I am confused with followed concepts:
string str="123"; Some books say that: using "=" is copy initialization,
but some articles say: string str="123" is same as string str("123"). There is no doubt str("123") is directly initialization.
So which style for string str="123";?
How to judge which is copy initialization or directly initialization?
It's simply a matter of grammar:
T x = y; is copy-initialization, and
T x(y); is direct-initialization.
This is true for any type T. What happens exactly depends on what sort of type T is. For primitive types (e.g. ints), the two are exactly the same. For class-types (such as std::string), the two are practically the same, though copy-initialization requires that a copy-constructor be accessible and non-explicit (though it will not actually be called in practice).
char const *. In principle, there could be two constructor calls, conversion followed by copying, but the second one is usually elided.Yes that is called copy initialization.
Instead of default constructing str and then constructing another string from "123" using string(const char*) and then assigning the two strings, the compiler just construct a string using string(const char*) with "123".
string str="123"is same asstring str("123"). There is no doubt str("123") is directly initial
However remember that is possible only if the corresponding constructor is not explicit.
explicit, private, or delete'ed, you might notice a difference. For std::string that is not the case.