CODEWITHSUNDEEP
c++string
mab0189
mab0189

Reputation: 136

Is std::string str(array.begin(), array.end()) adding the null character on its own?

I am very new to C++ and i was wondering if std::string str(array.begin(), array.end()) is adding a null character on its own at the end of the string? I looked up this reference but i could not figure it out. I appreciate your answers!

Edit: I am using C++11 and I have an boost::array<u_int8_t> of uknown size. My goal ist to print it afterwards with str.c_str() because %s is expecting a char*.

str = std::string(arr.begin(),arr.end());
ROS_INFO("Got response: \n %s", str.c_str());

Upvotes: 4

Views: 829

Answers (3)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122133

Since C++11 std::string must contain a terminating null character. However, a null character in a std::string does not necessarily terminate the std::string.

I hope it gets more clear with the following example:

#include <string>
#include <iostream>

int main() {
    std::string x{"Hello World"};

    std::cout << x.c_str() << "\n";

    x[5] = '\0';
    std::cout << x << "\n";
    std::cout << x.c_str();
}

prints:

Hello World
HelloWorld
Hello

We can get a null-terminated c-string via c_str to get the expected output. The << overload knows where to stop because there is a null-terminator. Though after adding a \0 in the middle of the std::string we can still print the whole string with the << overload for std::string. Calling c_str again, will again return a pointer to a c-string with 10 characters + the terminating \0, but this time the << overload for char* stops when it encounters the first \0, because thats what indicates the end of a c-string.

TL;DR: Unless you need to get a c-string from the std::string you need not worry about adding the null-terminator. std::string does that for you. On the other hand you should be aware that std::string can contain null characters also in the middle not only at their end.

Upvotes: 6

user14215102
user14215102

Reputation:

https://en.cppreference.com/w/cpp/string/basic_string

null-terminated (since C++11)CharT[] array.

C++11 standard and later guarantee null-termination.

Also here: https://en.cppreference.com/w/cpp/string/basic_string/c_str

c_str() and data() perform the same function. (since C++11)

As the return value of c_str, being a C-style string has to be null-terminated. In earlier standards, if you wanted null-termination, you would have to call c_str() on the string at least once to be sure.

Upvotes: 3

Sam Varshavchik
Sam Varshavchik

Reputation: 118292

The C++ standard specifies that c_str() returns a null-terminated string (the contents of the string followed by a '\0'). Depending on C++ standard version, you may or may not be guaranteed that str[str.size()] also gives you a '\0'.

That's the only thing that the C++ standard specifies. Whether the '\0' is originally added by its constructor, or not, including this specific constructor: this is unspecified.

However, since you are guaranteed to get that all-important '\0' when you go look for it, this is a moot point. It will be there.

Upvotes: 7

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