Bind function and placeholders in C++

C++Server Side ProgrammingProgramming

Here we will see the Bind function and the placeholders in C++. Sometimes we need to manipulate the operation of some functions as we need. We can use some default parameters to get some essence of manipulating.

In C++11, one new feature is introduced, called the bind function. This helps us to do such manipulating in some easier fashion. To use these features, we have to use <functional> header file.

Bind functions with the help of placeholders helps to determine the positions, and number of arguments to modify the function according to desired outputs.

Placeholders are namespaces which detect the position of a value in a function. Placeholders are represented by _1, _2, _3 etc.

Example

 Live Demo

#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() {
   std::function<void(int)> f_display = print_num;
   f_display(-9);
   std::function<void()> f_display_42 = []() { print_num(42); };
   f_display_42();
   std::function<void()> f_display_31337 = std::bind(print_num, 31337);
   f_display_31337();
   std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
   const Foo foo(314159);
   f_add_display(foo, 1);
   std::function<int(Foo const&)> f_num = &Foo::num_;
   std::cout << "num_: " << f_num(foo) << '\n';
   using std::placeholders::_1;
   std::function<void(int)> f_add_display2= std::bind( &Foo::print_add, foo, _1 );
   f_add_display2(2);
   std::function<void(int)> f_add_display3= std::bind( &Foo::print_add, &foo, _1 );
   f_add_display3(3);
   std::function<void(int)> f_display_obj = PrintNum();
   f_display_obj(18);
}

Output

-9
42
31337
314160
num_: 314159
314161
314162
18
raja
Published on 18-Jun-2019 08:02:02
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