Barry Allen keeps his secret identity as The Flash a closely guarded secret, but how quickly did he reveal this secret to his girlfriend, Iris West? From Barry's first appearance in Showcase #4, Barry was engaged to Iris West, a news reporter who wished that Barry would arrive to their dates on time. After gaining his powers and becoming the Flash however, Barry would become the fastest man alive, and fight numerous villains in Central City, such as Captain Cold, Mirror Master, Pied Piper, and the Reverse Flash.
Because of the life-threatening adventures Barry would embark on, he chose to keep his identity a secret from Iris, worried that she would leave him because of his lifestyle. In the early flash stories, the only two heroes who knew Flash's secret identity were Wally West, Iris's nephew who took the mantle Kid Flash, and Hal Jordan, the Green Lantern of Earth. Though Barry was torn over his decision to keep his dual-life a secret from Iris, one early Flash adventure would take the choice out of Barry's hands.
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In The Flash #156, Barry gets his secret identity outed by an alien species from the planet Gyr. Two factions of an alien species from the planet Gyr were waging war with one another, and the Tyrannical forces of Gyr, the Drdons, were losing. Scientists on Gyr discovered a means of time-travel, that would allow them to defeat the "new popular government" sworn to defeat the tyrants once and for all. Despite lacking the super-speed necessary to power this means of time-travel, the Drdons learned of the Flash and his fantastic powers, and planned to capture him to exploit his speed for their purposes. The Drdons then sent forth an emissary from Gyr, and demanded that Flash surrender himself to their planet, and even outed his secret identity to ensure that he could not go into hiding. Though Flash was able to defeat the tyrants, his identity was still known on Earth, with Iris being hurt that Barry did not trust her enough to share that part of his life with her. Barry told Iris that he would have to utilize time travel to wipe the earth's memory of his identity completely, and promised that he would share his secret identity with her again on their wedding day. In the aftermath of this adventure, Iris forgot Barry's identity, though it would be awhile before Barry revealed himself to his fiancée proper.
In The Flash #165, Barry and Iris were finally set to get married after years of engagement. Barry, the whole day leading up to the wedding was contemplating whether or not he should reveal his identity to Iris (despite having promised to do so). After nearly having his wedding sabotaged by Reverse-Flash, who attempted to steal Barry's life, Barry opted to not tell Iris his secret after all, and even asked the reader at the end of the issue whether he should tell his new wife about his alter-ego. For months afterwards, Barry would contemplate the issue further, until finally Jay Garrick, the original Flash, and his wife Joan from Earth 2 got involved. In Flash #173, Jay and Joan discuss the issue of Iris not knowing Barry's secret, with Joan feeling that Barry's dishonesty would hurt Iris in the long run, noting how Jay shared his secret with Joan long ago. The two travel to Earth 1 to discuss the issue with Barry and, after an adventure involving Jay, Barry, and Wally, Barry agrees to tell Iris on their one year wedding anniversary.
In Flash #174, Barry finally decides that it would be an appropriate time to tell Iris his secret, once and for all. After a long day fighting the rogues, Barry and Iris meet back together to celebrate their anniversary, when Iris suddenly reveals that she had known Barry's secret since their wedding night. As it turned out, the night of their wedding, Iris had heard Barry talk in his sleep, and he inadvertently revealed his identity to Iris in his sleep mutterings. Though Iris had known about Barry's secret for a year, she decided that, rather than shock or embarrass Barry, she would wait until he was comfortable revealing his secret to reveal this information. Though Iris may have been upset, she understood Barry's reasons for keeping his secret, but with Flash's identity out in the open, the two became closer than ever.
It's understandable that super-heroes like the Flash would keep their identity secret from the world at large, but keeping it a secret from your loved ones isn't always a great idea. On one hand, if a hero chooses to keep the truth from their friends or family, then they wouldn't have to burden their loved ones with the reality that, as a hero, they would be in constant danger, and might not return home one day. On the other hand, if a hero makes the choice to reveal the truth to their closest friends and family, they could better inform their loved ones about their double life and, in case a villain already knows the truth, protect them more efficiently.
While Iris ultimately accepted Barry's reasoning for keeping his secret, the first time she learned the truth she was more hurt that Barry did not trust her, rather than worried about his safety. Thankfully though, The Flash did not come between Barry and Iris, and the two remain one of the greatest couples in comic book history.