Create Simple DLL loadable by MatLab

Creating DLL

Create a new project in visual studio community 2017
File -> New -> Project

On New Project Dialog
Installed -> Visual C++ -> Windows Desktop -> Windows Desktop Wizard

On Wizard Dialog
Application type -> Dynamic Link Library (.dll)
Additional Info:
  check "Empty Project"
  uncheck "Precompiled Header"

Add Files to project
Project -> Add New Item -> .cpp/.h ... etc

Set x64 or Win32 Platform as Active Configuration
Solution Explorer -> right click project -> Properties -> Configuration Manager

Code
simple.c

#define EXPORT_FCNS
#include "helper.h"
#include "simple.h"

int sq(int x)
{
 return (x*x);
}

simple.h

#ifndef SIMPLEH_H
#define SIMPLEH_H

#include "helper.h"

#ifdef  __cplusplus
extern "C" {
#endif

 EXPORTED_FUNCTION int sq(int x);

#ifdef  __cplusplus
}
#endif

#endif

helper.h

#ifndef HELPER_H
#define HELPER_H

#ifdef _WIN32
#ifdef EXPORT_FCNS
#define EXPORTED_FUNCTION __declspec(dllexport)
#else
#define EXPORTED_FUNCTION __declspec(dllimport)
#endif
#else
#define EXPORTED_FUNCTION
#endif

#endif

Loading DLL on Matlab

Setup compiler

on command window

mex -setup

Script

loadlibrary('simple')
libfunctions simple
calllib('simple','sq', 3)

Reference 

https://stackoverflow.com/questions/18413118/visual-studio-dll-and-matlab