In C++ Builder 12, and modern C++ the std::variant is one of the powerful features that comes with C++17. The std::variant is a discriminated union that we can work with multiple data types. It represents a type-safe union and holds one of its types in definition.
What is the class template std::variant in C++ 17?
The std::variant is a class template defined in <variant> header that represents a disjoint union (or discriminated union). A value of variant<A, B, C> contains one of an A, a B, or a C at any one time. It can be used as a multi-type variable, for example, a variable can be a float, an int, or a string.
Here is the template definition since C++17:
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template< class... Types > class variant; |
Here is a simple example that shows how we can use it.
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std::variant< int, double, std::string > myvar; myvar = 100; // int |
To get value of a variant we can use std::get (std::variant), here is how we can use it:
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std::variant< int, double, std::string > myvar2; myvar2 = std::get<int>(myvar); |
std::variant has many useful methods and properties that can be used in modern C++, such as index, valueless_by_exception, emplace, swap, get_if, visit, variant_size, variant_size_v, variant_npos, monostate, std::hash, and operators ( =, ==, !=, <, >, <=,>=,<=> )
Is there a full example about the class template variant in C++ 17?
Here is an example.
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#include<iostream> #include <string> #include <variant> int main() { std::variant< int, double, std::string > myvar; //myvar = true; // bool myvar = 100; // int if ( std::holds_alternative<int>(myvar) ) std::cout << std::get<int>( myvar ) << std::endl; myvar = 9.81; // double if ( std::holds_alternative<double>(myvar) ) std::cout << std::get<double>( myvar ) << std::endl; myvar = "LearnCPlusPlus.org!"; // string if ( std::holds_alternative<std::string>(myvar) ) std::cout << std::get<std::string>( myvar ) << std::endl; system("pause"); return 0; } |
I should note that this feature can currently only be compiled with the new bcc64x compiler.
For more details about this feature in C++17 standard, please see these papers; P0088R3, P0393R3, P0032R3, P0504R0, P0510R0, LWG 2901DR
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