What Is Std::any In C++ 17 And How We Can Use It?
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
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
.
std::any <variable_definition>;
|
Here is a simple definition example.
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.
#include
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 #include #include #include #include
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
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
system(“pause”); return 0; }
|
Here is the output,.
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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