class MatcherInterface

Returns true if and only if the matcher matches x; also explains the

match result to 'listener' if necessary (see the next paragraph), in

the form of a non-restrictive relative clause ("which ...",

"whose ...", etc) that describes x. For example, the

MatchAndExplain() method of the Pointee(...) matcher should

generate an explanation like "which points to ...".

Implementations of MatchAndExplain() should add an explanation of

the match result *if and only if* they can provide additional

information that's not already present (or not obvious) in the

print-out of x and the matcher's description. Whether the match

succeeds is not a factor in deciding whether an explanation is

needed, as sometimes the caller needs to print a failure message

when the match succeeds (e.g. when the matcher is used inside

Not()).

For example, a "has at least 10 elements" matcher should explain

what the actual element count is, regardless of the match result,

as it is useful information to the reader; on the other hand, an

"is empty" matcher probably only needs to explain what the actual

size is when the match fails, as it's redundant to say that the

size is 0 when the value is already known to be empty.

You should override this method when defining a new matcher.

It's the responsibility of the caller (Google Test) to guarantee

that 'listener' is not NULL. This helps to simplify a matcher's

implementation when it doesn't care about the performance, as it

can talk to 'listener' without checking its validity first.

However, in order to implement dummy listeners efficiently,

listener->stream() may be NULL.