Make C++ std::string end at specified character?

Let's say I am traversing a string of length n. I want it to end at a specific character that fulfils some conditions. I know that C style strings can be terminated at the i'th position by simply assigning the character '\0' at position i in the character array.

Is there any way to achieve the same result in an std::string (C++ style string)? I can think of substr, erase, etc. but all of them are linear in their complexity, which I cannot afford to use.

TL;DR, is there any "end" character for an std::string? Can I make the end iterator point to the current character somehow?