Recursion is a powerful technique in programming that allows a function to call itself. In C++, recursion is a useful tool for solving problems that can be broken down into smaller, simpler problems. It is a way of solving a problem by breaking it down into smaller and smaller sub-problems until the solution becomes trivial.

Recursion is a process in which a function calls itself as a subroutine. This allows the function to be repeated several times, since the function is called within itself. Recursion is a powerful tool in programming, as it allows for the creation of complex algorithms that can solve problems that would be difficult or impossible to solve using other methods.

Recursion is often used in programming to solve problems that involve a repetitive process. For example, a function that calculates the factorial of a number can be written using recursion. The factorial of a number is the product of all the integers from 1 to that number. The factorial of 5, for example, is 5 x 4 x 3 x 2 x 1 = 120.

Here is an example of a C++ function that calculates the factorial of a number using recursion:

int factorial(int n) {
    if (n == 0) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

In this example, the function checks if the input parameter n is equal to 0. If it is, the function returns 1. If n is not equal to 0, the function calls itself with the parameter n - 1. This process continues until n is equal to 0, at which point the function returns 1.

Another example of recursion in C++ is the Fibonacci sequence. The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding numbers. The first two numbers in the sequence are 0 and 1. The sequence goes as follows: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269, 2178309, 3524578, 5702887, 9227465, 14930352, 24157817, 39088169, 63245986, 102334155, 165580141, 267914296, 433494437, 701408733, 1134903170, 1836311903, 2971215073, 4807526976, 7778742049, 12586269025, 20365011074, 32951280099, 53316291173, 86267571272, 139583862445, 225851433717, 365435296162, 591286729879, 956722026041, 1548008755920, 2504730781961, 4052739537881, 6557470319842, 10610209857723, 17167680177565, 27777890035288, 44945570212853, 72723460248141, 117669030460994, 190392490709135, 308061521170129, 498454011879264, 806515533049393, 1304969544928657, 2111485077978050, 3416454622906707, 5527939700884757, 8944394323791464, 14472334024676221, 23416728348467685, 37889062373143906, 61305790721611591, 99194853094755497, 160500643816367088, 259695496911122585, 420196140727489673, 679891637638612258, 1100087778366101931, 1779979416004714189, 2880067194370816120, 4660046610375530309, 7540113804746346429, 12200160415121876738, 19740274219868223167, 31940434634990099905, 51680708854858323072, 83621143489848422977, 135301852344706746049, 218922995834555169026, 354224848179261915075, 573147844013817084101, 927372692193078999176, 1500520536206896083277, 2427893228399975082453, 3928413764606871165730, 6356306993006846248183, 10284720757613717413913, 16641027750620563662096, 26925748508234281076009, 43566776258854844738105, 70492524767089125814114, 114059301025943970552219, 184551825793033096366333, 298611126818977066918552, 483162952612010163802885, 781774079430987230721437, 1264937032042997393488322, 2046711111473984623699759, 3311648143516982017188081, 5358359254990966640887840, 8670007398507948658055921, 14028366653498915298923761, 22698374052006863956979682, 36726740705505779255803443, 59425114757512643212783125, 96151855463018422468586568, 155576970220531065681649693, 251728825683549488150424261, 407305795904080553832073954, 659034621587630041982498215, 1066340417491710595814572169, 1725375039079340637797070384, 2791715456571051233611642553, 4517090495650391871408712937, 7308805952221443105020355490, 11825896447871834976429068427, 19134702400093278081449423917, 30960598847965113057878492344, 50095301248058391139327916261, 81055900096023504197206408605, 131151201344081895336534324866, 212207101440105399533740733471, 343358302784187294870275058337, 555565404224292694404015791808, 898923707008479989274290850145, 1454489111232772683678306641953, 2353416825501258432952597492098, 3807905936734031116630904134051, 6161322762235289549583501626149, 9969228708969320666214405760200, 16130531424904581415797907386349, 26099760133873802082012313146549, 42230291558778383497810220532898, 68330051692652185579822533679447, 110560307156090817237632754212345, 178890358848742002817455287891792, 289450666004832820055088042104137, 468341024853574822872543329995929, 757791690858407642927631372100066, 1226132595394188293000174702095995, 1983924286252595935927806074196061, 3210056881646784228927980776292056, 5193981167899380164855786850488117, 8404038049546164393783767626780173, 13598018856415543986439554477268290, 22002056905961708380223322104048463, 35600075762377252366662876581316753, 57602132668338960746886198685365216, 93202208430716213113549075266681969, 150804141499155273260935273952047185, 244006349929871486374484349218729154, 394810491429026759635419623170776339, 638816841358898246009903972389505493, 1033628323428189498226323595560281832, 1672445164787087744236227567949787325, 2706073488215277242462551163510069157, 4378518653002364986698778731459856482, 7084592141217642229161339894969925639, 11463159734250047265890128626419782121, 18547751875467689495051468521389707760, 30010911609717736760941597147809489881, 48558663485185426255993065669299197641, 