用于字符串比较和替换的C ++优化

I am trying to optimize this function I have written that compares two strings, then replaces the characters in the first string if they are not found in the second. Do you think, for example converting the string to a vector of chars when changing to uppercase would yield a performance boost? I can't see many ways around the 2 for loops however. Any general tips would be appreciated! I am trying to optimize this function I have wr