted as an end definition for a scope.
• TF_Formatting : Considered a part of the formatting
• TF_Literal : Anything considered a literal by C++.

I plan to replace IsAssign with a general flags field and properly keep track of all operator types instead of abstracting it away to ETokType::Operator.

Traversing the tokens is done with the following interface macros:

Macro	Description
currtok_noskip	Get the current token without skipping whitespace
currtok	Get the current token, skip any whitespace tokens
prevtok	Get the previous token (does not skip whitespace)
nexttok	Get the next token (does not skip whitespace)
eat( Token Type )	Check to see if the current token is of the given type, if so, advance Token's index to the next token
left	Get the number of tokens left in the token array
check_noskip	Check to see if the current token is of the given type, without skipping whitespace
check	Check to see if the current token is of the given type, skip any whitespace tokens

Parser

The parser has a limited user interface, only specific types of definitions or statements are expected to be provided by the user directly when using to construct an AST dynamically (See SOA for example). It however does attempt to provide capability to parse a full C/C++ from production codebases.

Each public user interface procedure has the following format:

<code type> parse_<definition type>( Str def )
{
    check_parse_args( def );
    using namespace Parser;

    TokArray toks = lex( def );
    if ( toks.Arr == nullptr )
        return CodeInvalid;

    // Parse the tokens and return a constructed AST using internal procedures
    ...
}

The most top-level parsing procedure used for C/C++ file parsing is parse_global_body:

It uses a helper procedure called parse_global_nspace.

Each internal procedure will have the following format:

internal
<code type> parse_<definition_type>( <empty or contextual params> )
{
    push_scope();

    ...

    <code type> result = (<code type>) make_code();
    ...

    Context.pop();
    return result;
}

Below is an outline of the general alogirithim used for these internal procedures. The intention is to provide a basic briefing to aid the user in traversing the actual code definitions. These appear in the same order as they are in the parser.cpp file

NOTE: This is still heavily in an alpha state. A large swaph of this can change, make sure these docs are up to date before considering them 1:1 with the repo commit your considering.

parse_array_decl

1. Check if its an array declaration with no expression.
   1. Consume and return empty array declaration
2. Opening square bracket
3. Consume expression
4. Closing square bracket
5. If adjacent opening bracket
   1. Repeat array declaration parse until no brackets remain

parse_assignment_expression

1. Eat the assignment operator
2. Make sure there is content or at least an end statement after.
3. Flatten the assignment expression to an untyped Code string.

parse_attributes

1. Check for standard attribute
2. Check for GNU attribute
3. Check for MSVC attribute
4. Check for a token registered as an attribute a. Check and grab the arguments of a token registered of an attribute if it has any.
5. Repeat for chained attributes. Flatten them to a single attribute AST node.

parse_class_struct

1. Check for export module specifier
2. class or struct keyword
3. parse_attributes
4. If identifier : parse_identifier
5. Parse inherited parent or interfaces
6. If opening curly brace : parse_class_struct_body
7. If not an inplace definition
   1. End statement
   2. Check for inline comment

parse_class_struct_body

1. Opening curly brace
2. Parse the body (Possible options):
   1. Ignore dangling end statements
   2. Newline : ast constant
   3. Comment : parse_comment
   4. Access_Public : ast constant
   5. Access_Protected : ast constant
   6. Access_Private : ast constant
   7. Decl_Class : parse_complicated_definition
   8. Decl_Enum : parse_complicated_definition
   9. Decl_Friend : parse_friend
   10. Decl_Operator : parse_operator_cast
   11. Decl_Struct : parse_complicated_definition
   12. Decl_Template : parse_template
   13. Decl_Typedef : parse_typedef
   14. Decl_Union : parse_complicated_definition
   15. Decl_Using : parse_using
   16. Operator == '~'
       1. parse_destructor
   17. Preprocess_Define : parse_define
   18. Preprocess_Include : parse_include
   19. Preprocess_Conditional (if, ifdef, ifndef, elif, else, endif) : parse_preprocess_cond or else/endif ast constant
   20. Preprocess_Macro : parse_simple_preprocess
   21. Preprocess_Pragma : parse_pragma
   22. Preprocess_Unsupported : parse_simple_preprocess
   23. StaticAssert : parse_static_assert
   24. The following compound into a resolved definition or declaration:
       1. Attributes (Standard, GNU, MSVC) : parse_attributes
       2. Specifiers (consteval, constexpr, constinit, explicit, forceinline, inline, mutable, neverinline, static, volatile, virtual)
       3. Possible Destructor : parse_destructor
       4. Possible User defined operator cast : parse_operator_cast
       5. Possible Constructor : parse_constructor
       6. Something that has the following: (identifier, const, unsigned, signed, short, long, bool, char, int, double)
          1. Possible Constructor parse_constructor
          2. Possible Operator, Function, or varaible : parse_operator_function_or_variable
   25. Something completely unknown (will just make untyped...) : parse_untyped

parse_comment

1. Just wrap the token into a cached string ( the lexer did the processing )

parse_compilcated_definition

This is a helper function used by the following functions to help resolve a declaration or definition:

