string object in c++

Question

I was reading a book on c++ when I encountered the following example:

#include <iostream>
#include <string>

using namespace std;

int main () {
 const char *message = "how do you do\n";

 string s(message); 
 cout << s << " and its size:" << s.size() << endl;
}

I wanted to know what exactly does it do. How can we pass a variable inte another variable as done in s(message)? Thanks in advance

Answer 1

s(message) actually calls the constructor of std::string, which constructs a new object of this type from the given character array pointed to by message. s is just an arbitary name given to the string object. std::string is C++'s idiomatic object for working with strings, it is usually preferred over raw C strings.

Consider this simple sample:

// Declare a fresh class
class A {

   public:

      // a (default) constructor that takes no parameters and sets storedValue to 0.
      A() {storedValue=0;}

      // and a constructor taking an integer
      A(int someValue) {storedValue=someValue;}

   // and a public integer member
   public:
      int storedValue;
};

// now create instances of this class:
A a(5);

// or
A a = A(5);

// or even
A a = 5;

// in all cases, the constructor A::A(int) is called.
// in all three cases, a.storedValue would be 5

// now, the default constructor (with no arguments) is called, thus
// a.storedValue is 0.
A a;

// same here
A a = A();

std::string declares various constructors, including one that accepts a const char* for initialization - the compiler automatically chooses the right constructor depending on the type and number of the arguments, so in your case string::string(const char*) is choosen.

Answer 2

That is actually one of the constructors of std::string.

In C++, you can create objects a few different ways.

std::string s = "Hello"; // implicit constructor using const char *
std::string s = std::string("Hello"); // invoke the const char* constructor of std::string
std::string s("Hello"); // another way to do the stuff above

There are more ways than that, but just to demonstrate how you could create this std::string object.