CPP

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

c++ define function in header

#ifndef FUNCTIONS_H_INCLUDED
#define FUNCTIONS_H_INCLUDED

int add(int a, int b)
{
   return a + b;
}

#endif

how to declare a function in c++ header file

//somefile.h
#ifndef SOMEFILE_H
#define SOMEFILE_H
int add(int a, int b);
#endif