Converting string into const char *

Question

Is there anything bad about the following code? Although it runs fine, but should I have allocated a memory to the character pointer first before initializing it?

const char *a;
string  b;
getline(cin, b) ;
a=&b[0u];

Answer 1

It is fine since C++11.

Before C++11, there was no formal guarantee that operator[] would return a reference to a character that would be part of a null-terminated character array (ie a C-style string). One consequence of that missing guarantee was that &b[0u] would have been undefined behaviour if b was an empty non- const string.

^{(Actual implementations typically behaved correctly anyway, because that's the only sane way of implementing std::string , but that's another story.)}

See also http://en.cppreference.com/w/cpp/string/basic_string/operator_at :

 reference operator[]( size_type pos ); 

(...)

If pos == size() , a reference to the character with value CharT() (the null character) is returned.

(since C++11)

Still, the code you've posted is not particularly good style. Why create a pointer with an uninitialised value and then assign it a value later on, and why bother with the more complicated syntax?

Here's an improved version of the code:

std::string b;
std::getline(std::cin, b);
auto const a = b.c_str();

In this version, a is const , so you cannot accidentally make it point to something else; you also make the compiler deduce the type ( char const* ) automatically. And c_str() is a clearer way of saying what your code actually means.

Answer 2

As suggested by @goodvibration you should use c_str -> Reason: If you're using a compiler that doesn't support C++11 and higher versions of the standard, the std::string object won't include an appropriate null termination, which signals the end of the string when working with c-like methods. c_str takes care about the correct "format". See this stackoverflow link for additional information.

Hope that helps.

Answer 3

Simply use the string::c_str() method to get the const char *

string strText = "My String";
const char * chText = strText.c_str ();

Answer 4

There are two methods for this

1. std::string::c_str
2. std::string::data

You can use anyone based on taste, readability, etc.

std::string s;
std::getline(std::cin, s);
const char * a = s.c_str();