README.md

How to use std::bind with a class callback

Sometimes we want to be able to invoke callbacks from our class. This can be difficult in C++ as we need to have the context (i.e. the class object) somehow associated with the function.

Thankfully, std::bind, can do this for us. Here we bind "account1" to one of its methods.

    auto method = std::bind(&BankAccount<int>::deposit, &account1, _1);
    method(100);

This is identical to:

    account1.deposit(100);

So not too earth shattering. However, suppose we want to call a method in our class and have it call another method within the same class (or potentially another class) i.e. a callback.

Here we define the ability for "check_transaction" to accept a callack function:

    using CheckTransactionCallback = std::function<void(T)>;
    int check_transaction (int cash, CheckTransactionCallback fn) {
        if (cash < 100) {
            throw std::string("transaction is too small for Mr Money Bags");
        } else {
            fn(cash);
        }
        return cash;
    }

And to invoke it:

    auto callback = std::bind(&BankAccount<int>::deposit, &account1, _1);
    account1.check_transaction(99, callback);

Here is the full example:

#include <algorithm>  // for std::move
#include <functional> // for _1, _2
#include <iostream>
#include <memory>
#include <sstream> // for std::stringstream
#include <string>
#include <utility>

using namespace std::placeholders; // for _1, _2, _3...

template < class T > class BankAccount;

template < class T > class BankAccount
{
private:
  const T no_cash {};
  T       cash {};

public:
  BankAccount() { std::cout << "default constructor " << to_string() << std::endl; }
  BankAccount(T cash) : cash(cash) { std::cout << "new cash " << to_string() << std::endl; }
  BankAccount(const BankAccount &o)
  {
    std::cout << "copy cash constructor called for " << o.to_string() << std::endl;
    cash = o.cash;
    std::cout << "copy cash constructor result is  " << to_string() << std::endl;
  }
  // Transfer of funds?
  BankAccount(BankAccount< T > &&o)
  {
    std::cout << "move cash called for " << o.to_string() << std::endl;
    cash   = o.cash;
    o.cash = no_cash;
    std::cout << "move cash result is  " << to_string() << std::endl;
  }
  ~BankAccount() { std::cout << "delete account " << to_string() << std::endl; }
  void deposit(const T &deposit)
  {
    cash += deposit;
    std::cout << "deposit cash called " << to_string() << std::endl;
  }
  using CheckTransactionCallback = std::function< void(T) >;
  int check_transaction(int cash, CheckTransactionCallback fn)
  {
    if (cash < 100) {
      throw std::string("transaction is too small for Mr Money Bags");
    } else {
      fn(cash);
    }
    return cash;
  }
  T    balance(void) const { return cash; }
  bool check_balance(T expected) const
  {
    if (cash == expected) {
      return true;
    } else {
      throw std::string("account has different funds " + to_string() + " than expected " + std::to_string(expected));
    }
  }
  friend std::ostream &operator<<(std::ostream &os, const BankAccount< T > &o)
  {
    os << "$" << std::to_string(o.cash);
    return os;
  }
  std::string to_string(void) const
  {
    auto              address = static_cast< const void              *>(this);
    std::stringstream ss;
    ss << address;
    return "BankAccount(" + ss.str() + ", cash $" + std::to_string(cash) + ")";
  }
};

int main(int, char **)
{
  try {
    // create account1 and try to deposit into it
    auto account1 = BankAccount< int >(0);

    auto deposit_method = std::bind(&BankAccount< int >::deposit, &account1, _1);
    account1.check_transaction(100, deposit_method);
    account1.check_transaction(100, deposit_method);
    account1.check_balance(200);
    std::cout << "SUCCESS: account1 1st deposit succeeded!" << std::endl;

    //
    // This will fail as we catch the 'small' transaction
    //
    account1.check_transaction(99, deposit_method);
    std::cout << "SUCCESS: account1 2nd deposit succeeded!" << std::endl;

    // end
  } catch (const std::string &e) {
    std::cerr << "FAILED: account1 deposit failed!: " << e << std::endl;
  }
}

To build:

cd std_bind_with_a_class_callback
rm -f *.o example
clang -std=c++2a -Werror -g -O3 -fstack-protector-all -ggdb3 -Wall -c -o main.o main.cpp
clang  main.o -lstdc++  -o example
./example

Expected output:

�[31;1;4mcreate account1 and try to deposit into it�[0m
new cash BankAccount(0x7ffcd444f308, cash $0)
deposit cash called BankAccount(0x7ffcd444f308, cash $100)
deposit cash called BankAccount(0x7ffcd444f308, cash $200)
�[0;35mSUCCESS: account1 1st deposit succeeded!�[0m
delete account BankAccount(0x7ffcd444f308, cash $200)
# FAILED: account1 deposit failed!: transaction is too small for Mr Money Bags