Check if one string is a prefix of another

This is both efficient and convenient:

str.compare(0, pre.size(), pre) == 0

compare is fast because it uses the fast traits::compare method and doesn't have to copy any data.

Here, it will compare std::min(str.size(), pre.size()) characters but if the characters in the two ranges are equal it also checks the length of pre and returns a non-zero value if pre is longer than this.

See the documentation at cplusplus.com.

I've written a test program that uses this code to compare prefixes and strings given on the command line.

If you know which string is shorter, the procedure is simple, just use std::equal with the shorter string first. If you don't, something like the following should work:

bool
unorderIsPrefix( std::string const& lhs, std::string const& rhs )
{
    return std::equal(
        lhs.begin(),
        lhs.begin() + std::min( lhs.size(), rhs.size() ),
        rhs.begin() );
}

std::string(X).find(Y) is zero if and only if Y is a prefix of X

Use std::mismatch. Pass in the shorter string as the first iterator range and the longer as the second iterator range. The return is a pair of iterators, the first is the iterator in the first range and the second, in the second rage. If the first is end of the first range, then you know the the short string is the prefix of the longer string e.g.

std::string foo("foo");
std::string foobar("foobar");

auto res = std::mismatch(foo.begin(), foo.end(), foobar.begin());

if (res.first == foo.end())
{
  // foo is a prefix of foobar.
}