"MyClass mc = MyClass()" or "MyClass mc"?

Question

What is the difference between

MyClass mc = MyClass();

and

MyClass mc;

in C++?

Answer 1

The first invokes copy constructor, with temporary object as parameter - MyClass() creates temporary.

The second invokes default constructor.

In reality, they are, in most cases, optimized out to the same code, but that's the semantical difference.

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As Negal mentioned, the case is a bit different with POD types; when "MyClass" is POD, the second snippet will not value-initialize mc, whereas first will.

Answer 2

The first one is copy initialization and the 2nd one is default initialization.

For example, the following code will not compile:

class MyC
{
public:
MyC(){}
private:
MyC(const MyC&) {}
};

int main()
{
  MyC myc = MyC();
  return 0;
}