How to retrieve the specific element from an array of std::strings as a LPCSTR?

Question

In C++, I've got a string array variable called:

...
/* set the variable */
string fileRows[500];
...
/* fill the array with a file rows */
while ( getline(infile,sIn ) )
{
    fileRows[i] = sIn;
    i++;
}

and an object which has this:

string Data::fileName(){
    return (fileRows);
}

I would like to make a function which return an array, and after that i would like to call it something like this:

Data name(hwnd);
MessageBox(hwnd, name.fileName(), "About", MB_OK);

But i get this error:

cannot convert 'std::string* {aka std::basic_string}' to 'LPCSTR {aka const char}' for argument '2' to 'int MessageBoxA(HWND, LPCSTR, LPCSTR, UINT)'

If i would like to show the 5. element of the array, how to convert it?

`MessageBox(hwnd, name.fileName().c_str(), "About", MB_OK);`? — Blake, Mar 13 '13 at 21:44

LihO · Answer 1 · 2013-03-13T22:12:07.797

5

LPCSTR is nothing else but an alias for const char*. The problem is that the Data::fileName() returns a std::string object and there's no implicit conversion to const char*.

To retrieve a string from the std::string in form of const char*, use c_str() method , :

MessageBox(hwnd, name.fileName().c_str(), "About", MB_OK);

Also note that you have created an array of std::string objects:

string fileRows[500];

but in the Data::fileName() you're trying to return it as a single std::string object:

string Data::fileName() {
    return fileRows;
}

I recommend you to use std::vector instead of C-style array though.

If i would like to show the 5. element of the array, how to convert it?

No matter whether you will use std::vector or keep using an array, it will look like this:

std::string Data::fileName() {
    return fileRows[4];
}

edited Mar 13 '13 at 22:12

answered Mar 13 '13 at 21:45

LihO

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2

`LPCSTR` is `char const*`. :-] – ildjarn Mar 13 '13 at 21:46
I got this error: error: could not convert '(std::string*)(&((Data*)this)->Data::fileRows)' from 'std::string* {aka std::basic_string*}' to 'std::string {aka std::basic_string}' – szuniverse Mar 13 '13 at 21:53
Do you know how to convert this error, If I would like to show the 5. element of the array? – szuniverse Mar 13 '13 at 21:58
@David: There's another error that I overlooked before. Check my answer now :) – LihO Mar 13 '13 at 21:59
yes i know because i tried several ways to return a string array. I tried with: string* Data::fileName()... Data::fileName().. but none of them worked, beacause i dont know how to display an array element in the messagebox.. – szuniverse Mar 13 '13 at 22:03
@David: `std::vector fileRows` will allow you to treat `fileRows` almost in the same way as an array, but it will be flexible in size. I also answered your last question, check my answer now :) – LihO Mar 13 '13 at 22:19

J. Calleja · Accepted Answer · 2013-03-13T22:34:23.877

2

fileRows is an array of 500 elements. If you want to return the array so that you can later access the n-th element, you should return the pointer to the beginning of the array. For example:

string* Data::fileName(){
        return fileRows;
}

Although it is probably better to use:

const string& Data::getFileName(size_t index){
        return fileRows[index];
}

Using the first method, you can access the n-th element using:

data.filename()[n];

So, if you want to access the 5th element of the array you should use:

data.filename()[4];

On the other hand, the function MessageBox needs a const char *. So you must call the c_str() method to get the pointer:

Data name(hwnd);
MessageBox(hwnd, name.fileName()[4].c_str(), "About", MB_OK);

edited Mar 13 '13 at 22:34

answered Mar 13 '13 at 22:03

J. Calleja

4,855
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Thanks a lot that was the problem! :-) Thank you! – szuniverse Mar 13 '13 at 22:10
-1 `fileName[5]` is not 5th element. – LihO Mar 13 '13 at 22:16
@LihO You are right, but I didn't say it was. Anyway, I will update the answer to clarify. – J. Calleja Mar 13 '13 at 22:24
1

Right, Milla Jovovich the 5th element. – Benjamin Lindley Mar 13 '13 at 22:25

score 1 · Answer 3 · answered Mar 13 '13 at 21:47

1

std::string::c_str will give you a pointer to an array that contains a null-terminated sequence of characters (i.e., a C-string) or LPCSTR

answered Mar 13 '13 at 21:47

Jacob Seleznev

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score 0 · Answer 4 · edited May 23 '17 at 11:57

0

use std:string's function c_str() ... take a look at this answer

edited May 23 '17 at 11:57

Community

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answered Mar 13 '13 at 21:45

A4L

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How to retrieve the specific element from an array of std::strings as a LPCSTR?

4 Answers4