string_view to C-style string

To convert a std::string_view to a C-style string safely, you need to ensure the resulting C-style string is null-terminated.

While std::string_view provides a data() method to access the underlying character array, this array may not be null-terminated.

The safest approach is to create a std::string from the std::string_view and then use the c_str() method. Here's an example:

1#include <iostream>
2#include <string>
3#include <string_view>
4
5void printCString(const char* cstr) {
6  std::cout << cstr << '\n';
7}
8
9int main() {
10  std::string_view view{"Hello, world"};
11
12  // Convert std::string_view to std::string,
13  // then to C-style string
14  std::string str{view};
15  const char* cstr = str.c_str();  
16
17  printCString(cstr);
18}

1Hello, world

Direct Use of data()

If you are certain that the std::string_view references a null-terminated string, you can use data() directly, but this approach is risky and not recommended for general use:

1#include <iostream>
2#include <string_view>
3
4void printCString(const char* cstr) {
5  std::cout << cstr << '\n';
6}
7
8int main() {
9  std::string_view view{"Hello, world"};
10  printCString(view.data());  
11}

1Hello, world

Ensuring Null-Termination

For cases where you need to ensure null-termination, using std::string is the safest method. Alternatively, you can manually create a null-terminated string if performance is critical and you know the string length:

1#include <iostream>
2#include <string>
3#include <string_view>
4#include <vector>
5
6void printCString(const char* cstr) {
7  std::cout << cstr << '\n';
8}
9
10int main() {
11  std::string_view view{"Hello, world"};
12  std::vector<char> buffer(
13    view.begin(), view.end()
14  );
15  buffer.push_back('\0');  
16  const char* cstr = buffer.data();
17
18  printCString(cstr);
19}

1Hello, world

In summary, converting std::string_view to a C-style string safely typically involves creating a std::string and using its c_str() method to ensure null-termination.