# Phases of translation

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The C++ source file is processed by the compiler as if the following phases take place, in this exact order:

### Phase 1

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. The basic source character set consists of 96 characters:
a) 5 whitespace characters (space, horizontal tab, vertical tab, form feed, new-line)
b) 10 digit characters from '0' to '9'
c) 52 letters from 'a' to 'z' and from 'A' to 'Z'
d) 29 punctuation characters: _ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ~ ! = , \ " '
2) Any source file character that cannot be mapped to a character in the basic source character set is replaced by its universal character name (escaped with `\u` or `\U`) or by some implementation-defined form that is handled equivalently.

3) Trigraph sequences are replaced by corresponding single-character representations. | (until C++17)

### Phase 2

1) Whenever backslash appears at the end of a line (immediately followed by the newline character), both backslash and newline 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. If a universal character name (\uXXX) is formed in this phase, the behavior is undefined.
2) If a non-empty source file does not end with a newline character after this step (whether it had no newline originally, or it ended with a backslash), the behavior is undefined (until C++11)a terminating newline character is added (since C++11).

### Phase 3

1) The source file is decomposed into comments, sequences of whitespace characters (space, horizontal tab, new-line, vertical tab, and form-feed), and preprocessing tokens, which are the following:
a) header names such as <iostream> or "myfile.h" (only recognized after `#include`)
b) identifiers
c) preprocessing numbers
d) character and string literals , including user-defined (since C++11)
e) operators and punctuators (including alternative tokens), such as +, <<=, new, <%, ##, or and
f) individual non-whitespace characters that do not fit in any other category

2) Any transformations performed during phases 1 and 2 between the initial and the final double quote of any raw string literal are reverted. | (since C++11)

3) Each comment is replaced by one space character.

Newlines are kept, and it's unspecified whether non-newline whitespace sequences may be collapsed into single space characters.

### Phase 4

1) The preprocessor is executed.
2) Each file introduced with the #include directive goes through phases 1 through 4, recursively.
3) At the end of this phase, all preprocessor directives are removed from the source.

### Phase 5

1) All characters in character literals and string literals are converted from the source character set to the execution character set (which may be a multibyte character set such as UTF-8, as long as the 96 characters of the basic source character set listed in phase 1 have single-byte representations).
2) Escape sequences and universal character names in character literals and non-raw string literals are expanded and converted to the execution character set. If the character specified by a universal character name isn't a member of the execution character set, the result is implementation-defined, but is guaranteed not to be a null (wide) character.

Note: the conversion performed at this stage 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 encodings of the execution character set in the string and character literals that don't have an encoding prefix (since C++11).

### Phase 6

Adjacent string literals are concatenated.

### Phase 7

Compilation takes place: each preprocessing token is converted to a token. The tokens are syntactically and semantically analyzed and translated as a translation unit.

### Phase 8

Each 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.

### Phase 9

Translation 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.

### Notes

Some compilers don't 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.

### References

- C++11 standard (ISO/IEC 14882:2011):

  - 2.2 Phases of translation [lex.phases]

- C++98 standard (ISO/IEC 14882:1998):

  - 2.1 Phases of translation [lex.phases]

### See also

C documentation for phases of translation

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