Why use std::bind over lambdas in C++14?

Question

Before C++11 I used boost::bind or boost::lambda a lot. The bind part made it into the standard library (std::bind) the other part became part of the core language (C++ lambdas) and made the use of lambdas a lot easier. Nowadays, I hardly use std::bind, since I can do almost anything with C++ lambdas. There's one valid use-case for std::bind that I can think of:

struct foo
{
  template < typename A, typename B >
  void operator()(A a, B b)
  {
    cout << a << ' ' << b;
  }
};

auto f = bind(foo(), _1, _2);
f( "test", 1.2f ); // will print "test 1.2"

The C++14 equivalent for that would be

auto f = []( auto a, auto b ){ cout << a << ' ' << b; }
f( "test", 1.2f ); // will print "test 1.2"

Much shorter and more concise. (In C++11 this does not work yet because of the auto parameters.) Is there any other valid use case for std::bind beating the C++ lambdas alternative or is std::bind superfluous with C++14?

Answer 1

For me, a valid use for std::bind is to make it clear that I'm using a member function as a predicate. That is, if all I do is call a member function, it's bind. If I do extra stuff with the argument (besides calling a memeber function), it's a lambda:

using namespace std;
auto is_empty = bind(&string::empty, placeholders::_1); // bind = just map member
vector<string> strings;
auto first_empty = any_of(strings.begin(), strings.end(), is_empty);

auto print_non_empty = [](const string& s) {            // lambda = more than member
    if(s.empty())                // more than calling empty
        std::cout << "[EMPTY]";  // more than calling empty
    else                         // more than calling empty
        std::cout << s;          // more than calling empty
};
vector<string> strings;
for_each(strings.begin(), strings.end(), print_non_empty);

Answer 2

Scott Meyers gave a talk about this. This is what I remember:

In C++14 there is nothing useful bind can do that can't also be done with lambdas.

In C++11 however there are some things that can't be done with lambdas:

1. You can't move the variables while capturing when creating the lambdas. Variables are always captured as lvalues. For bind you can write:

   auto f1 = std::bind(f, 42, _1, std::move(v));
   

2. Expressions can't be captured, only identifiers can. For bind you can write:

   auto f1 = std::bind(f, 42, _1, a + b);
   

3. Overloading arguments for function objects. This was already mentioned in the question.

4. Impossible to perfect-forward arguments

In C++14 all of these possible.

1. Move example:

   auto f1 = [v = std::move(v)](auto arg) { f(42, arg, std::move(v)); };
   

2. Expression example:

   auto f1 = [sum = a + b](auto arg) { f(42, arg, sum); };
   

3. See question

4. Perfect forwarding: You can write

   auto f1 = [=](auto&& arg) { f(42, std::forward<decltype(arg)>(arg)); };
   

Some disadvantages of bind:

• Bind binds by name and as a result if you have multiple functions with the same name (overloaded functions) bind doesn't know which one to use. The following example won't compile, while lambdas wouldn't have a problem with it:

  void f(int); void f(char); auto f1 = std::bind(f, _1, 42);
  

• When using bind functions are less likely to be inlined

On the other hand lambdas might theoretically generate more template code than bind. Since for each lambda you get a unique type. For bind it is only when you have different argument types and a different function (I guess that in practice however it doesn't happen very often that you bind several time with the same arguments and function).

What Jonathan Wakely mentioned in his answer is actually one more reason not to use bind. I can't see why you would want to silently ignore arguments.

Answer 3

Sometimes it is just less code. Consider this:

bool check(int arg1, int arg2, int arg3)
{
  return ....;
}

Then

wait(std::bind(check,a,b,c));

vs lambda

wait([&](){return check(a,b,c);});

I think that bind is easier to read here compared to the lambda which looks like a https://en.wikipedia.org/wiki/Brainfuck

Answer 4

Just expanding @BertR's comment to this answer to something testable, though I confess I couldn't quite get a solution using std::forward<> to work.

#include <string>
#include <functional>
using namespace std::string_literals;

struct F {
    bool        operator()(char c, int  i) { return c == i;  };
    std::string operator()(char c, char d) { return ""s + d; };
};

void test() {
    { // using std::bind
        auto f = std::bind(F(), 'a', std::placeholders::_1);
        auto b = f(1);
        auto s = f('b');
    }
    { // using lambda with parameter pack
        auto x = [](auto... args) { return F()('a', args...); };
        auto b = x(1);
        auto s = x('b');
    }
}

Test at Compiler Explorer

Answer 5

std::bind can still do one thing polymorphic lambdas can't: invoke overloaded functions

struct F {
  bool operator()(char, int);
  std::string operator()(char, char);
};

auto f = std::bind(F(), 'a', std::placeholders::_1);
bool b = f(1);
std::string s = f('b');

The call wrapper created by the bind expression calls different functions depending on the arguments you give it, the closure from a C++14 polymorphic lambda can take different types of arguments but can't take a different number of arguments, and always invokes (specializations of) the same function on the closure. Correction: see the comments below

The wrapper returned by std::bind can also be called with too many arguments and it will ignore them, whereas a closure created by a lambda will diagnose attempts to pass too many arguments ... but I don't consider that a benefit of std::bind :)

Answer 6

Another difference is that arguments to bind must be copied or moved, while a lambda can use variables captured by reference. See example below:

#include <iostream>
#include <memory>

void p(const int& i) {
    std::cout << i << '\n';
}

int main()
{
    std::unique_ptr<int> f = std::make_unique<int>(3);

    // Direct
    p(*f);

    // Lambda ( ownership of f can stay in main )
    auto lp = [&f](){p(*f);};
    lp();

    // Bind ( does not compile - the arguments to bind are copied or moved)
    auto bp = std::bind(p, *f, std::placeholders::_1);
    bp();
}

Not sure if it's possible to workaround the issue to use bind above without changing the signature of void p(const int&).