CODEWITHSUNDEEP
c++scope
db1234
db1234

Reputation: 807

Defining variable type based on a boolean in c++

Depending on a boolean variable myBool I would like to declare a variable mVect as a 1D or a 2D vector, i.e.:

  if (myBool){
       vector < vector< float > > myVect;
   }
   else{
       vector <float> myVect;
   }

  ///// later.....//////

 //call many instances of overloaded myFn( vector<float>), 
 //or  myFn ( vector<vector<float>)

   //// more stuff.....//
  // call more instances of myFn

I'd like the variables to have scope beyond the if statement. Using a ternary operator as in (Expand scope of a variable initialized in a if/else sequence) does not seem applicable since the variable types are different.

My motivation for this is that I have an overloaded function myFn, one version inputs a vector, another puts in a vector<vector>. I'd prefer to not put an if statement in myFn to distinguish the cases for performance reasons -- I'd like to call a single if statement when declaring and not call a trillion if statements when repeatedly calling myFn

edit:
This is for a physical simulation. I would like to keep the ability to do a 1D or a 2D calculation and myFn does a 1D or a 2D interpolation respectively. I don't want to modify any of the code-base beyond different initializing of variables and I thus want to keep the variable names the same so I can call the same overloaded interpolator, nor do I want an if statement in the interpolator

Upvotes: 1

Views: 769

Answers (2)

Slava
Slava

Reputation: 44248

One of the solution - OO. You create a base class with interface that used in your code and interpolator and then inherit 1d or 2d from it based on condition:

class MyVector {
public:
   virtual ~MyVector();
...
};

class MyVector1D : public MyVector {
};

class MyVector2D : public MyVector {
};

// usage
std::unique_ptr<MyVector> vector = myBool ? std::make_unique<MyVector1D>() 
                                          : std::make_unique<MyVector2D>();

myFn( vector );

Or you can use boost::variant with either visitor pattern or conditional code where that vector is used.

Upvotes: 0

Alexander Balabin
Alexander Balabin

Reputation: 2075

If I understand your question correctly you want a local variable to have different types based on runtime value of a boolean.

This is impossible to acheve in C++. The two options you have are:

1) If the boolean is const (or comes from a constexpr) you can convert you code into a template and use std::conditional to pick the right type.

2) Do all the work inside the respective scopes of your if, then the variables are different and no problem with them having different types:

  if (myBool){
       vector < vector< float > > myVect;
       ...
       myFn(myVect);
   }
   else{
       vector <float> myVect;
       ...
       myFn(myVect);
   }

Upvotes: 1

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