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std::function(3) C++ Standard Libary std::function(3) NAME std::function - std::function Synopsis Defined in header <functional> template< class > (since C++11) class function; /* undefined */ template< class R, class... Args > (since C++11) class function<R(Args...)>; Class template std::function is a general-purpose polymorphic func- tion wrapper. Instances of std::function can store, copy, and invoke any CopyCon- structible Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and point- ers to data members. The stored callable object is called the target of std::function. If a std::function contains no target, it is called empty. Invoking the target of an empty std::function results in std::bad_function_call exception being thrown. std::function satisfies the requirements of CopyConstructible and CopyAssignable. Member types Type Definition result_type R argument_type(deprecated in C++17)(removed in T if sizeof...(Args)==1 and T is the C++20) first and only type in Args... first_argument_type(deprecated in T1 if sizeof...(Args)==2 and T1 is the C++17)(removed in C++20) first of the two types in Args... second_argument_type(deprecated in T2 if sizeof...(Args)==2 and T2 is the C++17)(removed in C++20) second of the two types in Args... Member functions constructor constructs a new std::function instance (public member function) destructor destroys a std::function instance (public member function) operator= assigns a new target (public member function) swap swaps the contents (public member function) assign assigns a new target (removed in C++17) (public member function) operator bool checks if a target is contained (public member function) operator() invokes the target (public member function) Target access target_type obtains the typeid of the stored target (public member function) target obtains a pointer to the stored target (public member function) Non-member functions std::swap(std::function) specializes the std::swap algorithm (C++11) (function template) operator== compares a std::function with nullptr operator!= (function template) (removed in C++20) Helper classes std::uses_allocator<std::function> specializes the std::uses_alloca- tor type trait (C++11) (until C++17) (class template specialization) Deduction guides(since C++17) Notes Care should be taken when a 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 (until C++23) expression will always return a prvalue. Hence, the resulting reference will usually bind to a temporary whose lifetime ends when std::function::operator() returns. If a 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 (since C++23) because binding the returned referenced to a temporary object is forbidden. std::function<const int&()> F([]{ return 42; }); // Error since C++23: can't bind // the returned ref- erence to a temporary int x = F(); // Undefined behavior until C++23: the result of F() is a dangling reference std::function<int&()> G([]()->int& { static int i{0x2A}; return i; }); // OK std::function<const int&()> H([i{052}]->const int& { return i; }); // OK Example // Run this code #include <functional> #include <iostream> struct Foo { Foo(int num) : num_(num) {} void print_add(int i) const { std::cout << num_+i << '\n'; } int num_; }; void print_num(int i) { std::cout << i << '\n'; } struct PrintNum { void operator()(int i) const { std::cout << i << '\n'; } }; int main() { // store a free function std::function<void(int)> f_display = print_num; f_display(-9); // store a lambda std::function<void()> f_display_42 = []() { print_num(42); }; f_display_42(); // store the result of a call to std::bind std::function<void()> f_display_31337 = std::bind(print_num, 31337); f_display_31337(); // store a call to a member function std::function<void(const Foo&, int)> f_add_display = &Foo::print_add; const Foo foo(314159); f_add_display(foo, 1); f_add_display(314159, 1); // store a call to a data member accessor std::function<int(Foo const&)> f_num = &Foo::num_; std::cout << "num_: " << f_num(foo) << '\n'; // 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); // 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); // store a call to a function object std::function<void(int)> f_display_obj = PrintNum(); f_display_obj(18); auto factorial = [](int n) { // store a lambda object to emulate "recursive lambda"; aware of extra overhead 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 return fac(n); }; for (int i{5}; i != 8; ++i) { std::cout << i << "! = " << factor- ial(i) << "; "; } } Possible output: -9 42 31337 314160 314160 num_: 314159 314161 314162 18 5! = 120; 6! = 720; 7! = 5040; See also move_only_function wraps callable object of any type with specified function call (C++23) signature (class template) bad_function_call the exception thrown when invoking an empty std::function (C++11) (class) mem_fn creates a function object out of a pointer to a member (C++11) (function template) http://cppreference.com 2022.07.31 std::function(3)
NAME | Synopsis | Member types | Member functions | Target access | Non-member functions | Helper classes | Notes | Example | Possible output: | See also
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