= delete

Problem: Copy Is Not Always Wanted

A class that owns memory, for example …

  • Compiler generated default copy is buggy (only assigns pointer member)

  • All sorts of memory errors

    • Double free

    • Segmentation violation

    • …

code/c++11-delete-bogus-copy.cpp
class OwnsMemory
{
public:
    OwnsMemory(unsigned amount) : _mem{new char[amount]} {}
    ~OwnsMemory() { delete[] _mem; }

private:
    char* _mem;
};

int main()
{
    OwnsMemory om(100);
    OwnsMemory om_copy = om;
    return 0;
}

$ ./code/c++11-delete-bogus-copy
free(): double free detected in tcache 2
Aborted (core dumped)

C++ < 11 Solution: Make Copy private

Make those private (no implementation necessary)

  • Copy constructor

  • Assignment operator

⟶ Problem

  • Weird way of inhibiting bugs

  • Everyone has to know that idiom to understand what the compiler says

code/c++11-delete-private-copy.cpp
class OwnsMemory
{
public:
    OwnsMemory(unsigned amount) : _mem{new char[amount]} {}
    ~OwnsMemory() { delete[] _mem; }

private:
    char* _mem;

    OwnsMemory(const OwnsMemory&);
    void operator=(const OwnsMemory&);
};

int main()
{
    OwnsMemory om(100);
    OwnsMemory om_copy = om;
    return 0;
}

code/c++11-delete-private-copy.cpp:17:26: error: ‘OwnsMemory::OwnsMemory(const OwnsMemory&)’ is private within this context
   17 |     OwnsMemory om_copy = om;
      |                          ^~
code/c++11-delete-private-copy.cpp:10:5: note: declared private here
   10 |     OwnsMemory(const OwnsMemory&);
      |     ^~~~~~~~~~

C++ >= 11 Solution: = delete Copy

code/c++11-delete-delete-copy.cpp
class OwnsMemory
{
public:
    OwnsMemory(unsigned amount) : _mem{new char[amount]} {}
    ~OwnsMemory() { delete[] _mem; }

    OwnsMemory(const OwnsMemory&) = delete;
    void operator=(const OwnsMemory&) = delete;

private:
    char* _mem;
};

int main()
{
    OwnsMemory om(100);
    OwnsMemory om_copy = om;
    return 0;
}

code/c++11-delete-delete-copy.cpp:17:26: error: use of deleted function ‘OwnsMemory::OwnsMemory(const OwnsMemory&)’
   17 |     OwnsMemory om_copy = om;
      |                          ^~
code/c++11-delete-delete-copy.cpp:7:5: note: declared here
    7 |     OwnsMemory(const OwnsMemory&) = delete;
      |     ^~~~~~~~~~