CODEWITHSUNDEEP
c++
glaxy
glaxy

Reputation: 63

Are std::string with null-character possible?

I initialized a C++ string with a string literal and replaced a char with NULL.

When printed with cout << the full string is printed and the NULL char prints as blank.

When printed as c_str the string print stop at the NULL char as expected.

I'm a little confused. Does the action came from cout? or string?

int main(){
  std::string a("ab0cd");
  a[2] = '\0'; // '\0' is null char

  std::cout << a << std::endl; // abcd
  std::cout << a.c_str() << std::endl; // ab
}

Test it online.

I'm not sure whether the environment is related, anyway, I work with VSCode in Windows 10

Upvotes: 4

Views: 2424

Answers (3)

Lukas-T
Lukas-T

Reputation: 11360

First you can narrow down your program to the following:

#include <iostream>
#include <string>

int main(){
    std::string a("ab0cd");
    a[2] = '\0'; // replace '0' with '\0' (same result as NULL, just cleaner)

    std::cout << a << "->" << a.c_str();
}

This prints

abcd->ab

That's because the length of a std::string is known. So it will print all of it's characters and not stop when encountering the null-character. The null-character '\0' (which is equivalent to the value of NULL [both have a value of 0, with different types]), is not printable, so you see only 4 characters. (But this depends on the terminal you use, some might print a placeholder instead)

A const char* represents (usually) a null-terminated string. So when printing a const char* it's length is not known and characters are printed until a null-character is encountered.

Upvotes: 5

user12450543
user12450543

Reputation:

string end limit (boundary) is not 0 (NULL) like simple char* but its size keep internally in its member data as it's actually user-defined type (an instantiated object) as opposed to primitive type, so

int main(){
  string a("abc0d");
  a[3] = 0; // '\0' is null char
    a.resize(2);
  std::cout << a << std::endl; // ab
  std::cout << a.c_str() << std::endl; // ab
}

i'm sorry change your code to be more comfortable, watch as it results in

ab
ab

good learning: http://www.cplusplus.com/reference/string/string/find/index.html

Upvotes: 0

Orace
Orace

Reputation: 8359

Contrary to what you seem to think, C++ string are not null terminated.

The difference in behavior came from the << operator overloads.

This code:

cout << a.c_str(); // a.c_str() is char*

As explained here, use the << overloads that came with cout, it print a char array C style and stop at the first null char. (the char array should be null terminated).

This code:

cout << a; // a is string

As explained here, use the << overloads that came with string, it print a string object that internally known is length and accept null char.

Upvotes: 2

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