C++: initialize char array with a string?

Question

Let's say I have a string called garbage.

Whatever's in garbage, I want to make a char array out of it. Each element would be one char of the string.

So, code would be similar to:

const int arrSize = sizeof(garbage); //garbage is a string
char arr[arrSize] = {garbage};

But, this will give an error "cannot convert string to char in initialization".

What is the correct way to do this? I just want to feed the darn thing a string and make an array out of it.

Assuming by string, you mean std::string, sizeof(garbage) probably doesn't do what you think it does. It just gives you the size of a std::string object, which is a compile time constant that has no relation to the length of the string, which is a runtime value. — Benjamin Lindley
– Benjamin Lindley, Commented Dec 4, 2012 at 1:14
You are correct, Ben. sizeof(garbage) does not give me what I want. Good catch! garbage.size() is the correct method. — derp
– derp, Commented Dec 4, 2012 at 1:20
Yes, but then arrSize cannot be used to specify the size of arr, because it would no longer be a compile time constant. — Benjamin Lindley
– Benjamin Lindley, Commented Dec 4, 2012 at 1:22

Matt Kline · Accepted Answer · 2012-12-04 01:10:52Z

14

C++ std::string maintains an internal char array. You can access it with the c_str() member function.

#include <string>
std::string myStr = "Strings! Strings everywhere!";
const char* myCharArr = myStr.c_str();

Keep in mind that you cannot modify the internal array. If you want to do so, make a copy of the array and modify the copy.

answered Dec 4, 2012 at 1:10

Matt Kline

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4 Comments

Syntactic Fructose Over a year ago

why a std::string would need to be converted to a char* is beyond me, but a good answer to this question.

Justin Time - Reinstate Monica Sep 18 at 19:33

Most common reason is for passing it to a function that only accepts C strings. As long as the function doesn't need to change the string, you can just pass it your std::string's internal buffer directly.

derp Over a year ago

Exactly what I wanted. And to answer the question above, I'm implementing a string randomizer. I figure I can randomize to get a number in the range of the string size, and then pull a char from the string. Might be good for password generation if I have the entire alphanumeric set as the string.

Matt Kline Over a year ago

You could do what you want without getting the array - myStr.size() will return the string's length, and you can access any character in the string at index i with myStr[i].

double-beep · Accepted Answer · 2019-05-24 08:19:44Z

9

I think what you're after is a lot simpler:

char arr[] = "some interesting data (garbage)";

edited May 24, 2019 at 8:19

double-beep

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answered May 24, 2019 at 8:18

JJ on SE

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1 Comment

Mo Jo Over a year ago

Thank you, This answered the reason I came to this question. I am trying to convert a piece of C code to C++. In the C code, it declares a 'char * foo = "string literal" ', and foo is expected to be a char *, not a const char *, in a later function.

Kerrek SB · Accepted Answer · 2012-12-04 01:11:41Z

0

You could just use garbage.data(), which is already a pointer to the first element of an array of chars containing your string data.

answered Dec 4, 2012 at 1:11

Kerrek SB

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2 Comments

Luchian Grigore Over a year ago

I don't think the initialization would work, since arr is not const.

Kerrek SB Over a year ago

@LuchianGrigore: No, of course, but I wouldn't even initialize anything. Just use the data directly. If you need a copy, copy the string.

n4feng · Accepted Answer · 2015-11-12 20:00:30Z

-1

Or here is a heavy way to do so.

#include <string>
std::string temp = "something";
char* myChar = new char(temp.length());
for(int i = 0; i < temp.length(); ++i){
    a[i] = temp[i];
}

answered Nov 12, 2015 at 20:00

n4feng

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2 Comments

n4feng Over a year ago

Kind reminder: remember to delete "a" after finished using it.

DragonLord Over a year ago

and by "a[i]" he means "myChar[i]". But char arr[]="something"; is more concise.

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