Extraction from C++ Primer 5th. Editioin 3.2.1
C++ has several different forms of initialization, we should understand how these forms differ from one aother.
When we initialize a variable using =, we are asking the compiler to copy initialize the object by copying the initializer on the right-hand side into the object being created.
Ohterwise when we omit the =, we use direct intialization.When we have a single intializer, we use either the direct or copy form of intialization. When we intialize a variable from more than one value, such as:
string s4(, 'c');
we must use the direct form of initialization.
when we want to use several values, we can indirectly use the copy form of intialization be explicitly creating a (temporary) object to copy:
string s8=string(, 'c'); //copy initialization
The intializer of s8——string（10, 'c')——creates a string of the given size and character value and then copies that value into s8. it is as if we had written
string temp(, 'c');
string s8=temp;
Although the used to intialize s8 is legal, it is less readable and offers no compensating advantage over the way we intilize s4.