78569575094903163016934662817108687522, 127128238380888789072727928486407885163, 205697813475791952089662591303516572685, 332826051856680741162390519789924457848, 538523865332472693252053111093441030533, 871349917189153434414443630883365488381, 1405870803527629127661491741974806518914, 2277210720716782562075935372858172007295, 3683081524244411689737427114832978526209, 5960292244961194251813362487691150533504, 9643373769205605941550789602524139059713, 15615686084126740113314162090275249593217, 25259059853332346054864951692899388652930, 40874745937459086168179113783174638246147, 66133805790791432223044065476074026999077, 106908851728150418191823979459248665145224, 172042657518941850414868044935322692144301, 278951509247092268606692024394571357289525, 450994166766034119021560069329894049433826, 729945676013126387628252093724465406723351, 1181936842774161501645810169051358453155177, 1911882518787287889274062262775823869878528, 3093819361561449390929872431827182323033705, 5005701880348737280203934694603006192912233, 8099521241910186671133807126430188515945938, 13115223152278913971347741871013104708878171, 21214744394189000642481548997443293224824109, 34329967546467914613829290868456397933702280, 55544711940656915256310839865899691158526389, 89874679487124829870140130734356089092228669, 145118791727681245026150970200155680150155058, 234993471214806074896291100934511769242383727, 380112262942487319922442071134667449392538785, 615105734157293394818733172069179218634922512, 996217997099780714741175243203846668027461297, 1611323732253073101552908418274020882667382809, 2607541739352853816294083661477867550694844106, 4218865471605926917846992079751888433362226915, 6826407210958780734141075741229755984057071021, 11095292642504797681968017820921624417459217936, 17921799853463578416109093562151380401516288957, 29017092495968376098077111383073004818975506893, 46938892349431954514186204945224385220491795850, 75955984845300330612263316328297390039467302743, 122794877894632785926249821973421475459459298593, 198750862739933116538513138301718865498926601336, 321545740634565902464762960275140340958385899929, 520296603374499019003276098576859206457312501265, 841842344009064921468039058852999547415698401194, 1363136947385567949477312153424858754876015902459, 2204979291394632870945351212277858302291714303653, 3568116238780200820422663365702717057167730206112, 5773095530174833691368014577980575359459444519765, 9341211768955034511790677943683292416627174725877, 15114367299109818223138632501683837716016649205642, 24455579068064852734929310445367130132643823931519, 39569946367174670958067942947050967848660473137161, 64025525435239523692997253392418097981304297068680, 103985571402814794651665696839469265129764670105841, 167011096838054318344662950231887363111068967174521, 271996668240869113996328647071356628240833637280362, 439007765078923432341991597303243991351902604454883, 710004433319792546338320244374600619592736241735245, 1146018191398716978683316843676849617942636848197128, 1856022624718509525021637088051440237535373089932373, 3002040816117226503704953931728289855478009938129501, 4858063440835736028726591029779730093013383028061874, 7860104256952962532431544961508019948491392966191375, 12768167667778648581198145921237740061544715934273249, 20628271924731611113629690882745760010036108900464624, 33396439592510259694827836803983500071580824834737873, 54024711517241870808457527686729260081616933735202497, 87421151109752130503285364490712760153297758569940370, 141246662926594401311342792677442920234614692905842867, 228667814036346531814628157168155680387912451475783237, 369914476962940933125970949845598600622527144381626104, 598582290999287464940599107013754281010439595857409341, 968496767962228398066570056859352881632966740239035445, 1564073057968519864005160164877105167642405332099441786, 2532569825930748262071730221736458049275372072338477231, 4096642883899268126076890386613563216917777404437919017, 6629212709830016388148620608350021266193149476776396248, 10705865543789224544215580914983594423140926831294395265, 17335078253619240932364201523333615689334076308070791513, 28040943797408465476579782438317210112475003139365186778, 45376022051027706408943983961650825801809079447435978291, 73416965848436171885523766399968035914284082586801165069, 118792087599463178694167150361418861516593162534937943360, 192209053447899350579690916761386897430877245121739108429, 310001141047362529273858067122805758947470407656677051789, 502210194495261879853548983884192656378347652778416160218, 812211335542624409127407051007998415325818060435093212007, 1313422523038887286988959037892193074705169719211507377225, 2125633858581511696116366088890191490030987789646600589232, 3439056381620398983105325126782384564736157508858107966457, 5564690240201910679221691215672576054767145298504708555689, 9003746621822309662327016342454960619503302807362816522146, 14528466822014230371548797588187576634210488185877515017835, 23532213443836540033875813930642537253713790993240331539981, 38060680265850770405424611518830113887924279179117846557816, 61592893709687310439300425449472651141638070172358178197797, 99653573975538080844725036968302765029562349351476024755613, 161306467985825691684525162817475216271100819223934902253410, 260960041961363772529250199786777981300663168575410926009023, 422266509947189464213775362604253197571763987799345828262433, 683226551908553236743025562391031178872427156374