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Difference between revisions of "cpp/utility/any/make any"

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< cpp‎ | utility‎ | any
(Fixed editorially)
m (→‎Synopsis: +<any>)
 
(5 intermediate revisions by 5 users not shown)
Line 2: Line 2:
 
{{cpp/utility/any/navbar}}
 
{{cpp/utility/any/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++17 |
+
{{dcl header|any}}
 +
{{dcl|num=1|since=c++17|
 
template< class T, class... Args >
 
template< class T, class... Args >
 
std::any make_any( Args&&... args );
 
std::any make_any( Args&&... args );
 
}}
 
}}
−
{{dcl | num=2 | since=c++17 |
+
{{dcl|num=2|since=c++17|
 
template< class T, class U, class... Args >
 
template< class T, class U, class... Args >
 
std::any make_any( std::initializer_list<U> il, Args&&... args );
 
std::any make_any( std::initializer_list<U> il, Args&&... args );
Line 14: Line 15:
 
Constructs an {{tt|any}} object containing an object of type {{tt|T}}, passing the provided arguments to {{tt|T}}'s constructor.
 
Constructs an {{tt|any}} object containing an object of type {{tt|T}}, passing the provided arguments to {{tt|T}}'s constructor.
  
−
@1@ Equivalent to {{c|return std::any(std::in_place<T>, std::forward<Args>(args)...); }}
+
@1@ Equivalent to {{c|return std::any(std::in_place_type<T>, std::forward<Args>(args)...);}}
−
@2@ Equivalent to {{c|return std::any(std::in_place<T>, il, std::forward<Args>(args)...); }}
+
@2@ Equivalent to {{c|return std::any(std::in_place_type<T>, il, std::forward<Args>(args)...);}}
  
−
=== See also ===
+
===Example===
 +
{{example
 +
|code=
 +
#include <any>
 +
#include <complex>
 +
#include <functional>
 +
#include <iostream>
 +
#include <string>
 +
 
 +
int main()
 +
{
 +
    auto a0 = std::make_any<std::string>("Hello, std::any!\n");
 +
    auto a1 = std::make_any<std::complex<double>>(0.1, 2.3);
 +
 
 +
    std::cout << std::any_cast<std::string&>(a0);
 +
    std::cout << std::any_cast<std::complex<double>&>(a1) << '\n';
 +
 
 +
    using lambda = std::function<void(void)>;
 +
 
 +
    // Put a lambda into std::any. Attempt #1 (failed).
 +
    std::any a2 = [] { std::cout << "Lambda #1.\n"; };
 +
    std::cout << "a2.type() = \"" << a2.type().name() << "\"\n";
 +
   
 +
    // any_cast casts to <void(void)> but actual type is not
 +
    // a std::function..., but ~ main::{lambda()#1}, and it is
 +
    // unique for each lambda. So, this throws...
 +
    try
 +
    {
 +
        std::any_cast<lambda>(a2)();
 +
    }
 +
    catch (std::bad_any_cast const& ex)
 +
    {
 +
        std::cout << ex.what() << '\n';
 +
    }
 +
 
 +
    // Put a lambda into std::any. Attempt #2 (successful).
 +
    auto a3 = std::make_any<lambda>([] { std::cout << "Lambda #2.\n"; });
 +
    std::cout << "a3.type() = \"" << a3.type().name() << "\"\n";
 +
    std::any_cast<lambda>(a3)();
 +
}
 +
|p=true
 +
|output=
 +
Hello, std::any!
 +
(0.1,2.3)
 +
a2.type() = "Z4mainEUlvE_"
 +
bad any_cast
 +
a3.type() = "St8functionIFvvEE"
 +
Lambda #2.
 +
}}
 +
 
 +
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/utility/any/dsc constructor}}
+
{{dsc inc|cpp/utility/any/dsc constructor}}
 +
{{dsc inc|cpp/utility/any/dsc any_cast}}
 
{{dsc end}}
 
{{dsc end}}
 +
 +
{{langlinks|es|ja|ru|zh}}

Latest revision as of 06:26, 21 April 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::any
 
Defined in header <any>
template< class T, class... Args >
std::any make_any( Args&&... args );
 (1) (since C++17)
template< class T, class U, class... Args >
std::any make_any( std::initializer_list<U> il, Args&&... args );
 (2) (since C++17)

Constructs an any object containing an object of type T, passing the provided arguments to T's constructor.

1) Equivalent to return std::any(std::in_place_type<T>, std::forward<Args>(args)...);
2) Equivalent to return std::any(std::in_place_type<T>, il, std::forward<Args>(args)...);

[edit] Example

Run this code
#include <any>
#include <complex>
#include <functional>
#include <iostream>
#include <string>
 
int main()
{
    auto a0 = std::make_any<std::string>("Hello, std::any!\n");
    auto a1 = std::make_any<std::complex<double>>(0.1, 2.3);
 
    std::cout << std::any_cast<std::string&>(a0);
    std::cout << std::any_cast<std::complex<double>&>(a1) << '\n';
 
    using lambda = std::function<void(void)>;
 
    // Put a lambda into std::any. Attempt #1 (failed).
    std::any a2 = [] { std::cout << "Lambda #1.\n"; };
    std::cout << "a2.type() = \"" << a2.type().name() << "\"\n";
 
    // any_cast casts to <void(void)> but actual type is not
    // a std::function..., but ~ main::{lambda()#1}, and it is
    // unique for each lambda. So, this throws...
    try
    {
        std::any_cast<lambda>(a2)();
    }
    catch (std::bad_any_cast const& ex)
    {
        std::cout << ex.what() << '\n';
    }
 
    // Put a lambda into std::any. Attempt #2 (successful).
    auto a3 = std::make_any<lambda>([] { std::cout << "Lambda #2.\n"; });
    std::cout << "a3.type() = \"" << a3.type().name() << "\"\n";
    std::any_cast<lambda>(a3)();
}

Possible output: 

Hello, std::any!
(0.1,2.3)
a2.type() = "Z4mainEUlvE_"
bad any_cast
a3.type() = "St8functionIFvvEE"
Lambda #2.

[edit] See also

 constructs an any object
(public member function) [edit]
(C++17)
 type-safe access to the contained object
(function template) [edit]
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