README.md

How to use std::unique_ptr with a custom deleter

Often when dealing with legacy C code, memory is allocated for you which you would like to be able to manage to avoid leaks. One way of doing this is with a custom deleter.

    static void mydeleter (char* mem) noexcept {
        auto addr = static_cast<const void*>(mem);
        free((void*)mem);
    }
    std::unique_ptr<char, decltype(deleter) > retval(mem, deleter);

Or if you want, via a lambda:

    auto lambda_deleter = ( [](char *mem) {
        if (mem) {
            free((void*)mem);
        }
    } );

    std::unique_ptr<char, decltype(lambda_deleter) > retval(mem, lambda_deleter);

Note that make_unique cannot be used with this approach as the whole point of make_unique is to automatically invoke a deleter for the managed object. In our case, C is managing the object so we cannot use it and must provide a deleter.

#include <functional> // for _1, _2
#include <iostream>
#include <memory>
#include <sstream>
#include <string.h>

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

static auto cpp_strdup1(const char *s)
{
  char *mem  = strdup(s);
  auto  addr = static_cast< const void  *>(mem);
  std::cout << "copy " << mem << " addr " << addr << std::endl;

  auto lambda_deleter = ([](char *mem) {
    if (mem) {
      auto addr = static_cast< const void * >(mem);
      std::cout << "free " << mem << " addr " << addr << std::endl;
      free((void *) mem);
    }
  });

  std::unique_ptr< char, decltype(lambda_deleter) > retval(mem, lambda_deleter);
  return retval;
}

static void mydeleter(char *mem) noexcept
{
  auto addr = static_cast< const void * >(mem);
  std::cout << "free " << mem << " addr " << addr << std::endl;
  free((void *) mem);
}

static auto cpp_strdup2(const char *s)
{
  char *mem  = strdup(s);
  auto  addr = static_cast< const void  *>(mem);
  std::cout << "copy " << mem << " addr " << addr << std::endl;
  std::unique_ptr< char, std::function< void(char *) > > retval(mem, mydeleter);
  return retval;
}

static auto cpp_strdup3(const char *s)
{
  char *mem  = strdup(s);
  auto  addr = static_cast< const void  *>(mem);
  std::cout << "copy " << mem << " addr " << addr << std::endl;
  std::unique_ptr< char, decltype(&mydeleter) > retval(mem, mydeleter);
  return retval;
}

static auto cpp_strdup4(const char *s)
{
  char *mem  = strdup(s);
  auto  addr = static_cast< const void  *>(mem);
  std::cout << "copy " << mem << " addr " << addr << std::endl;
  auto                                  fn = std::bind(mydeleter, _1);
  std::unique_ptr< char, decltype(fn) > retval(mem, fn);
  return retval;
}

int main(void)
{
  // Cpp strdup wrapper with lambda deleter
  auto p1 = cpp_strdup1("hello");
  std::cout << p1.get() << std::endl;

  // Cpp strdup wrapper with std::function deleter
  auto p2 = cpp_strdup2("there");
  std::cout << p2.get() << std::endl;

  // Cpp strdup wrapper with decltype(&mydeleter) deleter
  auto p3 = cpp_strdup3("Zaphod");
  std::cout << p3.get() << std::endl;

  // Cpp strdup wrapper with std::bind deleter
  auto p4 = cpp_strdup4("Beeblebrox");
  std::cout << p4.get() << std::endl;

  // End, expect memory to be freed
}

To build:

cd std_unique_ptr_with_custom_deallocator
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;4mCpp strdup wrapper with lambda deleter�[0m
copy hello addr 0x55d875dccec0
hello

�[31;1;4mCpp strdup wrapper with std::function deleter�[0m
copy there addr 0x55d875dccee0
there

�[31;1;4mCpp strdup wrapper with decltype(&mydeleter) deleter�[0m
copy Zaphod addr 0x55d875dccf00
Zaphod

�[31;1;4mCpp strdup wrapper with std::bind deleter�[0m
copy Beeblebrox addr 0x55d875dccf20
Beeblebrox

�[31;1;4mEnd, expect memory to be freed�[0m
free Beeblebrox addr 0x55d875dccf20
free Zaphod addr 0x55d875dccf00
free there addr 0x55d875dccee0
free hello addr 0x55d875dccec0