Font locking of C++ constructors is somewhat inconsistent.  This is no
doubt complicated by the fact that unlike other function declarations
they "don't have a return type".

When a single argument is not used but named, the constructor is not
fontified (normally it's fontified with `font-lock-function-name-face').
If the keyword explicit is used, then the argument type is fontified as
a variable, and the constructor name is fontified as a type.  Perhaps
interestingly, naming the parameter or adding another parameter causes
fontification to work correctly (with or without explicit).

I have included a sample file below with comments on what I see in `emacs -q`


class Bob
{
    // string is `font-lock-type-face', Bob is `font-lock-function-name-face'
    Bob( string bob );
    // string and Bob are not fontified (though I sometimes see string fontified as a type)
    Bob( string );
    // string is `font-lock-variable-name-face', Bob is `font-lock-type-face'
    explicit Bob( string );
    // string is `font-lock-type-face', Bob is `font-lock-function-name-face'
    explicit Bob( string, string );
};