Variable length array of non-POD element type 'string' (aka 'basic_string') c++

Question

I get this error in my c++ code Variable length array of non-POD element type string (aka basic_string<char>).

string words[numWords];

If I get rid of numWords and enter a number this works fine but if i put the same number in a variable it gives me the Variable length array of non-POD element type 'string' (aka 'basic_string<char>') error, I have done it this way before and it worked in visual studio but I have now tried it in Xcode and it doesn't work. I have tried using vectors but I can't get them sot store any data and they just come back blank.

For those who asked this is my vector code should all be there

char ch;

ifstream repFile("//Users//bobthemac//Documents//c++asignment//c++asignment//test1.txt");

while(repFile.get(ch))
{
    if(ch == ' ' || ch == '\n' || ch == '\t')
    {
        numWords++;
    }
}

vector<string> words (numWords);

while(repFile >> x)
    words.push_back(x);
repFile.close();

This has *nothing* to do with Xcode. It has to do with the compiler you're using. Some version of gcc I presume? What does `gcc -version` say? — Irfy, Mar 03 '12 at 23:18
Using a variable for the size of an array is not standard. Using a vector should work, so the real issue is finding why that doesn't work for you. — Vaughn Cato, Mar 03 '12 at 23:20
possible duplicate of [Variable length arrays in C++?](http://stackoverflow.com/questions/1887097/variable-length-arrays-in-c) — Neil, Mar 03 '12 at 23:21
@bobthemac: lrfy meant to ask what `gcc --version` (two hyphens) says. — ruakh, Mar 03 '12 at 23:22

Carl Norum · Answer 1 · 2012-03-03T23:31:38.723

6

C++ doesn't have C99-style variable length arrays. Your compiler might support them as an extension, but they're not part of the language. In this specific case, your success with Visual Studio indicates that it does in fact have such an extension. clang++ will support VLAs, but only of POD types, so your attempt to make a VLA of string objects won't work. g++ does work on my machine if I leave off enough warning/error flags.

edited Mar 03 '12 at 23:31

answered Mar 03 '12 at 23:21

Carl Norum

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Or it may just be that the dynamic value was used to initialise a `const int` and VS failed to spot that the value didn't originate statically, in order to warn/error. I've seen it happen. – Lightness Races in Orbit Mar 03 '12 at 23:28
Could be. I know approximately nothing about Visual Studio. – Carl Norum Mar 03 '12 at 23:29
Well I've seen it in GCC too. Far more likely than accidentally using an extension, IMO. – Lightness Races in Orbit Mar 03 '12 at 23:42

hmjd · Accepted Answer · 2012-03-03T23:50:03.267

3

This initializes words with numWords empty strings then appends the actual strings afterwards:

vector<string> words (numWords);

while(repFile >> x)
    words.push_back(x);

Change to:

vector<string> words;

while(repFile >> x)
    words.push_back(x);

or:

vector<string> words (numWords);

int idx = 0;
while(repFile >> x /* && idx < numWords */)
    words[idx++] = x;

EDIT:

There is no reason to count the number of words before populating the vector:

vector<string> words;
ifstream repFile("//Users//bobthemac//Documents//c++asignment//c++asignment//test1.txt");
if (repFile.is_open())
{
    while(repFile >> x)
    {
        words.push_back(x);
    }
    repFile.close();
}

edited Mar 03 '12 at 23:50

answered Mar 03 '12 at 23:28

hmjd

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I have tried both of your solutions but one throws an exception and the other still just prints nothing, think it might be my computer or a daft mistake – bobthemac Mar 03 '12 at 23:43
I think you need to reopen `repFile` before the second `while`. – hmjd Mar 03 '12 at 23:45
Thanks that got it i had to close repFile and then open it again thanks for all the help from everyone. – bobthemac Mar 03 '12 at 23:51
1

Why the extra copy in your code example. `while(repFile >> words[idx++]) {}` – Martin York Mar 04 '12 at 03:17
@LokiAstari, oversight. Thanks. – hmjd Mar 04 '12 at 08:29

score 0 · Answer 3 · answered Mar 03 '12 at 23:29

Sorry, you need to write gcc --version to get the version.

As others state, you shouldn't use variable-length arrays, but GCC does support them as an extension in C++. My GCC 4.4.4 compiles just fine with the following code:

#include <string>
#include <iostream>
using namespace std;

int main() {
  int n;
  cin >> n;
  string s[n];
  return 0;
}

Does that code compile for you? If it does, then you need to give us the smallest piece of code that fails.

The best solution, though, is to use vector<string>.

Variable length array of non-POD element type 'string' (aka 'basic_string') c++

3 Answers3

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