可变参数模板在C++11中被引入。我发现printf函数可以使用它进行替换。 然而,在实现中使用了cout。我想知道是否有可能使用其他东西来实现类型安全,但不会牺牲太多性能。
void safe_printf(const char *s)
{
while (*s) {
if (*s == '%') {
if (*(s + 1) == '%') {
++s;
}
else {
throw "invalid format string: missing arguments";
}
}
std::cout << *s++;
}
}
template<typename T, typename... Args>
void safe_printf(const char *s, T& value, Args... args)
{
while (*s) {
if (*s == '%') {
if (*(s + 1) == '%') {
++s;
}
else {
std::cout << value;
safe_printf(s + 1, args...); // call even when *s == 0 to detect extra arguments
return;
}
}
std::cout << *s++;
}
throw "extra arguments provided to printf";
}
编辑:根据要求,我进行了一个测试来比较cout和printf的性能。 gcc版本为4.8.1,x86_64-redhat-linux
#include <iostream>
#include <cstdio>
#include <ctime>
#include <string>
#include <stdint.h>
#include <sstream>
using namespace std;
int32_t val = 10;
string s = "avcccc";
void TestOne(){
for(int i = 0; i < 1000000; i++){
printf("This is the value: %d, %s\n", i, s.c_str());
}
}
void TestTwo(){
ios_base::sync_with_stdio(false);
for(int i = 0; i < 1000000; i++){
cout << "This is the value: " << i << ", " << s << "\n";
}
}
int main( int argc, char * argcv[] )
{
timespec start;
clock_gettime(CLOCK_REALTIME, &start);
TestOne();
timespec end;
clock_gettime(CLOCK_REALTIME, &end);
timespec start2;
clock_gettime(CLOCK_REALTIME, &start2);
TestTwo();
timespec end2;
clock_gettime(CLOCK_REALTIME, &end2);
int64_t d1 = ((end.tv_sec - start.tv_sec) * 1000000000) + (end.tv_nsec - start.tv_nsec);
int64_t d2 = ((end2.tv_sec - start2.tv_sec) * 1000000000) + (end2.tv_nsec - start2.tv_nsec);
cout << "printf:" << d1 << " cout:" << d2 << ", diff(prinf-cout) " << d1-d2 << endl;
}
不关闭同步 stdio 的情况下。
printf:242862382 cout:275197806, diff(prinf-cout) -32335424
printf:201481941 cout:226177849, diff(prinf-cout) -24695908
printf:244915215 cout:275071790, diff(prinf-cout) -30156575
printf:235603694 cout:286859058, diff(prinf-cout) -51255364
printf:230076992 cout:248738879, diff(prinf-cout) -18661887
printf:223023056 cout:246486805, diff(prinf-cout) -23463749
printf:229922272 cout:269340309, diff(prinf-cout) -39418037
printf:228955230 cout:270675387, diff(prinf-cout) -41720157
printf:228390291 cout:228521421, diff(prinf-cout) -131130
printf:248419954 cout:268399737, diff(prinf-cout) -19979783
printf:229476729 cout:254635928, diff(prinf-cout) -25159199
printf:229646751 cout:252797729, diff(prinf-cout) -23150978
printf:227093478 cout:255390754, diff(prinf-cout) -28297276
printf:199161604 cout:224850870, diff(prinf-cout) -25689266
printf:198763304 cout:225709890, diff(prinf-cout) -26946586
printf:200445025 cout:223221944, diff(prinf-cout) -22776919
printf:202760174 cout:232363225, diff(prinf-cout) -29603051
printf:201989909 cout:225384923, diff(prinf-cout) -23395014
printf:202773400 cout:221465068, diff(prinf-cout) -18691668
printf:199076506 cout:223602064, diff(prinf-cout) -24525558
printf:200153338 cout:228610432, diff(prinf-cout) -28457094
printf:202794950 cout:223272108, diff(prinf-cout) -20477158
printf:209928648 cout:232300225, diff(prinf-cout) -22371577
printf:209722611 cout:233044140, diff(prinf-cout) -23321529
printf:248807680 cout:264406580, diff(prinf-cout) -15598900
printf:242916731 cout:246345645, diff(prinf-cout) -3428914
printf:221375038 cout:252251979, diff(prinf-cout) -30876941
printf:232892684 cout:273617728, diff(prinf-cout) -40725044
printf:238734317 cout:294238303, diff(prinf-cout) -55503986
printf:243180284 cout:284243039, diff(prinf-cout) -41062755
printf:245863056 cout:270413734, diff(prinf-cout) -24550678
printf:229300100 cout:259492696, diff(prinf-cout) -30192596
printf:227563904 cout:247802177, diff(prinf-cout) -20238273
printf:206847682 cout:230084237, diff(prinf-cout) -23236555
