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| − | {{cpp/title|function}}
| + | #REDIRECT [[enwiki:Rust_(programming_language)]] |
| − | {{cpp/utility/functional/function/navbar}}
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| − | {{dcl begin}}
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| − | {{dcl header|functional}}
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| − | {{dcl|since=c++11|
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| − | template< class >
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| − | class function; /* undefined */
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| − | }}
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| − | {{dcl|since=c++11|
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| − | template< class R, class... Args >
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| − | class function<R(Args...)>;
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| − | }}
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| − | {{dcl end}}
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| − | | + | |
| − | Class template {{tt|std::function}} is a general-purpose polymorphic function wrapper. Instances of {{tt|std::function}} can store, copy, and invoke any {{named req|CopyConstructible}} {{named req|Callable}} ''target'' -- functions (via pointers thereto), [[cpp/language/lambda|lambda expressions]], [[cpp/utility/functional/bind|bind expressions]], or other function objects, as well as pointers to member functions and pointers to data members.
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| − | The stored callable object is called the ''target'' of {{tt|std::function}}. If a {{tt|std::function}} contains no target, it is called ''empty''. Invoking the ''target'' of an ''empty'' {{tt|std::function}} results in {{lc|std::bad_function_call}} exception being thrown.
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| − | | + | |
| − | {{tt|std::function}} satisfies the requirements of {{named req|CopyConstructible}} and {{named req|CopyAssignable}}.
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| − | | + | |
| − | ===Member types===
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| − | {{dsc begin}}
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| − | {{dsc hitem|Type|Definition}}
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| − | {{dsc|{{tt|result_type}}|{{tt|R}}}}
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| − | {{dsc|{{tt|argument_type}}{{mark|deprecated in C++17}}{{mark until c++20|removed=yes}}|{{tt|T}} if {{c|1=sizeof...(Args)==1}} and {{tt|T}} is the first and only type in {{tt|Args...}}}}
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| − | {{dsc|{{tt|first_argument_type}}{{mark|deprecated in C++17}}{{mark until c++20|removed=yes}}|{{tt|T1}} if {{c|1=sizeof...(Args)==2}} and {{tt|T1}} is the first of the two types in {{tt|Args...}} }}
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| − | {{dsc|{{tt|second_argument_type}}{{mark|deprecated in C++17}}{{mark until c++20|removed=yes}}|{{tt|T2}} if {{c|1=sizeof...(Args)==2}} and {{tt|T2}} is the second of the two types in {{tt|Args...}}}}
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| − | {{dsc end}}
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| − | | + | |
| − | ===Member functions===
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| − | {{dsc begin}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc constructor}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc destructor}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc operator{{=}}}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc swap}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc assign}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc operator_bool}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc operator()}}
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| − | | + | |
| − | {{dsc h2|Target access}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc target_type}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc target}}
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| − | {{dsc end}}
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| − | | + | |
| − | ===Non-member functions===
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| − | {{dsc begin}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc swap2}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc operator_cmp}}
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| − | {{dsc end}}
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| − | | + | |
| − | ===Helper classes===
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| − | {{dsc begin}}
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| − | {{dsc inc|cpp/utility/functional/function/dsc uses_allocator}}
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| − | {{dsc end}}
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| − | | + | |
| − | ===[[cpp/utility/functional/function/deduction_guides|Deduction guides]]{{mark since c++17}}===
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| − | | + | |
| − | ===Notes===
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| − | {{rrev multi
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| − | |rev1=
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| − | Care should be taken when a {{tt|std::function}}, whose result type is a reference, is initialized from a lambda expression without a trailing-return-type. Due to the way auto deduction works, such lambda expression will always return a prvalue. Hence, the resulting reference will usually bind to a temporary whose lifetime ends when {{rlt|operator()|std::function::operator()}} returns.
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| − | |since2=c++23|rev2=
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| − | If a {{tt|std::function}} returning a reference is initialized from a function or function object returning a prvalue (including a lambda expression without a trailing-return-type), the program is ill-formed because binding the returned reference to a temporary object is forbidden.
