# Extra Clang Tools 21.1.8 documentation

## clang-tidy - modernize-avoid-bind

# modernize-avoid-bind

The check finds uses of `std::bind` and `boost::bind` and replaces them with lambdas. Lambdas will use value-capture unless reference capture is explicitly requested with `std::ref` or `boost::ref`.

It supports arbitrary callables including member functions, function objects, and free functions, and all variations thereof. Anything that you can pass to the first argument of `bind` should be diagnosable. Currently, the only known case where a fix-it is unsupported is when the same placeholder is specified multiple times in the parameter list.

Given:

    int add(int x, int y) { return x + y; }

Then:

    void f() {
      int x = 2;
      auto clj = std::bind(add, x, _1);
    }

is replaced by:

    void f() {
      int x = 2;
      auto clj = [=](auto && arg1) { return add(x, arg1); };
    }

`std::bind` can be hard to read and can result in larger object files and binaries due to type information that will not be produced by equivalent lambdas.

## Options

PermissiveParameterList

  If the option is set to true, the check will append `auto&&...` to the end of every placeholder parameter list. Without this, it is possible for a fix-it to perform an incorrect transformation in the case where the result of the `bind` is used in the context of a type erased functor such as `std::function` which allows mismatched arguments. Default is is false.

For example:

    int add(int x, int y) { return x + y; }
    int foo() {
      std::function<int(int,int)> ignore_args = std::bind(add, 2, 2);
      return ignore_args(3, 3);
    }

is valid code, and returns 4. The actual values passed to `ignore_args` are simply ignored. Without `PermissiveParameterList`, this would be transformed into

    int add(int x, int y) { return x + y; }
    int foo() {
      std::function<int(int,int)> ignore_args = [] { return add(2, 2); }
      return ignore_args(3, 3);
    }

which will not compile, since the lambda does not contain an `operator()` that accepts 2 arguments. With permissive parameter list, it instead generates

    int add(int x, int y) { return x + y; }
    int foo() {
      std::function<int(int,int)> ignore_args = [](auto&&...) { return add(2, 2); }
      return ignore_args(3, 3);
    }

which is correct.

This check requires using C++14 or higher to run.