Calling constructors in c++ without new

Question

I've often seen that people create objects in C++ using

Thing myThing("asdf");

Instead of

Thing myThing = myThing("asdf");

This seems to work (using gcc), at least as long as there are no templates involved. My question now, is the first line correct and if so should I use it?

Answer 1

Both lines are in fact correct but do subtly different things.

The first line creates a new object on the stack by calling a constructor of the format Thing(const char*).

The second one is a bit more complex. It essentially does the following

1. Create an object of type Thing using the constructor Thing(const char*)
2. Create an object of type Thing using the constructor Thing(const Thing&)
3. Call ~Thing() on the object created in step #1

Answer 2

Yes, the first line is correct. I would use that syntax instead of the second one, because depending on your compiler, the second syntax will construct a temporary myThing and then pass it into your named variable using the copy assignment operator. (Note that it doesn't have to do this, but compilers are allowed to)

Answer 3

Quite simply, both lines create the object on the stack, rather than on the heap as 'new' does. The second line actually involves a second call to a copy constructor, so it should be avoided (it also needs to be corrected as indicated in the comments). You should use the stack for small objects as much as possible since it is faster, however if your objects are going to survive for longer than the stack frame, then it's clearly the wrong choice.

Answer 4

i assume with the second line you actuall mean:

MyThing *myThing = new MyThing("uiae");

which would be the standard way of creating new dynamic objects (necessary for dynamic binding and polymorphism) and storing their address to a pointer. your code does what JaredPar described, namely creating two objects, assigning with operator= and deleting one of them

MyThing myThing("uiae");

creates a static object which is destroyed automatically upon exiting the current scope

Answer 5

Ideally, a compiler would optimize the second, but it's not required. The first is the best way. However, it's pretty critical to understand the distinction between stack and heap in C++, sine you must manage your own heap memory.

Answer 6

The compiler may well optimize the second form into the first form, but it doesn't have to.

#include <iostream>

class A
{
    public:
        A() { std::cerr << "Empty constructor" << std::endl; }
        A(const A&) { std::cerr << "Copy constructor" << std::endl; }
        A(const char* str) { std::cerr << "char constructor: " << str << std::endl; }
        ~A() { std::cerr << "destructor" << std::endl; }
};

void direct()
{
    std::cerr << std::endl << "TEST: " << __FUNCTION__ << std::endl;
    A a(__FUNCTION__);
    static_cast<void>(a);
}

void assignment()
{
    std::cerr << std::endl << "TEST: " << __FUNCTION__ << std::endl;
    A a = A(__FUNCTION__);
    static_cast<void>(a);
}

void prove_copy_constructor_is_called()
{
    std::cerr << std::endl << "TEST: " << __FUNCTION__ << std::endl;
    A a(__FUNCTION__);
    A b = a;
    static_cast<void>(b);
}

int main()
{
    direct();
    assignment();
    prove_copy_constructor_is_called();
    return 0;
}

Output from gcc 4.4:

TEST: direct
char constructor: direct
destructor

TEST: assignment
char constructor: assignment
destructor

TEST: prove_copy_constructor_is_called
char constructor: prove_copy_constructor_is_called
Copy constructor
destructor
destructor

Answer 7

I played a bit with it and the syntax seems to get quite strange when a constructor takes no arguments. Let me give an example:

#include <iostream> 

using namespace std;

class Thing
{
public:
    Thing();
};

Thing::Thing()
{
    cout << "Hi" << endl;
}

int main()
{
    //Thing myThing(); // Does not work
    Thing myThing; // Works

}

so just writing Thing myThing w/o brackets actually calls the constructor, while Thing myThing() makes the compiler thing you want to create a function pointer or something ??!!