C++ compare strings up to "%" char

Question

I would like to implement a string comparison in C++ comparing strings up to a "%" sign.

I could do it like this:

std::equal(str1.begin(), 
           std::find(str1.begin(), str1.end(), L'%'),
           str2.begin());

Since I'm doing this in a loop over many strings, I wonder if there is a method without two distinct string traversals for find and equal (maybe with a predicate that can abort the comparison at any point). Boost is ok.

Answer 1

You can try std::mismatch.
The following code will run with C++14 (it requires the template overload with two iterator pairs), but it works quite similar in C++11 (or 03, without lambdas though):

auto iters = std::mismatch( str1.begin(), str1.end(), str2.begin(), str2.end(), 
                     [] (char lhs, char rhs) {return lhs != '%' && lhs == rhs;});


if (iters.first  == str1.end()
 || iters.second == str2.end()
 || *iters.first == '%')
    // Success […]

Demo.

Answer 2

Here's a hackish way of doing it:

auto it = std::find_if(
  str1.begin(), str1.end(),
  [&str2](const char &c) {
    return c == '%' || str2[&c - &str1[0]] != c
  }
);
bool equal = (it == str1.end() || *it == '%');

The idea is to create a predicate "character is the same as in the other string and not a '%'."

It relies on str2 being long enough, but so does the code in the question.