gencpp/docs/Parser_Algo.md

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 StrC type. The tokens are stored (for now) in gen::parser::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>( StrC 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

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_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

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. Newline : ast constant
   2. Comment : parse_comment
   3. Access_Public : ast constant
   4. Access_Protected : ast constant
   5. Access_Private : ast constant
   6. Decl_Class : parse_complicated_definition
   7. Decl_Enum : parse_complicated_definition
   8. Decl_Friend : parse_friend
   9. Decl_Operator : parse_operator_cast
   10. Decl_Struct : parse_complicated_definition
   11. Decl_Template : parse_template
   12. Decl_Typedef : parse_typedef
   13. Decl_Union : parse_complicated_definition
   14. Decl_Using : parse_using
   15. Operator == '~'
       1. parse_destructor
   16. Preprocess_Define : parse_define
   17. Preprocess_Include : parse_include
   18. Preprocess_Conditional (if, ifdef, ifndef, elif, else, endif) : parse_preprocess_cond or else/endif ast constant
   19. Preprocess_Macro : parse_simple_preprocess
   20. Preprocess_Pragma : parse_pragma
   21. Preprocess_Unsupported : parse_simple_preprocess
   22. StaticAssert : parse_static_assert
   23. The following compound into a resolved definition or declaration:
       1. Attributes (Standard, GNU, MSVC) : parse_attributes
       2. Specifiers (consteval, constexpr, constinit, forceinline, inline, mutable, neverinline, static, volatile)
       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
   24. 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 any of the above is the case, parse_operator_function_or_variable
4. If the previous token was a closing curly brace, its a definition : parse_forward_or_definition
5. 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

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. NewLine : ast constant
   2. Comment : parse_comment
   3. Decl_Cass : parse_complicated_definition
   4. Decl_Enum : parse_complicated_definition
   5. Decl_Extern_Linkage : parse_extern_link
   6. Decl_Namespace : parse_namespace
   7. Decl_Struct : parse_complicated_definition
   8. Decl_Template : parse_template
   9. Decl_Typedef : parse_typedef
   10. Decl_Union : parse_complicated_definition
   11. Decl_Using : parse_using
   12. Preprocess_Define : parse_define
   13. Preprocess_Include : parse_include
   14. Preprocess_If, IfDef, IfNotDef, Elif : parse_preprocess_cond
   15. Preprocess_Else : ast constant
   16. Preprocess_Endif : ast constant
   17. Preprocess_Macro : parse_simple_preprocess
   18. Preprocess_Pragma : parse_pragma
   19. Preprocess_Unsupported : parse_simple_preprocess
   20. StaticAssert : parse_static_assert
   21. Module_Export : parse_export_body
   22. Module_Import : NOT_IMPLEMENTED
   23. 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. If its an operator cast (definition outside class) : parse_operator_cast
          2. Its an operator, function, or varaible : parse_operator_function_or_varaible
4. If its not a global body, consuem the closing curly brace

parse_identifier

parse_include

parse_operator_after_ret_type

parse_operator_function_or_variable

parse_pragma

parse_params

parse_preprocess_cond

parse_simple_preprocess

parse_static_assert

parse_template_args

parse_variable_after_name

parse_variable_declaration_list

parse_class

parse_constructor

parse_destructor

parse_enum

parse_export_body

parse_extern_link_body

parse_extern_link

parse_friend

parse_function

parse_namespace

parse_operator

parse_operator_cast

parse_struct

parse_template

parse_type

parse_typedef

1. Check for export module specifier
2. typedef keyword
3. If its a preprocess macro: Get the macro name

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