Comments disappear before C++ interprets your source

Comments remain for readers but disappear before C++ interprets the source

class Notes {
    int shown = 7; // 7 is active source.
    /*
    int hidden = 9;
    */
};

The compiler does not treat every character in this file as an instruction. The characters // and the text after them on the same line form a comment. The characters /* through the next */ form another comment. Those comment characters and the text between them are removed before C++ interprets the remaining source.

The line containing the literal 7 remains active source except for its comment. The declaration containing the literal 9 is entirely inside the block comment, so it is ignored. Its text can still help you understand the file, but it does not affect what the compiler interprets.

A // comment stops at the next line break

On the line containing 7, the // marker starts a line comment. The comment includes the space after the marker, the words 7 is active source., and every character after those words on that line. The next line break ends that comment. A line break is therefore part of the boundary: it does not belong to the comment text.

int shown = 7; // 7 is active source.
                 <------------------ comment ends at the line break

The /* marker on the following line begins independently. The earlier // comment cannot continue onto that line, because its line has already ended. This gives the source file two separate comment spans: one ending at a line break and one ending at a closing marker.

A /* comment continues until the first */ marker

The block comment starts at /* and continues across the line break, the line containing the declaration with 9, and the next line until it reaches */. Line breaks do not end a block comment. Every character in that span is comment text, including characters that look like C++ source.

/*
int hidden = 9;
*/
start                 end

A // marker inside this block span would also be ordinary comment text. The block comment has already started, so that line marker cannot change how the block ends. The first */ marker ends the block. C++ does not support nested block comments, so a second /* inside the span does not create a separate inner block that must be closed first.

class Notes {
    int shown = 7; // 7 is active source.
    /*
    int hidden = 9;
};

Comment delimiters can hide source or accidentally hide too much

The block around the declaration containing 9 can have either of two purposes. It can intentionally disable that source while keeping it in the file, or it can explain something to a reader. In both cases, the compiler sees only a comment span and does not interpret the characters inside it. The meaning for readers does not change the meaning for C++.

A comment delimiter is only useful when its pair is correct. If you delete the closing */, the block comment continues past the declaration containing 9 and consumes the rest of the file as comment text. Required source after that point can disappear from the compiler's input, which can cause compilation to fail.