C++ source files are processed by the compiler to produce C++ programs.
[edit] Translation processThe text of a C++ program is kept in units called source files.
C++ source files undergo translation to become a translation unit, consisting of the following steps:
A C++ program can be formed from translated translation units. Translated translation units and instantiated units (instantiated units are described in phase 8 below) can be saved individually or saved into a library. Multiple translation units communicate with each other through (for example) symbols with external linkage or data files. Translation units can be separately translated and then later linked to produce an executable program.
The process above can be organized into 9 translation phases.
[edit] Preprocessing tokensA preprocessing token is the minimal lexical element of the language in translation phases 3 through 6.
The categories of preprocessing token are:
The set of preprocessing tokens of preprocessing number is a superset of the union of the sets of tokens of integer literals and floating-point literals:
.(optional) digit pp-continue-seq (optional) digit - one of digits 0-9 pp-continue-seq - a sequence of pp-continue s
Each pp-continue is one of the following:
identifier-continue (1) exp-char sign-char (2). (3) â digit (4) (since C++14) â nondigit (5) (since C++14) identifier-continue - any non-first character of a valid identifier exp-char - one of P, p,(since C++11) E and e sign-char - one of + and - digit - one of digits 0-9 nondigit - one of Latin letters A/a-Z/z and underscore
A preprocessing number does not have a type or a value; it acquires both after a successful conversion an integer/floating-point literal token.
[edit] WhitespaceWhitespace consists of comments, whitespace characters, or both.
The following characters are whitespace characters:
Whitespace is usually used to separate preprocessing tokens, with the following exceptions:
#include "my header" // OK, using a header name containing whitespace #include/*hello*/<iostream> // OK, using a comment as whitespace #include <iostream> // Error: #include cannot span across multiple lines "str ing" // OK, a single preprocessing token (string literal) ' ' // OK, a single preprocessing token (character literal)[edit] Maximal munch
The maximal munch is the rule used in phase 3 when decomposing the source file into preprocessing tokens.
If the input has been parsed into preprocessing tokens up to a given character (otherwise, the next preprocessing token will not be parsed, which makes the parsing order unique), the next preprocessing token is generally taken to be the longest sequence of characters that could constitute a preprocessing token, even if that would cause subsequent analysis to fail. This is commonly known as maximal munch.
int foo = 1; int bar = 0xE+foo; // Error: invalid preprocessing number 0xE+foo int baz = 0xE + foo; // OK
In other words, the maximal munch rule is in favor of multi-character operators and punctuators:
int foo = 1; int bar = 2; int num1 = foo+++++bar; // Error: treated as âfoo++ ++ +bazâ, not âfoo++ + ++bazâ int num2 = -----foo; // Error: treated as â-- -- -fooâ, not â- -- --fooâ
The maximal munch rule has the following exceptions:
std::vector<int> x; // OK, âintâ is not a header name
struct Foo { static const int v = 1; };
std::vector<::Foo> x; // OK, <: not taken as the alternative token for [
extern int y<::>; // OK, same as âextern int y[];â
int z<:::Foo::value:>; // OK, same as âint z[::Foo::value];â
template<int i> class X { /* ... */ };
template<class T> class Y { /* ... */ };
Y<X<1>> x3; // OK, declares a variable âx3â of type âY<X<1> >â
Y<X<6>>1>> x4; // Syntax error
Y<X<(6>>1)>> x5; // OK
#define R "x" const char* s = R"y"; // ill-formed raw string literal, not "x" "y" const char* s2 = R"(a)" "b)"; // a raw string literal followed by a normal string literal(since C++11) [edit] Tokens
A token is the minimal lexical element of the language in translation phase 7.
The categories of token are:
Translation is performed as if in the order from phase 1 to phase 9. Implementations behave as if these separate phases occur, although in practice different phases can be folded together.
[edit] Phase 1: Mapping source characters 1)The individual bytes of the source code file are mapped (in implementation-defined manner) to the characters of the
basic source character set. In particular, OS-dependent end-of-line indicators are replaced by newline characters.
