来自GCC 4.3.4状态的basic_string.h中的注释：

The comments in basic_string.h from GCC 4.3.4 state:

    // The maximum number of individual char_type elements of an
    // individual string is determined by _S_max_size. This is the
    // value that will be returned by max_size().  (Whereas npos
    // is the maximum number of bytes the allocator can allocate.)
    // If one was to divvy up the theoretical largest size string,
    // with a terminating character and m _CharT elements, it'd
    // look like this:
    // npos = sizeof(_Rep) + (m * sizeof(_CharT)) + sizeof(_CharT)
    // Solving for m:
    // m = ((npos - sizeof(_Rep))/sizeof(CharT)) - 1
    // In addition, this implementation quarters this amount.

特别注意最后一行此外，

In particular, note the last line, "In addition, this implementation quarters this amount." I take that to mean that the division by four is in fact entirely arbitrary.

我试图在这里找到更多信息。

I tried to find more information in the checkin log for basic_string.h, but it only goes back to October 5, 2000, and this comment was already present as shown in that revision, and I'm not familiar enough with that code base to know where the file might have lived in the source tree before it was moved to its current location.