std::string comparison (check whether string begins with another string)

I need to check whether an std:string begins with "xyz". How do I do it without searching through the whole string or creating temporary strings with substr().

I would use compare method:

std::string s("xyzblahblah");
std::string t("xyz")

if (s.compare(0, t.length(), t) == 0)
{
// ok
}

An approach that might be more in keeping with the spirit of the Standard Library would be to define your own begins_with algorithm.

#include <algorithm>
using namespace std;


template<class TContainer>
bool begins_with(const TContainer& input, const TContainer& match)
{
	return input.size() >= match.size()
		&& equal(match.begin(), match.end(), input.begin());
}

This provides a simpler interface to client code and is compatible with most Standard Library containers.

Look to the Boost's String Algo library, that has a number of useful functions, such as starts_with, istart_with (case insensitive), etc. If you want to use only part of boost libraries in your project, then you can use bcp utility to copy only needed files

It seems that std::string::starts_with is inside C++20, meanwhile std::string::find can be used

std::string s1("xyzblahblah");
std::string s2("xyz")

if (s1.find(s2) == 0)
{
   // ok, s1 starts with s2
}

I feel I'm not fully understanding your question. It looks as though it should be trivial:

s[0]=='x' && s[1]=='y' && s[2]=='z'

This only looks at (at most) the first three characters. The generalisation for a string which is unknown at compile time would require you to replace the above with a loop:

// look for t at the start of s
for (int i=0; i<s.length(); i++)
{
  if (s[i]!=t[i])
    return false;
}