2) The set of source file characters accepted is implementation-defined(since C++11). Any source file character that cannot be mapped to a character in the
basic source character setis replaced by its
universal character name(escaped with
\u
or
\U
) or by some implementation-defined form that is handled equivalently.
(until C++17) (until C++23)Input files that are a sequence of UTF-8 code units (UTF-8 files) are guaranteed to be supported. The set of other supported kinds of input files is implementation-defined. If the set is non-empty, the kind of an input file is determined in an implementation-defined manner that includes a means of designating input files as UTF-8 files, independent of their content (recognizing the byte order mark is not sufficient).
1) If the first translation character is byte order mark (U+FEFF), it is deleted. (since C++23)Whenever backslash (\) appears at the end of a line (immediately followed by zero or more whitespace characters other than new-line followed by(since C++23) the newline character), these characters are deleted, combining two physical source lines into one logical source line. This is a single-pass operation; a line ending in two backslashes followed by an empty line does not combine three lines into one.
2) If a non-empty source file does not end with a newline character after this step (end-of-line backslashes are no longer splices at this point), a terminating newline character is added.
[edit] Phase 3: Lexing 1)The source file is decomposed into
preprocessing tokensand
whitespace:
// The following #include directive can de decomposed into 5 preprocessing tokens: // punctuators (#, < and >) // â // ââââââââââ¼âââââââââ // â â â #include <iostream> // â â // â âââ header name (iostream) // â // ââââââââââââ identifier (include)
If a source file ends in a partial preprocessing token or in a partial comment, the program is ill-formed:
// Error: partial string literal "abc
// Error: partial comment /* comment
As characters from the source file are consumed to form the next preprocessing token (i.e., not being consumed as part of a comment or other forms of whitespace), universal character names are recognized and replaced by the designated element of the
translation character set, except when matching a character sequence in one of the following preprocessing tokens:
Any transformations performed during
phase 1 and(until C++23)phase 2 between the initial and the final double quote of any
raw string literalare reverted.
(since C++11) 3)Whitespace is transformed:
Each file introduced with the
#includedirective goes through phases 1 through 4, recursively.
3) At the end of this phase, all preprocessor directives are removed from the source.
[edit] Phase 5: Determining common string literal encodings 2) Escape sequencesand universal character names in character literals and non-raw string literals are expanded and converted to the literal encoding.
If the character specified by a universal character name cannot be encoded as a single code point in the corresponding literal encoding, the result is implementation-defined, but is guaranteed not to be a null (wide) character.
(until C++23)For a sequence of two or more adjacent string literal tokens, a common encoding prefix is determined as described here. Each such string literal token is then considered to have that common encoding prefix. (Character conversion is moved to phase 3)
(since C++23) [edit] Phase 6: Concatenating string literalsAdjacent string literals are concatenated.
[edit] Phase 7: CompilingCompilation takes place: each preprocessing token is converted to a token. The tokens are syntactically and semantically analyzed and translated as a translation unit.
[edit] Phase 8: Instantiating templatesEach translation unit is examined to produce a list of required template instantiations, including the ones requested by explicit instantiations. The definitions of the templates are located, and the required instantiations are performed to produce instantiation units.
[edit] Phase 9: LinkingTranslation units, instantiation units, and library components needed to satisfy external references are collected into a program image which contains information needed for execution in its execution environment.
[edit] NotesSource files, translation units and translated translation units need not necessarily be stored as files, nor need there be any one-to-one correspondence between these entities and any external representation. The description is conceptual only, and does not specify any particular implementation.
The conversion performed at phase 5 can be controlled by command line options in some implementations: gcc and clang use -finput-charset to specify the encoding of the source character set, -fexec-charset and -fwide-exec-charset to specify the ordinary and wide literal encodings respectively, while Visual Studio 2015 Update 2 and later uses /source-charset and /execution-charset to specify the source character set and literal encoding respectively.
(until C++23)Some compilers do not implement instantiation units (also known as template repositories or template registries) and simply compile each template instantiation at phase 7, storing the code in the object file where it is implicitly or explicitly requested, and then the linker collapses these compiled instantiations into one at phase 9.
[edit] Defect reportsThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior CWG 787 C++98 the behavior was undefined if a non-empty source file doesRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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