CODEWITHSUNDEEP
c++
aryan
aryan

Reputation: 19

what is the difference between string a="hello" and string a=(char *)"hello" in c++?

What is the difference between these two string initializations in c++? i get the same output in both the programs. Program 1

void main(){
    string a = "hello";
    cout<<a;
}

program 2

void main(){
    string a = (char *)"hello";
    cout<<a;
}

Upvotes: 1

Views: 236

Answers (3)

Om Choudhary
Om Choudhary

Reputation: 512

std::string is a typedef for a specialization of basic_string class templated on a char (objects that represent sequences of characters).

The expression string a = "hello" corresponds to a stream of characters whose size is allocated statically.

Short, simple and typesafe std::cout is a typedef of ostream class templated on standard objects (provides support for high-level output operations).

cout means "the standard character output device", and the verb << means "output the object".

cout << a; sends the string like a stream to the stdout.

char * are special pointers to a constant character, they point to ASCII strings e.g.:

const char * s = "hello world";

The expression (char *)"hello" corresponds to a char * pointer, where you are casting away const, but then the constructor immediately puts the const back on the call.

cout, therefore, will print a string because it has a special operator for char * wich it will treat as a pointer to (the first character of) a C-style string that outputs strings.

char* or const char*, cout will treat the operand as a pointer to (the first character of) a C-style string, and prints the contents of that string:

Upvotes: 0

BoBTFish
BoBTFish

Reputation: 19757

"hello" is a string literal. Its type is

const char[N], where N is the size of the string [...] including the null terminator.

So in this case, the type is const char[6]. Note the const.

Now std::string can be constructed (constructor 5 in the link) from a const char*. Again, note the const.

In C++, you can pass a non-const object into a function that expects a const. In your case, your cast (char *) removes the const, but then immediately puts the const back on in the constructor call.

So basically, no difference. They will compile to exactly the same thing.

A few additional notes:

  1. casting away const-ness is very dangerous. If you had actually tried to change anything in the char array, your program would have had Undefined Behaviour.
  2. using namespace std; is widely considered to be bad practice.
  3. void main() is not a valid signature for the main function; it must return an int.
  4. Using C-style casts in C++ can also be considered a bad practice - it is harder to spot in code, and C++ provides safer equivalents for more specific situations: const_cast, static_cast, dynamic_cast, and (the most dangerous) reinterpret_cast.

Upvotes: 5

kiran Biradar
kiran Biradar

Reputation: 12732

Nothing actually, that additional char * type casting is not needed.

In both the cases copy initialization is done.

string (const char* s);

read more on copy initialization

Upvotes: 4

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