Question 38140 in Ubuntu Question posted in
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Custom deleter unique_ptr creates 'function not used' warnings in object code and error during linking – Ubuntu

I am creating an object file for my larger program. In this program I use a unique_ptr type with a custom deleter defined.
So I’m compiling my file like so:

g++ -std=c++20 tmp.cpp -c -Wall

The code looks as follows:

#include <memory>

using specialptr = std::unique_ptr<int, decltype([](int *f) {
                     delete f;
                   })>;

specialptr fun(){ 
  return specialptr{new int};
}

This generates this warning:

 tmp.cpp:12:12: warning: ‘specialptr fun()’ defined but not used [-Wunused-function]
   12 | specialptr fun(){ return specialptr{new int};}
      |

When I link this object file with a main.cpp that calls the function I get the following error:

In file included from main.cpp:1:
tmp.h:19:12: warning: ‘specialptr fun()’ used but never defined
   19 | specialptr fun();
      |            ^~~
/usr/bin/ld: /tmp/ccnuTTGC.o: in function `main':
main.cpp:(.text+0x4c): undefined reference to `fun()'
collect2: error: ld returned 1 exit status

What is going on here? My code works when I change the return value from a unique_ptr to a normal pointer. But intuitively, I defined the type and function and made a declaration in a header file.

g++ is version: g++ (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0

Tags:

2

Answers


  1. 0

    The decltype of a lambda is different for each translation unit.

    You can instead do something like:

    struct specialptr_deleter {
  void operator()(int *f) const {
    delete f;
  }
};

using specialptr = std::unique_ptr<int, specialptr_deleter>;

specialptr fun(){ 
  return specialptr{new int};
}
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  2. 0

    You can use object type as deleter. In this case you don’t need to set it every time you create std::unique_ptr.

    #include <memory>
#include <iostream>

struct MyDeleter {
    void operator()(int *f) {
        std::cout << "Deleter called";
        delete f;
    }
};

using specialptr = std::unique_ptr<int, MyDeleter>;

specialptr fun(){ 
  return specialptr{new int};
}

int main()
{
    auto x = fun();
}
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