printf:198457595 cout:227655859, diff(prinf-cout) -29198264
printf:200265832 cout:224559564, diff(prinf-cout) -24293732
printf:201723563 cout:221764167, diff(prinf-cout) -20040604
printf:203430800 cout:221937208, diff(prinf-cout) -18506408
printf:198619127 cout:222313750, diff(prinf-cout) -23694623
printf:197686981 cout:233131170, diff(prinf-cout) -35444189
printf:199909645 cout:222764002, diff(prinf-cout) -22854357
printf:198050669 cout:221853069, diff(prinf-cout) -23802400
printf:200002449 cout:222743999, diff(prinf-cout) -22741550
printf:198573481 cout:221842796, diff(prinf-cout) -23269315
printf:200349434 cout:221501830, diff(prinf-cout) -21152396
printf:202039344 cout:238001170, diff(prinf-cout) -35961826
printf:209269614 cout:223050036, diff(prinf-cout) -13780422
printf:202794419 cout:219673749, diff(prinf-cout) -16879330
printf:223558422 cout:248919110, diff(prinf-cout) -25360688
printf:204139028 cout:231116364, diff(prinf-cout) -26977336
关闭 sync_with_stdio
printf:228312005 cout:203814832, diff(prinf-cout) 24497173
printf:197367087 cout:170933621, diff(prinf-cout) 26433466
printf:203728802 cout:177833838, diff(prinf-cout) 25894964
printf:205758316 cout:180175924, diff(prinf-cout) 25582392
printf:251267252 cout:218081227, diff(prinf-cout) 33186025
printf:232029455 cout:199609356, diff(prinf-cout) 32420099
printf:246392438 cout:217010908, diff(prinf-cout) 29381530
printf:241149944 cout:202337266, diff(prinf-cout) 38812678
printf:222466515 cout:195872690, diff(prinf-cout) 26593825
printf:221633699 cout:206460840, diff(prinf-cout) 15172859
printf:229958105 cout:200284936, diff(prinf-cout) 29673169
printf:227201946 cout:195657372, diff(prinf-cout) 31544574
printf:244297134 cout:219042472, diff(prinf-cout) 25254662
printf:251881002 cout:216385940, diff(prinf-cout) 35495062
printf:227101117 cout:205227849, diff(prinf-cout) 21873268
printf:203951925 cout:177981416, diff(prinf-cout) 25970509
printf:201715048 cout:178496585, diff(prinf-cout) 23218463
printf:197549273 cout:175482278, diff(prinf-cout) 22066995
printf:239984907 cout:203745670, diff(prinf-cout) 36239237
printf:205235507 cout:181894912, diff(prinf-cout) 23340595
printf:203233202 cout:178961277, diff(prinf-cout) 24271925
printf:206434353 cout:173567831, diff(prinf-cout) 32866522
printf:202660160 cout:176737473, diff(prinf-cout) 25922687
printf:201831315 cout:175735846, diff(prinf-cout) 26095469
printf:200164787 cout:175192017, diff(prinf-cout) 24972770
printf:202584304 cout:177037255, diff(prinf-cout) 25547049
printf:204783650 cout:177513872, diff(prinf-cout) 27269778
printf:241033670 cout:220023173, diff(prinf-cout) 21010497
printf:247117709 cout:189569108, diff(prinf-cout) 57548601
printf:248105314 cout:219065335, diff(prinf-cout) 29039979
printf:236633133 cout:212964896, diff(prinf-cout) 23668237
printf:223496747 cout:195898087, diff(prinf-cout) 27598660
printf:224936154 cout:199397373, diff(prinf-cout) 25538781
printf:234572132 cout:216805022, diff(prinf-cout) 17767110
printf:246738504 cout:216305235, diff(prinf-cout) 30433269
printf:250904282 cout:217312285, diff(prinf-cout) 33591997
printf:208721377 cout:181195577, diff(prinf-cout) 27525800
printf:236245042 cout:203088785, diff(prinf-cout) 33156257
printf:204058026 cout:179401451, diff(prinf-cout) 24656575
printf:201038295 cout:176858089, diff(prinf-cout) 24180206
printf:204590031 cout:186586920, diff(prinf-cout) 18003111
printf:200774025 cout:177748984, diff(prinf-cout) 23025041
printf:203636203 cout:179084720, diff(prinf-cout) 24551483
printf:202479901 cout:179167171, diff(prinf-cout) 23312730
printf:202329892 cout:180674842, diff(prinf-cout) 21655050
printf:201305734 cout:179149029, diff(prinf-cout) 22156705
