如何在c ++中使用NULL（或0）初始化静态字符数组

Question

I attempted to initialize char array with NULL like this syntax.

char str[5] = NULL;

But it returned error.. How can I initialize char array with NULL or 0 in C++?

Concretely, I want to print "Hello" in this example code.

#include <iostream>
int main()
{
 char str[5] = NULL;
 if(!str)
  std::cout << "Hello" << std::endl;
 return 0;
}

This code will return error because of incorrect initialization. Then, what initializing sentence should I replace sentence with?

Answer 1

An array can not be null. Null is state of a pointer, and an array is not a pointer.

In the expression !str , the array will decay to the address of the first element. The type of this decayed address is a pointer, but you cannot modify the decayed pointer. Since the array is never stored in the address 0 (except maybe in some special case on an embedded system where an array might actually be stored at that memory location), !str will never be true.

What you can do, is initialize all of the sub-objects of the array to zero. This can be achieved using the value-initialization syntax:

char str[5]{};

As I explained earlier, !str is still not meaningful, and is always false. One sensible thing that you might do is check whether the array contains an empty string. An empty string contains nothing except the terminator character (the array can have elements after the terminator, but those are not part of the string).

Simplest way to check whether a string is empty is to check whether the first character is the terminator (value-initialized character will be a terminator, so a value initialized character array does indeed contain the empty string):

if (!str[0]) // true if the string is empty

Answer 2

//...
char str[5] = {'\0'};
if (str[0] != '\0')
//...

If you now put some characters into str it will print str up to the last '\\0'. Every string literal ends with '\\0', you must make sure your array ends with '\\0' too, if not, data will be read beyond your array (until '\\0' is encountered) and possibly beyond your application's memory space in which case your app will crash.

You should use std::string or QString however if you need a string.

Answer 3

You can't initialise a char array with NULL , arrays can never be NULL . You seem to be mixing up pointers and arrays. A pointer could be initialised with NULL .

You can initialise the chars of your array with 0, that would be

char str[5] = {0};

but I don't think that's what you're asking.

Answer 4

#include <stdio.h>
using namespace std;

int main() {

char str[5];
for(int i=0;i<5;i++){
    str[i]=NULL;
}
printf("success");
return 0;
}

Hope this helps.