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];
4 Answers
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 (i.e. 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 valueCharT()(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.
Comments
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.
3 Comments
Christian Hackl
You should read all answers to your own link! stackoverflow.com/a/11752722/3313064 explains that your first sentence is incorrect since C++11.
Vishal Sharma
so std::string in C++11 and above is terminated by null character?
Moerwald
It is stated under this link: en.cppreference.com/w/cpp/string/basic_string/data
Simply use the string::c_str() method to get the const char *
string strText = "My String";
const char * chText = strText.c_str ();
Comments
There are two methods for this
You can use anyone based on taste, readability, etc.
std::string s;
std::getline(std::cin, s);
const char * a = s.c_str();
a = &b.c_str()[0](or simplya = b.c_str()in the case of index0).operator[]returns a reference and no, it is a guarantee, so using&b[0]orb.c_str()orb.data()is largely a matter of taste, at least since C++11.const char*simple useb.c_str().