C++17 standard is amazing with a lot of new features, and one of the interesting features was the new type std::any. std::any is a type-safe container to store a single value of any variable type. In this post, we explain std::any in modern C++.
What is std::any in C++ 17 ?
The any class (std::any) is a new class defined in the <any> header in C++17 and it is used for any type definition, it is a safe type container for single values that are copy constructible. The std::any is a container type that is used to store any value in it without worrying about the type safety of the variable. It has been designed based on boost::any from the boost library. It is very useful, when you have a variable, and you want to change its type (int to float) on runtime.
Here is the simplified syntax for std::any.
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std::any <variable_definition>; |
Here is a simple definition example.
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std::any a; |
The std::any is a type-safe container that has properties such as has_value(), type(), type().name(); it has modifiers such as emplace, reset, swap; it has bad_any_cast helper class, and it can be used with other methods such as make_any, any_cast, std::swap.
How can we use std::any in C++ 17?
Here is a simple example how we can use the std::any with different types in C++17 and beyond.
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#include <any> int main() { std::any a; a = true; // boolean a = 100; // integer a = 9.81; // double } |
In some definitions, we can use literals to define type of the variable, let’s see example below.
Is there a full example about how can we use std::any in C++ 17?
Here is a full example about std::any that shows different any definitions.
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#include <iostream> #include <string> #include <bitset> #include <vector> #include <any> using namespace std::literals; int main() { std::any a; std::cout << "it is " << a.type().name() << std::endl; a = true; // boolean std::cout << std::boolalpha << std::any_cast<bool>(a) << " is " << a.type().name() << std::endl; a = 100; // integer std::cout << std::any_cast<int>(a) << " is " << a.type().name() << std::endl; a = 9.81; //double std::cout << std::any_cast<double>(a) << " is " << a.type().name() << std::endl; //std::cout << std::any_cast<float>(a) << a.type().name() << std::endl; std::any a0; // empty std::any a1{}; // empty std::any a2 = {}; // empty std::any x0{ 77 }; // int std::any x1 = 4.5; // double std::any x2{ "LearnCPlusPlus.org" }; // const char * std::any x3{ "This is a string"s }; // std::string std::any x4{ std::bitset<8>{} }; //std::bitset<8> std::any x5{ std::vector<double>{9.2, 8.1, 7.3} }; //std::vector<double> system("pause"); return 0; } |
Here is the output,.
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it is v true is b 100 is i 9.81 is d |
v shows it is any variable (empty),
b shows it is a boolean,
i shows it is an integer
d shows it is a double;
We should note that, in this example, the output could be different in different compilers in different operating systems.
For more details about this feature in C++17 standard, please see these papers; P0220R1, P0032R3, P0504R0
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