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c++msys

find_first_of with multiple characters doesn't return string::npos when substring shouldn't be present

I've written a small snippet like this:

#include <string>
#include <iostream>

int main() {
    std::string s1{"abcdefghijklmnop"};

    std::cout << "s1(fg) = " << s1.find_first_of("fg") << '\n';
    std::cout << "s1(fo) = " << s1.find_first_of("fo") << '\n';
}

I expect that it returns 5 and string::npos, but it actually returns

s1(fg) = 5
s1(fo) = 5

Why does it work like this?

My compile flags and g++ version:

$ g++ -std=c++11 main.cpp

$ g++ --version
g++.exe (Rev1, Built by MSYS2 project) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Solution

  • std::string::find_first_of returns the position of any of the characters you supply to it. In your case, it finds f both times.

    You should be using std::string::find.