Question

In C++, what is the difference between something like:

int i = 3;

and

int i(3);

and which is better?

Answer 1

For intrinsic types, they're identical. Even for your own defined classes, they should result in the same code if no or an empty constructor is explicitly defined. However, consider this:

class A
{
public:
A()
{
for(int i=0; i<100000; i++)
m_data += sqrtf(m_data);
}

A(const A &other)
{
m_data = other.m_data;
}

A &operator = (float val)
{
m_data = val;
}

private:
float m_data;
};


It's nonsensical, but illustrates the point. In this case, if you do for example

A myA;
myA = someOtherA;

you're executing the default constructor, which does a metric ton of work, only to then throw away that work by assigning via the = operator.
If instead you do

A myA(someOtherA);

it uses the copy constructor, which simply assigns the m_data value of the object using m_data of someOtherA.
Even if you don't explicitly define a copy constructor, C++ will create one for you (called the implicitly defined copy constructor), that simply copies the value of each member variable over using their own copy constructor. There's a pitfall here, because if the member variables of a class are complex types themselves, they may have time intensive copy constructors that are invoked by the implicitly generated copy constructor of your class without you noticing.