[clang] Cannot create object with new operator when the delete operator of a class is explicitly deleted.

[yanjiew1]

Hello Clang maintainers,

If the delete operator in a class is explicitly deleted, the class cannot be created on the heap.

The following code cannot be compiled with Clang, but it ca be compiled with GCC.

#include <cstddef>
#include <cstdlib>

class TestClass {
public:
    void* operator new(std::size_t count) {
        return malloc(count);
    }

  void  operator delete(void* p) = delete;
  void  operator delete [](void* p) = delete;
    
};

int main(void)
{
    TestClass *test = new TestClass();

    return 0;
}

Please see this Compiler Explorer:

The is the error shown in the compiler explorer.

<source>:17:23: error: attempt to use a deleted function
   17 |     TestClass *test = new TestClass();
      |                       ^
<source>:10:9: note: 'operator delete' has been explicitly marked deleted here
   10 |   void  operator delete(void* p) = delete;
      |         ^
1 error generated.
Compiler returned: 1

[keinflue]

I think Clang's behavior is correct.

In a new expression that uses operator new the compiler has to look for a matching operator delete which will be called if the construction of the new object throws an exception in order to free the allocated memory again.

The matching deallocation function in this case is void operator delete(void* p) = delete; in the class.

[basic.def.odr]p4.2 also clarifies that this implies that the operator delete is named for the purpose of odr-use, so it being defined as deleted should make the program ill-formed.

There currently doesn't seem to be an exception for the case where the construction of the new object can't fail with an exception. See open CWG 901 discussing this.

[keinflue]

Related, potential duplicate: #67405

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Yan-Jie Wang (yanjiew1)

Hello Clang maintainers,

If the delete operator in a class is explicitly deleted, the class cannot be created on the heap.

The following code cannot be compiled with Clang, but it ca be compiled with GCC.

#include <cstddef>
#include <cstdlib>

class TestClass {
public:
    void* operator new(std::size_t count) {
        return malloc(count);
    }

  void  operator delete(void* p) = delete;
  void  operator delete [](void* p) = delete;
    
};

int main(void)
{
    TestClass *test = new TestClass();

    return 0;
}

Please see this Compiler Explorer:

The is the error shown in the compiler explorer.

<source>:17:23: error: attempt to use a deleted function
   17 |     TestClass *test = new TestClass();
      |                       ^
<source>:10:9: note: 'operator delete' has been explicitly marked deleted here
   10 |   void  operator delete(void* p) = delete;
      |         ^
1 error generated.
Compiler returned: 1

[yanjiew1]

I found that setting the compiler flag -fno-exceptions would make that C++ program compiled with Clang.
However, explicitly defining a constructor with noexcept without setting the compiler flag -fno-exceptions does not work.

[cor3ntin]

@ojhunt

[shafik]

I am closing as a duplicate b/c the previous question has a lot of discussion already and we don't want to duplicate this here.