std::common_type<T1, T2>::type function in C++

In this article, we will discuss the std::common_type<T1, T2>::type function in C++ with its syntax, parameters, key concepts, and example.

What is the std::common_type<T1, T2>::type function in C++?

In C++, the common type among a list of types is identified via the std::common_type trait. It performs this by determining the type to which all of the given types can be implicitly transformed (T...). This capability is especially helpful when we are working with templates or generic code and when operations involving many types must yield a consistent and legitimate result type.

Syntax:

It has the following syntax:

Parameters:

1. Std::common_type: The C++ Standard Library provides this template class in the header. It establishes the common type of one or more template parameters (types) that are accepted.
2. Type: The type is a type member or type alias in the class template std::common_type. It stands for the common type that T1 and T2's common_type trait inferred.
3. Typename: The typename keyword is necessary to specify that the ::type is a dependent name, which means that it is reliant on the template parameters T1 and T2 when accessing a type member (:: type) of a template class. This keyword is required because the compiler cannot determine whether the ::type is a type or something else (such as a static member) until the template is instantiated with specific types.

Key Concepts of std::common_type:

Several key concepts of the std::common_type function are as follows:

1. Implicit Conversion: The type to which all types in the pack T... can be implicitly converted is known as the common type. Because all operands in expressions like arithmetic operations must have compatible types, this conversion ensures that the types can coexist.
2. Custom Type Specialization: By allowing developers to specify the interactions between types in their own libraries or classes, the std::common_type trait can be customized for custom types. It is possible to specialize std::chrono::time_point, which represents a point in time, so that operations between various time points or durations provide a clearly defined type.
3. Application in Arithmetic Expressions: The result type of an expression, such as T0() + T1() +... + Tn(), can be thought as the common type for arithmetic types. In mixed-mode arithmetic, where calculations may use many kinds like integers, floating-point numbers, or even bespoke numeric types, this is essential. Given the promotion and conversion restrictions of C++, the result type must be able to handle all operand types.
4. For example, adding an int and a double would result in a double because the integer is promoted to a floating-point type in mixed arithmetic expressions.
5. General Use in Generic Code and Templates: To determine the results type of operations carried out on several template parameters, generic programming frequently uses std::common_type. For example, std::common_type can be used to identify the return type that is safe and compatible with all operations on the types that are accepted by the function you are developing.
6. Rules Based on Ternary Operator (?:): Similar guidelines apply to std::common_type for non-specialized types because they perform to the ternary conditional operator (?:). The type that would come from the expression is specifically used to identify the common type between types T1 and T2.
7. cond? T1(): T2();
8. For the ternary operator, the common type is determined by the rules governing implicit conversions for the second and third operands. It ensures the ability to blend types in the expression without the need for explicit casting.

Example:

Let us take an example to illustrate the std::common_type function in C++.

Output:

 
The result is: 13.14   

Explanation:

The template function added is defined by the provided C++ code that accepts two parameters of possibly different types (T1 and T2). For the addition operation, it leverages std::common_type to infer a common return type, guaranteeing that the function may handle various types with ease. An integer p and a double q are initialized with values of 10 and 3.14 in the main function. The implicit conversion rules of arithmetic operations cause the result to be stored in ans, which is of type double after the add function is invoked with p and q. The result of the addition, 13.14, is the last output of the function.