Check if one string is a prefix of another
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.
}

bool isPrefix(string const& s1, string const&s2)
{
    const char*p = s1.c_str();
    const char*q = s2.c_str();
    while (*p&&*q)
        if (*p++!=*q++)
            return false;
    return true;
}

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() );
}

// Yields true if the string 's' starts with the string 't'.
bool startsWith( const std::string &s, const std::string &t )
{
    return strncmp( s.c_str(), t.c_str(), t.size() ) == 0;
}

// Yields true if the string 's' starts with the string 't'.
template <class T>
bool startsWith( const T &s, const T &t )
{
    return std::equal( t.begin(), t.end(), s.begin() );
}

#include <string>
#include <iostream>

// does str1 have str2 as prefix?
bool StartsWith(const std::string& str1, const std::string& str2)
{
    return (str1.find(str2)) ? false : true;
}

// is one of the strings prefix of the another?
bool IsOnePrefixOfAnother(const std::string& str1, const std::string& str2)
{
    return (str1.find(str2) && str2.find(str1)) ? false : true;
}

int main()
{
    std::string str1("String");
    std::string str2("String:");
    std::string str3("OtherString");

    if(StartsWith(str2, str1))
    {
        std::cout << "str2 starts with str1" << std::endl;
    }
    else
    {
        std::cout << "str2 does not start with str1" << std::endl;
    }

    if(StartsWith(str3, str1))
    {
        std::cout << "str3 starts with str1" << std::endl;
    }
    else
    {
        std::cout << "str3 does not start with str1" << std::endl;
    }

        if(IsOnePrefixOfAnother(str2, str1))
        {
            std::cout << "one is prefix of another" << std::endl;
        }
        else
        {
            std::cout << "one is not prefix of another" << std::endl;
        }

        if(IsOnePrefixOfAnother(str3, str1))
        {
            std::cout << "one is prefix of another" << std::endl;
        }
        else
        {
            std::cout << "one is not prefix of another" << std::endl;
        }

    return 0;
}

str2 starts with str1
  str3 does not start with str1
  one is prefix of another
  one is not prefix of another

string a = "String";
string b = "String:";

if(b.find(a) == 0)
{
// Prefix

}
else
{
// No Prefix
}

bool prefix(const std::string& a, const std::string& b) {
  if (a.size() > b.size()) {
    return a.substr(0,b.size()) == b;
  }
  else {
    return b.substr(0,a.size()) == a;
  }
}

#include <string>
#include <iostream>

bool prefix(const std::string& a, const std::string& b);

int main() {
  const std::string t1 = "test";
  const std::string t2 = "testing";
  const std::string t3 = "hello";
  const std::string t4 = "hello world";
  std::cout << prefix(t1,t2) << "," << prefix(t2,t1) << std::endl;
  std::cout << prefix(t3,t4) << "," << prefix(t4,t3) << std::endl;
  std::cout << prefix(t1,t4) << "," << prefix(t4,t1) << std::endl;
  std::cout << prefix(t1,t3) << "," << prefix(t3,t1) << std::endl;

}