Binding and Const Member Functions

When binding member functions using std::bind, the constness of the member function is preserved. If you bind a const member function, the resulting functor will also be const-qualified.

Consider the following example:

1#include <functional>
2#include <iostream>
3
4struct Player {
5  std::string GetName() const { return Name; }  
6  std::string Name{"Anna"};
7};
8
9int main() {
10  const Player ConstPlayer;  
11
12  auto GetConstName{std::bind(
13    &Player::GetName, &ConstPlayer)};  
14
15  std::cout << GetConstName();
16}

1Anna

In this example, GetName() is a const member function of the Player struct. We create a const instance of Player called ConstPlayer.

When we bind &Player::GetName to &ConstPlayer using std::bind, the resulting GetConstName functor is const-qualified. This means that it can be called on const instances of Player.

Whilst we can bind a non-const member function to a const object, we will get a compilation error if we attempt invoke the function:

1#include <functional>
2#include <iostream>
3
4struct Player {
5  void SetName(const std::string& NewName) {
6    Name = NewName;
7  }  
8  std::string Name;
9};
10
11int main() {
12  const Player ConstPlayer;
13
14  auto SetConstName{std::bind(
15    &Player::SetName, &ConstPlayer, "Aria")};
16
17  SetConstName();
18}

1error: attempting to reference a deleted function

In this case, binding &Player::SetName to &ConstPlayer will result in a compilation error once we invoke the function, because SetName() is a non-const member function and cannot be called on a const object.

Therefore, when binding member functions, it's important to consider the constness of the member function and the object it is being bound to. std::bind ensures that the constness is preserved and enforced appropriately.