C++ for_each loop can't find pointed to map element - c++

Question

I'm building a huffman coding program and when I try and run the code I have so far it just gives me a warning that the freq map object .begin() doesn't exist.
Huff.h
#ifndef HuffPuff_Huff_h
#define HuffPuff_Huff_h
//---Include---
#include <iostream>
#include <vector>
#include <set>
using namespace std;
//---Node---
struct Node {
int weight;
char litteral;
string symbol;
Node* childL;
Node* childR;
void set_node(int w, char l, Node* L, Node* R){
weight = w;
litteral = l;
childL = L;
childR = R;
}
bool operator>(Node & r){
if(this->weight > r.weight)
return true;
return false;
}
};
//---Code---
struct Code {
string symbol;
char content;
};
//---HuffClass---
class Huff {
private:
typedef pair<char, int> c_pair;
vector<Code> code;
string content;
void copy_to(c_pair c);
public:
Huff(string);
~Huff();
string compress();
bool set_content();
string get_content();
string get_compress();
};
#endif
Huff.cpp
//---Include---
#include <iostream>
#include <vector>
#include "Huff.h"
#include <map>
#include <set>
using namespace std;
//---|+ -|---
Huff::Huff(string c): content(c){}
Huff::~Huff(){}
//---Compress---
struct CopyTo {
vector<Node*>* & nodes;
CopyTo(vector<Node*>* & c):nodes(c){}
void operator()(pair<char, int> c){
Node * n = new Node;
n->set_node(c.second, c.first, NULL, NULL);
nodes->push_back(n);
}
};
void show_freq(pair<char, int> p) {
cout << p.first << "\t" << p.second << endl;
}
/*void show_freq(Node* p) {
cout << p->litteral << "\t" << p->weight << endl;
}*/
string Huff::compress(){
vector<Node *>* nodes; // Vector of nodes for later use
map<char, int>* freq = new map<char, int>; // Map to find weight of nodes
for(int i = 0; i < content.length(); i++)
(*freq)[content[i]]++;
for_each(freq->begin(), freq->end(), show_freq);
CopyTo copyto(nodes); //Copy map elements to nodes in this and next one
for_each(freq->begin(), freq->end(), copyto);
delete freq;
Node p;
while(nodes->size() != 1){ //Sorts nodes by weight and then removes two of them and replaces them with one
sort(nodes->begin(), nodes->end());
vector<Node *>::iterator beg = nodes->begin();
int w= (**beg).weight + (**beg++).weight;
Node* p = new Node;
p->set_node(w, '*', *nodes->begin(), *(nodes->begin()++));
nodes->erase(nodes->begin(), nodes->begin()+2);
nodes->push_back(p);
//for_each(nodes->begin(), nodes->end(), show_freq);
cout << "--------------" << endl;
}
Node* root = *nodes->begin();
return "110";
}
Main.cpp
int main(){
Huff mike("Testing-");
mike.compress();
}

Answer 1

Where is including of algorithm header?
online compiler
results
Compilation output:
source.cpp: In member function 'std::string Huff::compress()':
source.cpp:76:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
source.cpp:94:11: warning: unused variable 'root' [-Wunused-variable]
Execution output:
- 1
T 1
e 1
g 1
i 1
n 1
s 1
t 1