• parse_class_struct_body
• parse_global_nspace
• parse_union

A portion of the code in parse_typedef is very similar to this as both have to resolve a similar issue.

1. Look ahead to the termination token (End statement)
2. Check to see if it fits the pattern for a forward declare
3. If the previous token was an identifier ( token[-1] ):
   1. Look back one more token : [-2]
   2. If the token has a closing brace its an inplace definition
   3. If the token[-2] is an identifier & token[-3] is the declaration type, its a variable using a namespaced type.
   4. If the token[-2] is an indirection, then its a variable using a namespaced/forwarded type.
   5. If the token[-2] is an assign classifier, and the starting tokens were the which type with possible class token after, its an enum forward declaration.
   6. If any of the above is the case, parse_operator_function_or_variable
4. If the token[2] is a vendor fundamental type (builtin) then it is an enum forward declaration.
5. If the previous token was a closing curly brace, its a definition : parse_forward_or_definition
6. If the previous token was a closing square brace, its an array definition : parse_operator_function_or_variable

parse_define

1. Define directive
2. Get identifier
3. Get Content (Optional)

parse_forward_or_definition

• Parse any of the following for either a forward declaration or definition:
  1. Decl_Class : parse_class
  2. Decl_Enum : parse_enum
  3. Decl_Struct : parse_struct
  4. Decl_Union : parse_union

parse_function_after_name

This is needed as a function defintion is not easily resolvable early on, as such this function handles resolving a function after its been made ceratin that the type of declaration or definition is indeed for a function signature.

By the point this function is called the following are known : export module flag, attributes, specifiers, return type, & name

1. parse_parameters
2. parse postfix specifiers (we do not check if the specifier here is correct or not to be here... yet)
3. If there is a body : parse_body
4. Otherwise :
   1. Statment end
   2. Check for inline comment

parse_function_body

Currently there is no actual parsing of the function body. Any content with the braces is shoved into an execution AST node. In the future statements and expressions will be parsed.

1. Open curly brace
2. Grab all tokens between the brace and the closing brace, shove them in a execution AST node.
3. Closing curly brace

parse_global_nspace

1. Make sure this is being called for a valid type (namespace, global body, export body, linkage body)
2. If its not a global body, consume the opening curly brace
3. Parse the body (Possible options):
   1. Ignore dangling end statements
   2. NewLine : ast constant
   3. Comment : parse_comment
   4. Decl_Cass : parse_complicated_definition
   5. Decl_Enum : parse_complicated_definition
   6. Decl_Extern_Linkage : parse_extern_link
   7. Decl_Namespace : parse_namespace
   8. Decl_Struct : parse_complicated_definition
   9. Decl_Template : parse_template
   10. Decl_Typedef : parse_typedef
   11. Decl_Union : parse_complicated_definition
   12. Decl_Using : parse_using
   13. Preprocess_Define : parse_define
   14. Preprocess_Include : parse_include
   15. Preprocess_If, IfDef, IfNotDef, Elif : parse_preprocess_cond
   16. Preprocess_Else : ast constant
   17. Preprocess_Endif : ast constant
   18. Preprocess_Macro : parse_simple_preprocess
   19. Preprocess_Pragma : parse_pragma
   20. Preprocess_Unsupported : parse_simple_preprocess
   21. StaticAssert : parse_static_assert
   22. Module_Export : parse_export_body
   23. Module_Import : NOT_IMPLEMENTED
   24. The following compound into a resolved definition or declaration:
       1. Attributes ( Standard, GNU, MSVC, Macro ) : parse_attributes
       2. Specifiers ( consteval, constexpr, constinit, extern, forceinline, global, inline, internal_linkage, neverinline, static )
       3. Is either ( identifier, const specifier, long, short, signed, unsigned, bool, char, double, int)
          1. Attempt to parse as construtor or destructor : parse_global_nspace_constructor_destructor
          2. If its an operator cast (definition outside class) : parse_operator_cast
          3. Its an operator, function, or varaible : parse_operator_function_or_varaible
4. If its not a global body, consume the closing curly brace

parse_global_nspace_constructor_destructor

1. Look ahead for the start of the arguments for a possible constructor/destructor
2. Go back past the identifier
3. Check to see if its a destructor by checking for the ~
4. Continue the next token should be a ::
5. Determine if the next valid identifier (ignoring possible template parameters) is the same as the first identifier of the function.
6. If it is we have either a constructor or destructor so parse using their respective functions (parse_constructor, parse_destructor).

