Why does `std::string::find()` not return the end iterator on failures?

user_1547699 0
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I find the behaviour of std::string::find to be inconsistent with standard C++ containers.

E.g.

std::map<int, int> myMap = {{1, 2}};
auto it = myMap.find(10);  // it == myMap.end()

But for a string,

std::string myStr = "hello";
auto it = myStr.find('!');  // it == std::string::npos

Why shouldn't the failed myStr.find('!') return myStr.end() instead of std::string::npos?

Since the std::string is somewhat special when compared with other containers, I am wondering whether there is some real reason behind this. (Surprisingly, I couldn't find anyone questioning this anywhere).

stdstring c++-standard-library c++
2 Answers

To begin with, the std::string interface is well known to be bloated and inconsistent, see Herb Sutter's Gotw84 on this topic. But nevertheless, there is a reasoning behind std::string::find returning an index: std::string::substr. This convenience member function operates on indices, e.g.

const std::string src = "abcdefghijk";

std::cout << src.substr(2, 5) << "\n";

You could implement substr such that it accepts iterators into the string, but then we wouldn't need to wait long for loud complaints that std::string is unusable and counterintuitive. So given that std::string::substr accepts indices, how would you find the index of the first occurence of 'd' in the above input string in order to print out everything starting from this substring?

const auto it = src.find('d'); // imagine this returns an iterator

std::cout << src.substr(std::distance(src.cbegin(), it));

This might also not be what you want. Hence we can let std::string::find return an index, and here we are:

const std::string extracted = src.substr(src.find('d'));

If you want to work with iterators, use <algorithm>. They allow you to the above as

auto it = std::find(src.cbegin(), src.cend(), 'd');

std::copy(it, src.cend(), std::ostream_iterator<char>(std::cout));
user_95935960

This is because std::string have two interfaces:

  • The general iterator based interface found on all containers
  • The std::string specific index based interface

std::string::find is part of the index based interface, and therefore returns indices.

Use std::find to use the general iterator based interface.

Use std::vector<char> if you don't want the index based interface (don't do this).

user_22617170
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