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

问题:

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?

回答1:

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.

回答2:

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.

回答3:

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).

回答4:

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++).

回答5:

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.