How to make a conditional typedef in C++

Question

I am trying to do something like this:

#include 
#include 

typedef int Integer;

#if sizeof(Integer) <= 4
    typedef std::         


        

Answer 1

Use the std::conditional meta-function from C++11.

#include   //include this

typedef std::conditional::type Engine;

Note that if the type which you use in sizeof is a template parameter, say T, then you have to use typename as:

typedef typename std::conditional::type Engine;

Or make Engine depend on T as:

template
using Engine = typename std::conditional::type;

That is flexible, because now you can use it as:

Engine  engine1;
Engine engine2;
Engine    engine3; // where T could be template parameter!