CODEWITHSUNDEEP
c++program-entry-point
Frustrated Coder
Frustrated Coder

Reputation: 4687

Why is main() argument argv of type char*[] rather than const char*[]?

When I wrote the following code and executed it, the compiler said

deprecated conversion from string constant to char*

int main()  
{  
  char *p;  
  p=new char[5];  
  p="how are you";  
  cout<< p;  
  return 0;  
}

It means that I should have written const char *.

But when we pass arguments into main using char* argv[] we don't write const char* argv[].

Why?

Upvotes: 31

Views: 17084

Answers (5)

Brian Roach
Brian Roach

Reputation: 76908

Because ... argv[] isn't const. And it certainly isn't a (static) string literal since it's being created at runtime.

You're declaring a char * pointer then assigning a string literal to it, which is by definition constant; the actual data is in read-only memory.

int main(int argc, char **argv)  {
    // Yes, I know I'm not checking anything - just a demo
    argv[1][0] = 'f';
    std::cout << argv[1] << std::endl;
}

Input:

g++ -o test test.cc

./test hoo

Output:

foo

This is not a comment on why you'd want to change argv, but it certainly is possible.

Upvotes: 13

Šimon T&#243;th
Šimon T&#243;th

Reputation: 36433

If you look at execution functions like execve, you will see that they actually don't accept const char* as parameters, but do indeed require char*, therefore you can't use a string constant to invoke main.

Upvotes: 0

Xeo
Xeo

Reputation: 131799

why is it required for char* to be constant while assigning it to a string

Because such literal strings (like "hi", "hello what's going on", etc), are stored in the read-only segment of your exe. As such, the pointers that point to them need to point to constant characters (eg, can't change them).

Upvotes: 1

Andre Holzner
Andre Holzner

Reputation: 18675

You are assigning a string constant (const char*) to a pointer to a non-constant string (char *p). This would allow you to modify the string constant, e.g. by doing p[0] = 'n'.

Anyway, why don't you use std::string instead ? (you seem to be using C++).

Upvotes: 1

user2100815
user2100815

Reputation:

Historical reasons. Changing the signature of main() would break too much existing code. And it is possible that some implementations allow you to change the parameters to main from your code. However code like this:

char * p = "helllo";
* p = 'x';

is always illegal, because you are not allowed to mess with string literals like that, so the pointer should be to a const char.

Upvotes: 12

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