parse_identifier

This is going to get heavily changed down the line to have a more broken down "identifier expression" so that the qualifier, template args, etc, can be distinguished between the targeted identifier.
The function can parse all of them, however the AST node compresses them all into a string.

1. Consume first identifier
2. parse_template_args
3. While there is a static symbol accessor ( :: )
   1. Consume ::
   2. Consume member identifier
   3. parse_template args (for member identifier)
   4. If a ~ is encounted and the scope is for a destructor's identifier, do not consume it and return with what parsed.

parse_include

1. Consume include directive
2. Consume the path

parse_operator_after_ret_type

This is needed as a operator defintion is not easily resolvable early on, as such this function handles resolving a operator after its been made ceratin that the type of declaration or definition is indeed for a operator signature.

By the point this function is called the following are known : export module flag, attributes, specifiers, return type

1. If there is any qualifiers for the operator, parse them
2. Consume operator keyword
3. Determine the operator type (This will be offloaded to the lexer moreso than how it is now) & consume
4. parse_params
5. If there is no parameters this is operator is a member of pointer if its symbols is a *.
6. Parse postfix specifiers
7. If there is a opening curly brace, parse function_body
8. Otherwise: consume end statement, check for inline comment.

parse_operator_function_or_variable

When this function is called, attribute and specifiers may have been resolved, however what comes next can still be either an operator, function, or varaible.

1. Check for preprocessor macro, if there is one : parse_simple_preprocess
2. parse_type (Does the bulk of the work)
3. Begin lookahead to see if we get qualifiers or we eventually find the operator declaration
4. If we find an operator keyword : parse_operator_after_ret_type
5. otherwise :
   1. parse_identifier
   2. If we se a opening parenthesis (capture start), its a function : parse_function_after_name
   3. Its a variable : parse_variable_after_name

parse_pragma

1. Consume pragma directive
2. Process the token content into cached string

parse_params

1. Consume either a ( or < based on use_template_capture arg
2. If the we immdiately find a closing token, consume it and finish.
3. If we encounter a varadic argument, consume it and return a param_varadic ast constant
4. parse_type
5. If we have a macro, parse it (Unreal has macros as tags to parameters and or as entire arguments).
6. So long as next token isn't a comma a. If we have an identifier
   1. Consume it
   2. Check for assignment: a. Consume assign operator b. Parse the expression
7. While we continue to encounter commas a. Consume them b. Repeat steps 3 to 6.2.b
8. Consume the closing token

parse_preprocess_cond

1. Parse conditional directive
2. Process directive's content expression

parse_simple_preprocess

There is still decent room for improvement in this setup. Right now the entire macro's relevant tokens are shoved into an untyped AST. It would be better to store it instead in an AST_Macro node instead down the line.

1. Consume the macro token
2. Check for an opening curly brace
   1. Consume opening curly brace
   2. Until the closing curly is encountered consume all tokens.
   3. If the parent context is a typedef
      1. Check for end stement
         1. Consume it
         2. Consume potential inline comment
3. Otherwise do steps 3 to 3.1.2
4. Shove it all in an untyped string

parse_static_assert

1. Consume static assert and opening curly brace
2. Consume all tokens until the the closing brace is reached.
3. Consume curly brace and end statement
4. Place all tokens within braces into a content for the assert.

parse_template_args

This will get changed heavily once we have better support for typename expressions

1. Consume opening angle bracket
2. Consume all tokens until closing angle bracket
3. Consme closing angle bracket
4. Return the currtok with the ammended length.

parse_variable_after_name

This is needed as a variable defintion is not easily resolvable early on, it takes a long evaluation period before its known that the declaration or definition is a variable. As such this function handles resolving a variable.

