Confused with direct initialization and copy initialization

问题

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?

回答1:

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).

回答2:

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 as string str("123"). There is no doubt str("123") is directly initial

However remember that is possible only if the corresponding constructor is not explicit.

来源：https://stackoverflow.com/questions/16209540/confused-with-direct-initialization-and-copy-initialization