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A single-line comment in C++ is a lexical construct that instructs the compiler to ignore all text following the comment delimiter up to the end of the current line. During the early translation phases of C++, the preprocessor strips these comments from the source code, typically replacing them with a single whitespace character before syntactic analysis begins. 
// comment_text
Syntactic Mechanics
  • Initiation: The comment is initiated by two consecutive forward slashes (//) with no intervening whitespace.
  • Termination: The comment is implicitly terminated by the next newline character (\n). No explicit closing delimiter is required.
  • Parsing: The compiler evaluates the code strictly from left to right. Once the // token is encountered, all subsequent characters on that physical line are treated as part of the comment, regardless of whether they resemble valid C++ syntax.
// This is a standalone single-line comment.
int x = 10; // This comment follows a valid statement.
Line Splicing (Continuation) Because C++ processes physical source lines into logical lines during phase 2 of translation (before comments are removed in phase 3), a single-line comment can be extended to the next physical line using a backslash (\). If the backslash is immediately followed by a newline character, the compiler splices the lines together, extending the single-line comment. 
// This comment is extended to the next physical line \
and the compiler will ignore this text as well.
int y = 20;
Nesting Behavior Single-line comments do not possess nesting semantics. Because everything after the // is treated as raw text, placing another // or a block comment initiator (/*) inside a single-line comment has no syntactic effect; it is simply absorbed as part of the ignored text. 
// // This second delimiter is ignored.
// /* This block comment initiator is also ignored.
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