By the point this function is called the following are known : export module flag, attributes, specifiers, value type, name

1. If its an assignment, parse the assignment expression (currently to an untyped string)
2. If its an opening curly brace, parse the expression within (currnelty to an untyped stirng).
   1. Consume the closing curly brace
3. If its a :, we're dealing with bitfield definition:
   1. Consume the assign classifier
   2. Consume the expression (currently to an untyped string)
4. If a comma is encountered : parse_variable declaration_list
5. Consume statement end
6. Check for inline comment

parse_variable_declaration_list

1. Consume the comma
2. Parse specifiers
3. parse_variable_after_name

parse_class

1. parse_class_struct

parse_constructor

This currently doesn't support postfix specifiers (planning to in the future)

1. parse_identifier
2. parse_parameters
3. If currtok is a :
   1. Consume :
   2. Parse the initializer list
   3. parse_function_body
4. If currtok is an opening curly brace
   1. parse_function_body
5. Otherwise:
   1. Consume statement end
   2. Check for inline comment

parse_destructor

1. Check for and consume virtual specifier
2. Check for the ~ operator
3. parse_identifier
4. Consume opening and closing parenthesis
5. Check for assignment operator:
   1. Consume assignment op
   2. Consume pure specifier 0
6. If not pure virtual & currtok is opening curly brace:
   1. parse_function_body
7. Otherwise:
   1. Consume end statement
   2. If currtok is comment : parse_comment

parse_enum

1. Consume enum token
2. Check for and consume class token
3. parse_attributes
4. If there is an identifier consume it
5. Check for a :
   1. Consume :
   2. parse_type
6. If there is a body parse it (Consume {):
   1. Newline : ast constant
   2. Comment : parse_comment
   3. Preprocess_Define : parse_define
   4. Preprocess_Conditional (if, ifdef, ifndef, elif ) : parse_preprocess_cond
   5. Preprocess_Else : ast constant
   6. Preprocess_Endif : ast constant
   7. Preprocess_Macro : parse_simple_preprocess
   8. Preprocess_Pragma : parse_pragma
   9. Preprocess_Unsupported : parse_smple_preprocess
   10. An actual enum entry
       1. Consume identifier
       2. If there is an assignment operator:
          1. Consume operator
          2. Consume the expression (assigned to untyped string for now)
          3. If a macro is encountered consume it (Unreal UMETA macro support)
       3. If there is a comma, consume it

parse_export_body

1. parse_global_nspace

parse_extern_link_body

1. parse_global_nspace

parse_extern_link

1. Consume Decl_Extern_Linkage
2. Consume the linkage identifier
3. parse_extern_link_body

parse_friend

1. Consume friend
2. parse_type
3. If the currok is an identifier its a function declaration or definition
   1. parse_function_after_name
4. Consume end statement so long as its not a function definion
5. Check for inline comment, parse_comment if exists

parse_function

1. Check and parse for export
2. parse_attributes
3. Parse specifiers
4. parse_type
5. parse_identifier
6. parse_function_after_name

parse_namespace

1. Consume namespace declaration
2. Parse identifier
3. parse_global_namespace

parse_operator

1. Check for and parse export declaration
2. parse_attributes
3. Parse specifiers
4. parse_type
5. parse_operator_after_ret_type

parse_operator_cast

1. Look for and parse a qualifier namespace for the cast (in-case this is defined outside the class's scope)
2. Consume operator declaration
3. parse_type
4. Consume opening and closing parethesis
5. Check for a const qualifiying specifier
6. Check to see if this is a definition ({)
   1. Consume {
   2. Parse body to untyped string (parsing statement and expressions not supported yet)
   3. Consume }
7. Otherwise:
   1. Consume end statement
   2. Check for and consume comment : parse_comment

parse_struct

1. parse_class_struct

parse_template

Note: This currently doesn't support templated operator casts (going to need to add support for it)

1. Check for and parse export declaration
2. Consume template declaration
3. parse_params
4. Parse for any of the following:
   1. Decl_Class : parse_class
   2. Decl_Struct : parse_struct
   3. Decl_Union : parse_union
   4. Decl_Using : parse_using
   5. The following compound into a resolved definition or declaration:
      1. parse_attributes
      2. Parse specifiers
      3. Attempt to parse as constructor or destructor: parse_global_nspace_constructor_destructor
      4. Otherwise: parse_operator_function_or_variable

parse_type

This function's implementation is awful and not done correctly. It will most likely be overhauled in the future as I plan to segement the AST_Type into several AST varaints along with sub-types to help produce robust type expressions.
Hopefully I won't need to make authentic type expressions as I was hopeing to avoid that...

