What Are Integral_constant And () Operator In C++?

Modern C++ has base class features that can be used with other modern features of C++. The std::integral_constant is the base class for the C++ type traits in C++11, and in C++14, std::integral_constant gained an operator () overload to return the constant value. In this post, we explain what integral_constant and () operator are in C++14.

What is integral_constant in C++?

The std::integral_constant  is the base class for the C++ type traits in <type_traits> header that wraps a static constant of specified type. The behavior in a code part that adds specializations for std::integral_constant is undefined.

Here is the definition in <type_traits> header since C++11,

Here is a very simple example to how can we use std::integral_constant, in C++11 we can use ::value to retrieve its value,

What is the integral_constant () operator in C++?

In C++ 14, std::integral_constant gained an operator() overload to return the constant value that can be used with some other useful modern features like templates, lambdas, etc. In C++14, std::integral_constant gained an operator() overload to return the constant value. This () operator enables std::integral_constant to serve as a source of compile-time function objects.

In C++14, operator() is defined as shown below.

Here is a simple example how we can use () operator in C++14 and above.

Is there a full example of how to use integral_constant in C++?

Here is a full example about integral_constant in modern C++.

and the output will be as below:

Is there a full example of how to use the integral_constant () operator in C++?

Here is an example of the () operator of integral_constant, we define a myfactorial template struct that returns factorial value in compilation, which means there is no calculation about this operator on run time.

and the output will be as we show below.

For more details about this feature, please see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3545.pdf

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