为什么我们不需要对象来存储字符串数据？

Question

In C++, if string is a class, why do we not need the dot operator or an object to store data in a string?

Classic string:

string str = "ABC";

Why can we directly pass ABC using " " instead of doing it like

string str;

str.data = "ABC";

But we need to use objects to access the functions. Example:

str.length();

Why do we do this?

Is string some special kind of class?

Answer 1

string str = "ABC"; is not assignment. It is construction. Specifically it calls the std::string constructor taking a const char * argument.

It's the same as doing

string str("ABC");

just different syntax.

Assignment also works. You do this:

string str;
str = "ABC";

See also:

Copy initialization

std::string constructors

std::basic_string::operator=

Answer 2

std::basic_string has a constructor like this:

basic_string( const CharT* s, const Allocator& alloc = Allocator() );

Constructs the string with the contents initialized with a copy of the null-terminated character string pointed to by s.

But the important point to note is that this constructor is not explicit , thus the compiler can do implicit conversion of null terminated character string during constructor call.

For example, following code compiles without any issue:

class Foo {
public:
    Foo(int) {}
};

int main() {
    Foo f = 10;
}

It won't compile if the constructor is written as:

explicit Foo(int) {}

Answer 3

In C++ a string literal is not a std::string , but a C style character array( char[N] ). And yes std::string or any other 3rd party string type that you may see is a class with a converting constructor accepting character arrays as input. More precisely, std::string is a type alias for an instansiation of the template std::basic_string . In short words, before you can do anything with a string literal, you'd better convert it to a string:

std::string{"ABC"}.size()

Or you will have to switch to C API which is not recommended for beginners:

strlen( "ABC")