Current Algorithim

Anything that is in the qualifier capture of the function typename is treated as an expression abstracted as an untyped string

1. parse_attributes
2. Parse specifiers
3. If the parse_type was called from a template parse, check to see if class was used instead of typname and consume as name.
4. This is where things get ugly for each of these depend on what the next token is.
   1. If its an in-place definition of a class, enum, struct, or union:
   2. If its a decltype (Not supported yet but draft impl there)
   3. If its a compound native type expression (unsigned, char, short, long, int, float, dobule, etc )
   4. Ends up being a regular type alias of an identifier
5. Parse specifiers (postfix)
6. We need to now look ahead to see If we're dealing with a function typename
7. If wer're dealing with a function typename:
   1. Shove the specifiers, and identifier code we have so far into a return type typename's Name (untyped string)
      1. Reset the specifiers code for the top-level typeanme
   2. Check to see if the next token is an identifier:
      1. parse_identifier
   3. Check to see if the next token is capture start and is not the last capture ("qualifier capture"):
      1. Consume (
      2. Consume expresssion between capture
      3. Consume )
   4. parse_params
   5. Parse postfix specifiers
8. Check for varaidic argument (param pack) token:
   1. Consume varadic argument token

WIP - Alternative Algorithim

Currently wrapped up via macro: GEN_USE_NEW_TYPENAME_PARSING Anything that is in the qualifier capture of the function typename is treated as an expression abstracted as an untyped string

1. parse_attributes
2. Parse specifiers (prefix)
3. This is where things get ugly for each of these depend on what the next token is.
   1. If its an in-place definition of a class, enum, struct, or union:
   2. If its a decltype (Not supported yet but draft impl there)
   3. If its a compound native type expression (unsigned, char, short, long, int, float, dobule, etc )
   4. Ends up being a regular type alias of an identifier
4. Parse specifiers (postfix)
   1. If any specifiers are found populate specifiers code with them.
5. We need to now look ahead to see If we're dealing with a function typename
6. If wer're dealing with a function typename:
   1. Shove the specifiers, and identifier code we have so far into a return type typename's Name (untyped string)
      1. Reset the specifiers code for the top-level typename
   2. Check to see if the next token is an identifier:
      1. parse_identifier
   3. Check to see if the next token is capture start and is not the last capture ("qualifier capture"):
      1. Consume (
      2. Parse binding specifiers
      3. parse_identifier
      4. parse_parameters -> params_nested
      5. Consume )
      6. Construct a nested function typename definition for the qualifier Name
   4. parse_params - > params
   5. Parse postfix specifiers
7. Check for varaidic argument (param pack) token:
   1. Consume varadic argument token

Later: Algorithim based on typename expressions

parse_typedef

1. Check for export module specifier
2. typedef keyword
3. If its a preprocess macro: Get the macro name
4. Else:
   1. Check to see if its a complicated definition (in-place enum, class, struct, union)
   2. If its a complicated definition:
      1. Perform the look ahead similar to parse_complicated_definition's implementation
      2. Check to see if its a forward declaration : parse_forward_declaration
      3. If end[-1] is an identifier:
         1. Its either an in-place, varaible type qualified identifier, or indirection type:
            1. parse_foward_or_definition
      4. else if end[-1] is a closing curly brace
         1. Its a definition: parse_forward_or_definition
      5. else if end[-1] is a closing square brace 2. Its an array definition: parse_type
   3. Else : parse-type
   4. Check for identifier : Consume the token
   5. parse_array_decl
5. Consume end statement
6. Check for inline comment : parse_comment

parse_union

1. Check for export module specifier
2. union keyword
3. parse_attributes
4. Check for identifier
5. Parse the body (Possible options):
   1. Newline
   2. Comment
   3. Decl_Class
   4. Decl_Enum
   5. Decl_Struct
   6. Decl_Union
   7. Preprocess_Define
   8. Preprocess_Conditional (if, ifdef, ifndef, elif, else, endif)
   9. Preprocess_Macro
   10. Preprocess_Pragma
   11. Unsupported preprocess directive
   12. Variable
6. If its not an inplace definiton: End Statement

parse_using

1. Check for export module specifier
2. using keyword
3. Check to see if its a using namespace
4. Get the identifier
5. If its a regular using declaration:
   1. parse_attributes
   2. parse_type
   3. parse_array_decl
6. End statement
7. Check for inline comment

parse_variable

1. Check for export module specifier
2. parse_attributes
3. parse specifiers
4. parse_type
5. parse_identifier
6. parse_variable_after_name