Last time we covered pointer arguments, and that article was applicable to C and C++. C++ introduced true pass-by-reference semantics.<\/p>\n
f( O & )<\/code><\/li>\nf( const O & )<\/code><\/li>\n<\/ul>\nf(O &)<\/code><\/h2>\nPreviously we were careful to note that passing a pointer to a function was still only pass-by-value. The pointer was copied, and then the function was free to use that pointer to modify the underlying object. That was how C provided the utility of pass-by-reference without actually having to implement it. C++ lets us declare a real reference type. We can then use the variable in the function as we would if it were inline in the caller.<\/p>\n
void<\/span> f(O &p) {\n p.modify();\n }<\/code><\/pre>\nBarring the different syntax (\u00e2\u20ac\u0153.<\/code>\u00e2\u20ac\u009d instead of \u00e2\u20ac\u0153-><\/code>\u00e2\u20ac\u009d), there is one fundamental difference between pass-by-reference and pass-by-pointer: null is not a valid value. You can only pass a reference to an object that exists. It\u00e2\u20ac\u2122s not possible to call f(nullptr)<\/code> since nullptr<\/code> is a pointer not an object. It\u00e2\u20ac\u2122s not possible to call f(*nullptr)<\/code>, since dereferencing nullptr<\/code> is illegal. Therefore f()<\/code> doesn\u00e2\u20ac\u2122t need to check for nullptr<\/code> itself.<\/p>\nWe\u00e2\u20ac\u2122re also cutting off any ability for f()<\/code> to affect the lifetime of p<\/code>. Whereas in the f(O*)<\/code> case, f()<\/code> could modify the object and<\/em> delete it; in the f(O&)<\/code> case, f()<\/code> cannot delete<\/code> the object, since delete<\/code> requires a pointer (I\u00e2\u20ac\u2122m ignoring the insane case of delete &p<\/code>).<\/p>\nThat fact makes f(O&)<\/code> far more attractive as a \u00e2\u20ac\u0153modifies O\u00e2\u20ac\u009d function declaration that f(O*)<\/code>. In fact, if you find yourself writing f(O*)<\/code>, think very carefully about whether you really have got the correct form.<\/p>\nf(const O &)<\/code><\/h2>\nThis is an easy one; excepting the change of syntax, this is identical to f(const O *)<\/code> saving the fact that it\u00e2\u20ac\u2122s not possible to pass nullptr<\/code> to it.<\/p>\nLike f(O*)<\/code> and f(O&)<\/code>, there is reason to pick between f(const O*)<\/code> and f(const O&)<\/code>:<\/p>\n\nf(const O&)<\/code><\/dt>\n- \n
f()<\/code> queries an O<\/code><\/p>\n<\/dd>\nf(const O*)<\/code><\/dt>\n- \n
f()<\/code> queries an O<\/code> with a fallback if this O<\/code> doesn\u00e2\u20ac\u2122t exist.<\/p>\n<\/dd>\n<\/dl>\nChoose between them as appropriate for your application. On the whole though, you should prefer f(const O&)<\/code>, since you should only work with objects that exist \u00e2\u20ac\u201c why would you even call f()<\/code> if you had nothing to call it with?<\/p>\n
\nNext time, C++11.<\/p>\n","protected":false},"excerpt":{"rendered":"
Last time we covered pointer arguments, and that article was applicable to C and C++. C++ introduced true pass-by-reference semantics. f( O & ) f( const O & ) f(O &) Previously we were careful to note that passing a pointer to a function was still only pass-by-value. The pointer was copied, and then the\u2026 Read More »<\/a><\/span><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[66,42,6],"_links":{"self":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1180"}],"collection":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1180"}],"version-history":[{"count":3,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1180\/revisions"}],"predecessor-version":[{"id":1191,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1180\/revisions\/1191"}],"wp:attachment":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1180"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1180"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1180"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}