Calling constructors in c++ without new

Question

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

Thing myThing("asdf");

Instead of this:

Thing myThing = Thing("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

I assume with the second line you actually mean:

Thing *thing = new Thing("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 (one passed a const char*, the other passed a const Thing&), and then calling the destructor (~Thing()) on the first object (the const char* one).

By contrast, this:

Thing thing("uiae");

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

Answer 3

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); // avoid warnings about unused variables
}

void assignment()
{
    std::cerr << std::endl << "TEST: " << __FUNCTION__ << std::endl;
    A a = A(__FUNCTION__);
    static_cast<void>(a); // avoid warnings about unused variables
}

void prove_copy_constructor_is_called()
{
    std::cerr << std::endl << "TEST: " << __FUNCTION__ << std::endl;
    A a(__FUNCTION__);
    A b = a;
    static_cast<void>(b); // avoid warnings about unused variables
}

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 4

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 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

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 ??!!

Answer 7

In append to JaredPar answer

1-usual ctor, 2nd-function-like-ctor with temporary object.

Compile this source somewhere here http://melpon.org/wandbox/ with different compilers

// turn off rvo for clang, gcc with '-fno-elide-constructors'

#include <stdio.h>
class Thing {
public:
    Thing(const char*){puts(__FUNCTION__ );}
    Thing(const Thing&){puts(__FUNCTION__ );}   
    ~Thing(){puts(__FUNCTION__);}
};
int main(int /*argc*/, const char** /*argv*/) {
    Thing myThing = Thing("asdf");
}

And you will see the result.

From ISO/IEC 14882 2003-10-15

8.5, part 12

Your 1st,2nd construction are called direct-initialization

12.1, part 13

A functional notation type conversion (5.2.3) can be used to create new objects of its type. [Note: The syntax looks like an explicit call of the constructor. ] ... An object created in this way is unnamed. [Note: 12.2 describes the lifetime of temporary objects. ] [Note: explicit constructor calls do not yield lvalues, see 3.10. ]

---

Where to read about RVO:

12 Special member functions / 12.8 Copying class objects/ Part 15

When certain criteria are met, an implementation is allowed to omit the copy construction of a class object, even if the copy constructor and/or destructor for the object have side effects.

Turn off it with compiler flag from comment to view such copy-behavior)