What is type deduction in C++?

Type inference (or type deduction) refers that the compiler can figure out the datatype of a variable or expression automatically, without the programmer needing to write it out.

This feature is available in many strongly typed languages like C++, where the type system is strict but the compiler helps to reduce the typing effort.

Following is the simple example to understand the type deduction in C++.

#include<iostream>
using namespace std;
int main() {
   // Let the compiler deduce the type from the value
   auto number = 50;     // int
   auto pi = 3.14;       // double
   cout<<"number = "<<number<<endl;
   cout<<"pi = "<<pi<<endl;
   return 0;
}

Following is the output to the above program:

number = 50
pi = 3.14

In C++, we use the type deduction with two keywords: "auto" and "decltype".

Type Deduction using auto Keyword

The auto keyword is used for automatic type deduction. For example, you want to create an iterator to iterate over a vector, you can simply use auto for that purpose.

Syntax of the auto keyword

Following is the syntax to deduce the auto keyword and the compiler deduces the type of variable_name from the value.

auto variable_name = value;

Example

This program uses 'auto' keyword to automatically deduce the type of the vector iterator and prints the first element of the vector using that iterator.

#include<iostream>
#include<vector>
using namespace std;
int main() {
   vector<int> v = {10, 20, 30, 40};
   // Using 'auto' to simplify iterator declaration
   auto it = v.begin();  // Type deduced as vector<int>::iterator
   cout<<"First element in the vector: "<<*it<<endl;
   return 0;
}

Following is the output to the above program:

First element in the vector: 10

Type Deduction using decltype Keyword

The decltype keyword is used to get the type of a variable or expression without actually using or changing the value. For example, if you have a variable int x = 5;, then writing decltype(x) y; will declare y as an int, the same type as x.

You can say that "the same deduced type as x" for decltype to clarify that decltype doesn't deduce the value, only the type.

Syntax of the decltype keyword

Following is the syntax to deduce the decltype keyword and the compiler deduces the type of variable_name from the type of the expression, without evaluating it.

decltype(expression) variable_name;

Example

This program uses 'decltype' keyword to create variables with the same type as existing variables or expressions, and prints their values.

#include<iostream>
using namespace std;
int main() {
   int a = 5;
   float b = 6.5;
   // Use decltype to get the type of a
   decltype(a) x = 100;         // x is int
   decltype(a + b) result = a + b; // result is float (a + b)
   cout<<"x = "<<x<<endl;
   cout<<"result = "<<result<<endl;
   return 0;
}

Following is the output to the above program:

x = 100
result = 11.5