C++ std::bind(f, ARGS)

bind binds one or more arguments to a function ...

Synopsis (since C++11, constexpr since C++20)

template< class F, class... Args >
/* unspecified */ bind( F&& f, Args&&... args );

template< class R, class F, class... Args >
/* unspecified */ bind( F&& f, Args&&... args );

The function template std::bind generates a forwarding call wrapper for f. Calling this wrapper is equivalent to invoking f with some of its arguments bound to args.

Example

#include <functional>
#include <iostream>
#include <memory>
#include <random>

void f(int n1, int n2, int n3, const int& n4, int n5)
{
    std::cout << n1 << ' ' << n2 << ' ' << n3 << ' ' << n4 << ' ' << n5 << '\n';
}

int g(int n1)
{
    return n1;
}

struct Foo
{
    void print_sum(int n1, int n2)
    {
        std::cout << n1 + n2 << '\n';
    }

    int data = 10;
};

int main()
{
    using namespace std::placeholders;  // for _1, _2, _3...

    std::cout << "1) argument reordering and pass-by-reference: ";
    int n = 7;
    // (_1 and _2 are from std::placeholders, and represent future
    // arguments that will be passed to f1)
    auto f1 = std::bind(f, _2, 42, _1, std::cref(n), n);
    n = 10;
    f1(1, 2, 1001); // 1 is bound by _1, 2 is bound by _2, 1001 is unused
                    // makes a call to f(2, 42, 1, n, 7)

    std::cout << "2) achieving the same effect using a lambda: ";
    n = 7;
    auto lambda = [&ncref = n, n](auto a, auto b, auto /*unused*/)
    {
        f(b, 42, a, ncref, n);
    };
    n = 10;
    lambda(1, 2, 1001); // same as a call to f1(1, 2, 1001)

    std::cout << "3) nested bind subexpressions share the placeholders: ";
    auto f2 = std::bind(f, _3, std::bind(g, _3), _3, 4, 5);
    f2(10, 11, 12); // makes a call to f(12, g(12), 12, 4, 5);

    std::cout << "4) bind a RNG with a distribution: ";
    std::default_random_engine e;
    std::uniform_int_distribution<> d(0, 10);
    auto rnd = std::bind(d, e); // a copy of e is stored in rnd
    for (int n = 0; n < 10; ++n)
        std::cout << rnd() << ' ';
    std::cout << '\n';

    std::cout << "5) bind to a pointer to member function: ";
    Foo foo;
    auto f3 = std::bind(&Foo::print_sum, &foo, 95, _1);
    f3(5);

    std::cout << "6) bind to a mem_fn that is a pointer to member function: ";
    auto ptr_to_print_sum = std::mem_fn(&Foo::print_sum);
    auto f4 = std::bind(ptr_to_print_sum, &foo, 95, _1);
    f4(5);

    std::cout << "7) bind to a pointer to data member: ";
    auto f5 = std::bind(&Foo::data, _1);
    std::cout << f5(foo) << '\n';

    std::cout << "8) bind to a mem_fn that is a pointer to data member: ";
    auto ptr_to_data = std::mem_fn(&Foo::data);
    auto f6 = std::bind(ptr_to_data, _1);
    std::cout << f6(foo) << '\n';

    std::cout << "9) use smart pointers to call members of the referenced objects: ";
    std::cout << f6(std::make_shared<Foo>(foo)) << ' '
              << f6(std::make_unique<Foo>(foo)) << '\n';
}

template< class F, class... Args >
constexpr /* unspecified */ bind_front( F& f, Args&... args ); // since C++20
template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_front( Args&... args ); // since C++26

template< class F, class... Args >
constexpr /* unspecified */ bind_back( F& f, Args&... args ); // since C++23
template< auto ConstFn, class... Args >
constexpr /* unspecified */ bind_back( Args&... args ); // since C++26

#include <cassert>
#include <functional>

int minus(int a, int b)
{
    return a - b;
}

struct S
{
    int val;
    int minus(int arg) const noexcept { return val - arg; }
};

int main()
{
    auto fifty_minus = std::bind_front(minus, 50);
    assert(fifty_minus(3) == 47); // equivalent to: minus(50, 3) == 47

    auto member_minus = std::bind_front(&S::minus, S{50});
    assert(member_minus(3) == 47); //: S tmp{50}; tmp.minus(3) == 47

    // Noexcept-specification is preserved:
    static_assert(!noexcept(fifty_minus(3)));
    static_assert(noexcept(member_minus(3)));

    // Binding of a lambda:
    auto plus = [](int a, int b) { return a + b; };
    auto forty_plus = std::bind_front(plus, 40);
    assert(forty_plus(7) == 47); // equivalent to: plus(40, 7) == 47

#if __cpp_lib_bind_front >= 202306L
    auto fifty_minus_cpp26 = std::bind_front<minus>(50);
    assert(fifty_minus_cpp26(3) == 47);

    auto member_minus_cpp26 = std::bind_front<&S::minus>(S{50});
    assert(member_minus_cpp26(3) == 47);

    auto forty_plus_cpp26 = std::bind_front<plus>(40);
    assert(forty_plus(7) == 47);
#endif

#if __cpp_lib_bind_back >= 202202L
    auto madd = [](int a, int b, int c) { return a * b + c; };
    auto mul_plus_seven = std::bind_back(madd, 7);
    assert(mul_plus_seven(4, 10) == 47); //: madd(4, 10, 7) == 47
#endif

#if __cpp_lib_bind_back >= 202306L
    auto mul_plus_seven_cpp26 = std::bind_back<madd>(7);
    assert(mul_plus_seven_cpp26(4, 10) == 47);
#endif
}