printf:200157089 cout:178860913, diff(prinf-cout) 21296176
printf:202803518 cout:177353141, diff(prinf-cout) 25450377
printf:204768180 cout:175608340, diff(prinf-cout) 29159840
printf:237805444 cout:205725465, diff(prinf-cout) 32079979
如果将sync_with_stdio关闭,cout可以比printf更快,但我之前没有听说过sync_with_stdio。我不知道关闭它是否会有副作用。
上面的可变参数printf已包含在测试中。
void TestThree(){
ios_base::sync_with_stdio(false);
for(int i = 0; i < 1000000; i++){
safe_printf("This is the value: %d, %s\n", i, s.c_str());
}
}
再次打印仍然比关闭sync_with_stdio更快,但我不太理解。
printf:204099118 cout:684881656, diff(prinf-cout) -480782538
printf:227662054 cout:758110546, diff(prinf-cout) -530448492
printf:221534729 cout:732899502, diff(prinf-cout) -511364773
printf:203223553 cout:669386010, diff(prinf-cout) -466162457
printf:199923091 cout:662735504, diff(prinf-cout) -462812413
printf:207555026 cout:676522292, diff(prinf-cout) -468967266
printf:204846108 cout:655571171, diff(prinf-cout) -450725063
printf:199512948 cout:660717898, diff(prinf-cout) -461204950
printf:204251614 cout:676688426, diff(prinf-cout) -472436812
printf:201601655 cout:677217080, diff(prinf-cout) -475615425
printf:200311160 cout:674022044, diff(prinf-cout) -473710884
printf:202562272 cout:658126890, diff(prinf-cout) -455564618
printf:203100080 cout:660348312, diff(prinf-cout) -457248232
printf:228058969 cout:743305386, diff(prinf-cout) -515246417
printf:223976399 cout:742205352, diff(prinf-cout) -518228953
printf:204651603 cout:681893639, diff(prinf-cout) -477242036
printf:222386690 cout:745000014, diff(prinf-cout) -522613324
printf:217610811 cout:790129902, diff(prinf-cout) -572519091
printf:224156133 cout:758849874, diff(prinf-cout) -534693741
printf:204485998 cout:660430561, diff(prinf-cout) -455944563
printf:201259591 cout:667987480, diff(prinf-cout) -466727889
printf:204473955 cout:658096409, diff(prinf-cout) -453622454
printf:201986282 cout:670503647, diff(prinf-cout) -468517365
printf:204846379 cout:687089511, diff(prinf-cout) -482243132
printf:204962342 cout:681630774, diff(prinf-cout) -476668432
printf:202546744 cout:662300255, diff(prinf-cout) -459753511
printf:220856194 cout:757583221, diff(prinf-cout) -536727027
printf:232926622 cout:837024111, diff(prinf-cout) -604097489
printf:234187491 cout:801665699, diff(prinf-cout) -567478208
printf:244214945 cout:787558503, diff(prinf-cout) -543343558
printf:220227724 cout:788444837, diff(prinf-cout) -568217113
printf:227440252 cout:754596137, diff(prinf-cout) -527155885
printf:238158449 cout:814421312, diff(prinf-cout) -576262863
printf:205372147 cout:676782112, diff(prinf-cout) -471409965
printf:197566880 cout:662635516, diff(prinf-cout) -465068636
printf:199640024 cout:726088581, diff(prinf-cout) -526448557
printf:201249659 cout:660196078, diff(prinf-cout) -458946419
printf:201578923 cout:676444130, diff(prinf-cout) -474865207
printf:201034436 cout:690638993, diff(prinf-cout) -489604557
printf:208951246 cout:682032486, diff(prinf-cout) -473081240
printf:199928991 cout:673437083, diff(prinf-cout) -473508092
printf:233538795 cout:810890687, diff(prinf-cout) -577351892
printf:243872766 cout:847098807, diff(prinf-cout) -603226041
printf:243341748 cout:769877622, diff(prinf-cout) -526535874
printf:230232172 cout:733867319, diff(prinf-cout) -503635147
printf:220371109 cout:709704232, diff(prinf-cout) -489333123
printf:251803876 cout:790624221, diff(prinf-cout) -538820345
printf:202999188 cout:910149931, diff(prinf-cout) -707150743
printf:201931656 cout:669276274, diff(prinf-cout) -467344618
printf:200511370 cout:677748018, diff(prinf-cout) -477236648
std::cout
比printf
慢吗?我希望你有相关数据 ;) 上次我尝试时,例如std::cin
比scanf
快...不过需要注意的是ios_base::sync_with_stdio
等因素。 - J.N.