c++ should i write header files before cpp files code example

Example 1: c++ header files

// You should use header files when you wan't to split your
// program across multiple files. Use it like this:

// vec2.hpp
class vec2 {
	public:
  		void printVec(); // Decleration
  		float x, y;
}
// vec2.cpp
#include "vec2.hpp"
void vec2::printVec() { // Implementation
	std::cout << x << y << std::endl;
}

Example 2: c++ header files example

//headers 
#ifndef TOOLS_hpp 
#define TOOLS_hpp
#include<vector> 
#include<iostream>
#include<fstream>
#include<string>
using std::ifstream;
using std::cout; 
using std::cin;
using std::endl;
using std::cerr;
using std::vector;
using std::string;
// functions prototypes 
inline vector<int> merge(const vector<int>&a,const vector<int>& b);//merge function prototype with Formal Parameters 
std::vector<int> sort(size_t start, size_t length, const std::vector<int>& vec);//sort function prototype with formal parameters 
#endif