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| − | }}
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| − | | + | |
| − | {{source|1=
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| − | std::function<const int&()> F([] { return 42; }); // Error since C++23: can't bind
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| − | // the returned reference to a temporary
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| − | int x = F(); // Undefined behavior until C++23: the result of F() is a dangling reference
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| − | | + | |
| − | std::function<int&()> G([]() -> int& { static int i{0x2A}; return i; }); // OK
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| − | | + | |
| − | std::function<const int&()> H([i{052}] -> const int& { return i; }); // OK
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| − | }}
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| − | | + | |
| − | ===Example===
| + | |
| − | {{example
| + | |
| − | |code=
| + | |
| − | #include <functional> | + | |
| − | #include <iostream>
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| − |
| + | |
| − | struct Foo
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| − | {
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| − | Foo(int num) : num_(num) {}
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| − | void print_add(int i) const { std::cout << num_ + i << '\n'; }
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| − | int num_;
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| − | };
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| − |
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| − | void print_num(int i)
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| − | {
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| − | std::cout << i << '\n';
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| − | }
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| − | | + | |
| − | struct PrintNum
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| − | {
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| − | void operator()(int i) const
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| − | {
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| − | std::cout << i << '\n';
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| − | }
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| − | };
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| − |
| + | |
| − | int main()
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| − | {
| + | |
| − | // store a free function
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| − | std::function<void(int)> f_display = print_num;
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| − | f_display(-9);
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| − |
| + | |
| − | // store a lambda
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| − | std::function<void()> f_display_42 = []() { print_num(42); };
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| − | f_display_42();
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| − |
| + | |
| − | // store the result of a call to std::bind
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| − | std::function<void()> f_display_31337 = std::bind(print_num, 31337);
| + | |
| − | f_display_31337();
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| − |
| + | |
| − | // store a call to a member function
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| − | std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
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| − | const Foo foo(314159);
| + | |
| − | f_add_display(foo, 1);
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| − | f_add_display(314159, 1);
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| − |
| + | |
| − | // store a call to a data member accessor
| + | |
| − | std::function<int(Foo const&)> f_num = &Foo::num_;
| + | |
| − | std::cout << "num_: " << f_num(foo) << '\n';
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| − |
| + | |
| − | // store a call to a member function and object
| + | |
| − | using std::placeholders::_1;
| + | |
| − | std::function<void(int)> f_add_display2 = std::bind(&Foo::print_add, foo, _1);
| + | |
| − | f_add_display2(2);
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| − |
| + | |
| − | // store a call to a member function and object ptr
| + | |
| − | std::function<void(int)> f_add_display3 = std::bind(&Foo::print_add, &foo, _1);
| + | |
| − | f_add_display3(3);
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| − | | + | |
| − | // store a call to a function object
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| − | std::function<void(int)> f_display_obj = PrintNum();
| + | |
| − | f_display_obj(18);
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| − | | + | |
| − | auto factorial = [](int n)
| + | |
| − | {
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| − | // store a lambda object to emulate "recursive lambda"; aware of extra overhead
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| − | std::function<int(int)> fac = [&](int n) { return (n < 2) ? 1 : n * fac(n - 1); };
| + | |
| − | // note that "auto fac = [&](int n) {...};" does not work in recursive calls
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| − | return fac(n);
| + | |
| − | };
| + | |
| − | for (int i{5}; i != 8; ++i)
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| − | std::cout << i << "! = " << factorial(i) << "; ";
| + | |
| − | std::cout << '\n';
| + | |
| − | }
| + | |
| − | |p = true|output=
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| − | -9
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| − | 42
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| − | 31337
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| − | 314160
| + | |
| − | 314160
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| − | num_: 314159
| + | |
| − | 314161
| + | |
| − | 314162
| + | |
| − | 18
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| − | 5! = 120; 6! = 720; 7! = 5040;
| + | |
| − | }}
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| − | | + | |
| − | ===See also===
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| − | {{dsc begin}}
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| − | {{dsc inc|cpp/utility/functional/dsc move_only_function}}
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| − | {{dsc inc|cpp/utility/functional/dsc bad_function_call}}
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| − | {{dsc inc|cpp/utility/functional/dsc mem_fn}}
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| − | {{dsc|'''{{ltt | cpp/language/typeid|typeid}}'''|Queries information of a type, returning a {{tt|std::type_info}} object representing the type.}}
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| − | {{dsc end}}
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| − | {{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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