Is there a difference between copy initialization and direct initialization?

Question

Suppose I have this function:

void my_test()
{
    A a1 = A_factory_func();
    A a2(A_factory_func());

    double b1 = 0.5;
    double b2(0.5);

    A c1;
         


        

Answer 1

double b1 = 0.5; is implicit call of constructor.

double b2(0.5); is explicit call.

Look at the following code to see the difference:

#include 
class sss { 
public: 
  explicit sss( int ) 
  { 
    std::cout << "int" << std::endl;
  };
  sss( double ) 
  {
    std::cout << "double" << std::endl;
  };
};

int main() 
{ 
  sss ffffd( 7 ); // calls int constructor 
  sss xxx = 7;  // calls double constructor 
  return 0;
}

If your class has no explicit constuctors than explicit and implicit calls are identical.