bool in_string_permutation(string &s1, string &s2) {
	std::unordered_map<int,int> ht;
	auto s_it = s2.begin();
	bool check = true;

	if(s1.length() != s2.length()) {
		return false;
	}

	for_each(s1.begin(), s1.end(), [&](char &s)
	{ 
		// inc count
		auto it = ht.find(s);
		if (it != ht.end()) {
			ht[s]++;
		}
		else {
			ht.insert(std::make_pair(s, 1));
		}

		// dec count
		auto it2 = ht.find(*s_it);
		if (it2 != ht.end()) {
			ht[*s_it]--;
		}
		else {
			ht.insert(std::make_pair(*s_it, -1));
		}

		s_it++;
	});

	for_each(ht.begin(), ht.end(), [&](std::pair<const int, int> &p)
	{ 
		if(p.second != 0) {
			check = false;
		}
	});

	return check;
}

bool is_string_permutation(string s1, string s2){
    //maybe do a length check first
    sort(s1.begin(), s1.end());
    sort(s2.begin(), s2.end());
    return s1 == s2;
}

Different approach:

bool is_string_permutation(const string s1, const string s2){
    //maybe do a length check first
    map<char, size_t> m1;
    for (const auto &c : s1)
        m1[c]++;
    map<char, size_t> m2;
    for (const auto &c : s2)
        m2[c]++;
    return m1 == m2;
}

//dammit, too late