A program using class template, pair, vector - class

Question

I'm trying to program the following:
A template class map having a pointer to a vector that contains elements std::pair<T,Q>, where T and Q are template types. It's supposed to work similarly to std::map and T is 'key' type, whereas Q stands for 'value' type. Besides the following should be implemented:
1. Constructor & destructor.
2. Function empty returning bool (if the object is empty).
3. Function size (using count_if)
4. Function clear that deletes all vector records.
5. Operator [] which allows for: map["PI_value"] = 3.14; It should use function find
6. Operators =, ==, !=, >> (using equal function)
I've been trying to code the above task, but have stuck on the code below.
Do you have any ideas to repair this mess?
#include <iostream>
#include <tuple>
#include <utility>
#include <algorithm>
#include <vector>
using namespace std;
template <typename T, typename Q>
class mapa
{
private:
vector<std::pair<T,Q>>* ptr;
public:
/**< DEFAULT CONSTRUCTOR/////////////////////////// */
mapa()
{
ptr = new vector<std::pair<T,Q>>;
ptr->push_back(std::pair<T,Q>(0,0));
}
/**< DESTRUCTOR////////////////////////////////////// */
~mapa(){ delete ptr;}
/**< EMPTY()////////////////////////////// */
bool empty()
{
if(ptr)
return false;
else
return true;
}
/**< SIZE()///////////////////////////////// */
int size()
{
return ptr->size();
}
/**< CLEAR()///////////////////////////////// */
void clear()
{
ptr->clear(ptr->begin(), ptr->end());
}
/**< OPERATOR[]/////////////////////////////////////////// */
vector<std::pair<T,Q>>* & operator[](T key)
{
auto ptr2 = ptr;
if(empty())
{
std::pair<T,Q> para;
para.first = key;
para.second = 0;
ptr2->push_back(para);
//ptr2->push_back(std::pair<T,Q>(key,0));
}
else
{
auto ptr2 = find_if( ptr->begin(), ptr->end(),
[](std::pair<T,Q> example,T key)
{
return(example.first==key);
}
);
}
return ptr2;
}
}; //class end

Answer 1

The lambda provided to std::find_if is declared wrong.
If you see e.g. this reference for std::find_if, you will see that the functions should be like
bool pred(const Type &a)
That means the lambda should be something like
[&key](const std:pair<T, Q>& element) { return element.first == key }
There are also other problems with your operator[] function, like that it should return Q& instead of a reference to the vector pointer. You should also remember that std::find_if returns an iterator to the found element, or end() if not found.