How to make std::string compatible with char*?

Question

I usually find some functions with char* as its parameter.But I heard that std::string is more recommended in C++. How can I use a std::string object with functions taking char*s as parameters? Till now I have known the c_str(), but it doesn't work when the content of string should be modified.

Answer 1

For that purpose, you use std::string::data(). Returns a pointer to the internal data. Be careful not to free this memory or anything like that as this memory is managed by the string object.

Answer 2

You can use the address of the first element after C++11 like this:

void some_c_function(char* s, int n);

// ...

std::string s = "some text";

some_c_function(&s[0], s.size());

Before C++11 there was no guarantee that the internal string was stored in a contiguous buffer or that it would be null terminated. In those cases making a copy of the string was the only safe option.

After C++17 (the current standard) you can use this:

some_c_function(s.data(), s.size());

In C++17 a non-const return value of std::string::data() was added in addition to the const version.

Answer 3

Since C++17 std::string::data() returns a pointer to the underlying char array that does allow to modify the contents of the string. However, as usual with strings, you may not write beyond its end. From cppreference:

Modifying the past-the-end null terminator stored at data()+size() to any value other than CharT() has undefined behavior.