c++ conditional typedef

Explanation of conditional typedef in C++

In C++, a conditional typedef is a construct that allows the programmer to define a new type based on a condition. It is typically used to create aliases for types based on certain compile-time conditions.

The syntax for a conditional typedef in C++ looks like this:

typedef typename std::conditional<condition, type1, type2>::type type;

Let's break down the above syntax into its components:

1. std::conditional: This is a template class provided by the C++ Standard Library. It takes three template arguments: the condition, the type to be used if the condition is true, and the type to be used if the condition is false.

2. condition: This is a compile-time boolean expression that determines which type to select.

3. type1: This is the type that is selected if the condition evaluates to true.

4. type2: This is the type that is selected if the condition evaluates to false.

5. ::type: This is a nested typedef named type within the std::conditional template class. It represents the final selected type based on the condition.

Example

Here is an example that demonstrates the usage of a conditional typedef in C++:

#include <type_traits>

template<bool Condition>
struct MyType {
  typedef typename std::conditional<Condition, int, double>::type type;
};

int main() {
  MyType<true>::type var1;    // var1 has type int
  MyType<false>::type var2;   // var2 has type double
  return 0;
}

In this example, the MyType struct defines a nested typedef called type based on the condition provided as a template argument. When the condition is true, the typedef selects the type int, and when the condition is false, it selects the type double. The main function demonstrates the usage of MyType by declaring variables var1 and var2, which have types based on the condition.

This is a basic explanation and example of conditional typedef in C++. Let me know if you have any further questions.