I want to construct a std::string object like an array:
std::string str("");
str[0] = 'A';
str[1] = 'b';
str[2] = 'h';
str[3] = 'i';
str[4] = '\0';
std::cout<<str;
But it doesnt print the string. What am i missing?
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5 Answers
Firstly, std::string is not a C-string. You do not need to NULL-terminate it. Secondly, the [] operator is only valid for indices which are < std::string::length(), meaning that at least N elements must be allocated in advance before you can access an element between 0 and N-1.
std::string str(4); // construct a string of size 4
str[0] = 'A';
str[1] = 'b';
str[2] = 'h';
str[3] = 'i';
std::cout << str;
Edit: But also see Johnsyweb's answer. The big advantage of std::string over C-strings is that you don't have to worry about memory allocation. You can use the += operator or push_back member function, and can build the string character-by-character without worrying about how much memory to reserve.
2 Comments
badmaash
shouldn't it be std::string(4, ' '); ?
David Rodríguez - dribeas
@Abhi: the second argument is optional, if not present it will default to
'\0' (instead of the space). It is up to you to determine whether you prefer uninitialized elements to be spaces or NUL characters.Try
std::string (4, ' ');
instead of
std::string("");
basic_string's operator[] returns a reference to the specified character, but since your string is empty, it contains no characters.
2 Comments
CashCow
Actually I don't like that constructor of std::string because if you can't remember the order and put the character first it will still compile and give you the wrong thing. If you do std::string(' ', 4) it will give you a string of length 32 (ASCII value of space) filled with ASCII 4 characters. Unfortunate problem of char having implicit conversions with size_t.
Matthieu M.
@CashCow: I agree, it's unseemly that
char would both represent a character and behave as an integer...What am i missing?
You are missing the whole point of using a std::string. That approach may work for arrays of char, but not for strings.
Consider std::string::operator += instead.
2 Comments
Charles Salvia
In all likelihood, this is actually the best answer.
johnsyweb
To be honest, I cannot even understand why anyone would want to take that approach with a
char array.You allocated the string to be "", that is, exactly 0 bytes long.
You are then trying to write chars outside of bounds of the string - which doesn't work.
2 Comments
badmaash
I see. Isnt there a way to modify the string declared above then except using insert()?
David Rodríguez - dribeas
@Abhi: Look at the documentation for string ( SGI, C++Reference ). There are ( too ) many different member functions that operate on the string, including
resize() that will, well, resize the string so you can make it grow.you should create a space in the memory for your array
using namespace std;
char str[5];
str[0] = 'A';
str[1] = 'b';
str[2] = 'h';
str[3] = 'i';
str[4] = '\0';
cout << str ;
1 Comment
badmaash
i want to use std::string. I know i can do that with a char array.
