A function for find if a string is a substring of another string

gistfile1.cpp

#include <iostream>
#include <string>

bool is_substring_of(const std::string& substr, const std::string& str)
{
    auto it = substr.begin();
                          
    for (auto const& c : str)
    {
        if (c == *it)
            ++it;
        else
            it = substr.begin();
            
        if (it == substr.end())
            return true;
    }
    return false;
}

int main()
{
    std::string apple = "apple";
    std::string sause = "myapplesause";
    
    std::cout << std::boolalpha << is_substring_of(apple, sause);
}