Following the first answer to this question, I ran into an issue with overriding variables. This is the code I wrote to find the root of the problem:

#include <iostream>

using namespace std;

class foo
{
private:
    int *bar;
    int l;
public:
    foo(int bar);
    ~foo();
    void print_bar() {
        cout << "bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
    }
};


foo::foo(int l) {
    this->l = l;
    cout << "Creating with length " << l << endl;
    bar = new int[l];
    cout << "{"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
    for(size_t i = 0; i < l;   i) {bar[i] = 0;}
    cout << "Created with bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
}
foo::~foo() {
    cout << "Deleted with bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
    delete[] bar;
}


int main() {
    foo a = foo(1);
    a = foo(2);
    a.print_bar();
    return 0;
}

This outputs:

Creating with length 1
{0, }
Created with bar = {0, }
Creating with length 2
{0, 0, }
Created with bar = {0, 0, }
Deleted with bar = {0, 0, }
bar = {1431655787, 5, }
Deleted with bar = {1431655787, 5, }

Instead of what I expected:

Creating with length 1
{0, }
Created with bar = {0, }
Creating with length 2
{0, 0, }
Created with bar = {0, 0, }
Deleted with bar = {0, }
bar = {0, 0, }
Deleted with bar = {0, 0, }

How can I avoid this, or is there a better way to have a class with a variable length array as a parameter?

(Please keep in mind when answering that I'm very new to c .)

CodePudding user response：

in the line, a=foo(2), the default assignment was called, but the service it provided is not right in your case(it just copy memory byte to byte). You need to supply a customised copy of assignment operator, in which you need to destruct *this first.

class foo{
...
public:
   foo& operator= (const foo& f)
   {
       this->~foo(); // call the destructor explicitly !
       bar = nullptr; l= 0; // for exception safety.
       new (this)(f); // call the copy constructor to construct
                      // a foo object on this which can be regarded as
                      // raw memory (it's still a valid foo object)
                      // but treating it as raw memory has no harm
                      // as all resources (here memory) has been released.

                     // btw, this is placement new, 
                     // #include <new> 
                     // before you can use it.
       return *this;
   }

   // better yet in your case, a move assignment will be more efficient
   foo& operator = (foo&& f)
   {
         std::swap(*this, f); // this lazy way will do for your purpose
                             // experiment and see what actually happened
                             // and figure out why it's like that
         return *this;
   }

My bad. You don't have a copy constructor defined yet. You can define one like this.

      foo(const foo& f):foo(f.l)
      {
          for(int i=0; i<l;   i)
             bar[i] = f.bar[i];
      }

And in the main function

int main() {
    foo a = foo(1);
    foo b = foo(3);
    a = foo(2); // this will call the move assignment operator
    a = b; // this will call the copy assignment operator
    a = std::move(b); // this will call the move assignment operator
             // make sure you experiment with all likelyhood
            // and understand the logic behind.
           // don't trust me or anybody without actual trying
           // and playing
    a.print_bar();
    return 0;
}

CodePudding user response：

Thanks to PaulMcKenzie and Eljay in the comments, I switched to std::vector to use as the variable length array. Here's the updated (and working as expected) code for anyone confused:

#include <iostream>
#include <vector>

using namespace std;

class foo
{
private:
    vector<int> bar;
    int l;
public:
    foo(int bar);
    ~foo();
    void print_bar() {
        cout << "bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
    }
};


foo::foo(int l) {
    this->l = l;
    cout << "Creating with length " << l << endl;
    for(size_t i = 0; i < l;   i) {bar.push_back(0);}
    cout << "Created with bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
}
foo::~foo() {
    cout << "Deleted with bar = {"; for (int i = 0; i < l; i  ) {cout <<   bar[i] << ", ";} cout << "}" << endl;
}


int main() {
    foo a = foo(1);
    a = foo(2);
    a.print_bar();
    return 0;
}

(foo::~foo() doesn't have to be defined, I just left it there for consistency)