how to check string start in C++

You can do this with string::compare(), which offers various options for comparing all or parts of two strings. This version compares smallString with the appropriate size prefix of bigString (and works correctly if bigString is shorter than smallString):

bigString.compare(0, smallString.length(), smallString) == 0

I tend to wrap this up in a free function called startsWith(), since otherwise it can look a bit mysterious.

UPDATE: C++20 is adding new starts_with and ends_with functions, so you will finally be able to write just bigString.starts_with(smallString).

The correct solution, as always, comes from Boost: boost::algorithm::starts_with.

std::string s("Hello world");

if (s.find("Hello") == 0)
{
    std::cout << "String starts with Hello\n";
}

The approaches using string::find() or string::substr() are not optimal since they either make a copy of your string, or search for more than matches at the beginning of the string. It might not be an issue in your case, but if it is you could use the std::equal algorithm. Remember to check that the "haystack" is at least as long as the "needle".

#include <string>    

using namespace std;

bool startsWith(const string& haystack, const string& needle) {
    return needle.length() <= haystack.length() 
        && equal(needle.begin(), needle.end(), haystack.begin());
}